DRAFT - DO NOT CITE OR QUOTE
SEFft
EPA/635/R-08/002
www.epa.gov/iris
TOXICOLOGICAL REVIEW
OF
Cerium Oxide and Cerium
Compounds
(CAS No. 1306-38-3)
In Support of Summary Information on the
Integrated Risk Information System (IRIS)
May 2008
NOTICE
This document is an External Review draft. This information is distributed solely for the
purpose of pre-dissemination peer review under applicable information quality guidelines. It has
not been formally disseminated by EPA. It does not represent and should not be construed to
represent any Agency determination or policy. It is being circulated for review of its technical
accuracy and science policy implications.
U.S. Environmental Protection Agency
Washington, DC
-------
1 DISCLAIMER
2
3
4 This document is a preliminary draft for review purposes only. This information is distributed
5 solely for the purpose of pre-dissemination peer review under applicable information quality
6 guidelines. It has not been formally disseminated by EPA. It does not represent and should not
7 be construed to represent any Agency determination or policy. Mention of trade names or
8 commercial products does not constitute endorsement or recommendation for use.
May, 2008
li
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
CONTENTS—TOXICOLOGICAL REVIEW OF CERIUM OXIDE
AND CERIUM COMPOUNDS (CAS NO. 1306-38-3)
LIST OF TABLES v
LIST OF ACRONYMS vii
FOREWORD ix
AUTHORS, CONTRIBUTORS, AND REVIEWERS x
1. INTRODUCTION 1
2. CHEMICAL AM) PHYSICAL INFORMATION 3
3. TOXICOKINETICS 8
3.1. ABSORPTION 8
3.1.1. Oral Exposure 8
3.1.2. Inhalation Exposure 11
3.2. DISTRIBUTION 12
3.2.1. Oral Exposure 12
3.2.2. Inhalation Exposure 12
3.3. METABOLISM 14
3.4. ELIMINATION 14
3.5. PHYSIOLOGICALLY BASED TOXICOKINETIC MODELS 15
4. HAZARD IDENTIFICATION 16
4.1. STUDIES IN HUMANS—EPIDEMIOLOGY, CASE REPORTS, CLINICAL
CONTROLS 16
4.1.1. Oral Exposure 16
4.1.2. Inhalation Exposure 17
4.2. SUBCHRONIC AND CHRONIC STUDIES AND CANCER BIO ASSAYS IN
ANIMALS—ORAL AM) INHALATION 19
4.2.1. Oral Exposure 19
4.2.2. Inhalation Exposure 21
4.3. REPRODUCTIVE/DEVELOPMENTAL STUDIES—ORAL AND INHALATION ... 29
4.4. OTHER DURATION- OR ENDPOINT-SPECIFIC STUDIES 29
4.4.1. Acute Toxicity Studies (Oral and Inhalation) 29
4.4.2. Acute Studies (Injection) 30
4.4.2.1. Neurobehavioral and Neurodevelopmental Effects 30
4.4.2.2. Neurological Effects 31
4.4.2.3. Hepatic Effects 31
4.4.2.4. Cardiovascular Effects 33
4.4.2.5. Hematological Effects 34
4.4.2.6. Renal Effects 35
4.4.3. Genotoxi city 35
May, 2008
in
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
4.5. MECHANISTIC DATA AND OTHER STUDIES IN SUPPORT OF THE
MODE OF.ACTION 36
4.5.1. In Vitro Studies 36
4.5.2. Ex Vivo Studies 41
4.6. SYNTHESIS OF MAJOR NONCANCER EFFECTS 41
4.6.1. Oral 41
4.6.2. Inhalation 42
4.6.3. Mode-of-Action Information 42
4.6.3.1. Respiratory Tissues 42
4.6.3.2. Other Tissues 46
4.7. EVALUATION 01 CARCINOGENICITY 47
4.7.1. Summary of Overall Weight of Evidence 47
4.7.2. Synthesis of Human, Animal, and Other Supporting Evidence 47
4.8. SUSCEPTIBLE POPULATIONS AND LIFE STAGES 48
4.8.1. Possible Childhood Susceptibility 48
4.8.2. Possible Gender Differences 48
4.8.3. Possible Susceptible Populations 48
5. DOSE-RESPONSE ASSESSMENTS 50
5.1. ORAL REFERENCE DOSE (RID) 50
5.1.1. Choice of Principal Study and Critical Effect—with Rationale and
Justification 50
5.2. INHALATION REFERENCE CONCENTRATION (RfC) 51
5.2.1. Choice of Principal Study and Critical Effect—with Rationale and
Justification 51
5.2.2. Methods of Analysis—Including Models (PBTK, BMD, etc.) 53
5.2.3. RfC Derivation—Including Application of Uncertainty Factors (UFs) 54
5.2.4. RfC Comparison Information 57
5.2.5. Previous RfC Assessment 59
5.3. UNCERTAINTIES IN THE INHALATION REFERENCE CONCENTRATION (RfC)
60
5.4. CANCER ASSESSMENT 62
6. MAJOR CONCLUSIONS IN THE CHARACTERIZATION OF HAZARD 63
AND DOSE RESPONSE 63
6.1. HUMAN HAZARD POTENTIAL 63
6.2. DOSE RESPONSE 65
6.2.1. Noncancer/Oral 65
6.2.2. Noncancer/Inhalation 66
6.2.3. Cancer/Oral and Inhalation 68
7. REFERENCES 69
May, 2008
iv
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
LIST OF TABLES
Table 2-1. Physical properties of cerium and selected cerium compounds 4
Table 2-2. Major uses of selected cerium compounds 7
Table 4-1. Hematological changes in male and female Sprague-Dawley rats following
inhalation of cerium oxide aerosol 6 hours/day, 5 days/week for 13 weeks 23
Table 4-2. Absolute lung weight in rats exposed to cerium oxide aerosol 6 hours/day,
5 days/week for 13 weeks 24
Table 4-3. Relative lung weight in rats exposed to cerium oxide aerosol 6 hours/day,
5 days/week for 13 weeks 24
Table 4-4. Relative spleen weight in rats exposed to cerium oxide aerosol 6 hours/day,
5 days/week for 13 weeks 24
Table 4-5. Results of gross pathological examination of lungs of rats exposed to cerium
oxide 6 hours/day, 5 days/week for 13 weeks 25
Table 4-6. Results of gross pathological examination of bronchial, mediastinal, and
pancreatic lymph nodes of rats exposed to cerium oxide 6 hours/day,
5 days/week for 13 weeks 25
Table 4-7. Incidences of histopathologic effects in rats exposed to cerium oxide aerosol
6 hours/day, 5 days/week for 13 weeks 27
Table 4-8. NOAEL and LOAEL values for toxicological effects 27
Table 5-1. Output from RDDR v.2.3 used in the analysis in Section 5.2.2 54
May, 2008
v
DRAFT - DO NOT CITE OR QUOTE
-------
1 LIST OF FIGURES
2
3 Figure 5-1. Exposure-response array of selected toxicity effects from the BRL (1994) study.... 58
4 Figure 5-2. Points of Departure for selected endpoints from Figure 5-1 with corresponding
5 applied uncertainty factors and derived chronic inhalation RfVs 59
6
7
8
May, 2008
vi
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
LIST OF ABBREVIATIONS AND ACRONYMS
3
4
ALT
alanine aminotransferase
5
AST
aspartate aminotransferase
6
BMC
benchmark concentration
7
BMD
benchmark dose
8
CASRN
Chemical Abstract Service Registry Number
9
CFU
colony-forming unit
10
CI
confidence interval
11
COH
coumarin 7-hydroxylase
12
CT
computer tomography
13
CYP
cytochrome P450 (in connection with isozyme abbreviations)
14
CYP450
cytochrome P450
15
DAF
dosimetric adjustment factor
16
DCF
dichlorodihydrofluorescein diacetate
17
DPO
dendriform pulmonary ossification
18
EDTA
ethylenediamine tetraacetic acid
19
EGTA
ethylene glycol bis(2-aminoethylether)tetraacetic acid
20
EPA
Environmental Protection Agency
21
GD
gestation day
22
GI
gastrointestinal
23
GSD
geometric standard deviation
24
GSH
glutathione
25
HEC
human equivalent concentration
26
HEI
Health Effects Institute
27
i.v.
intravenous
28
IRIS
Integrated Risk Information System
29
LC50
median lethal concnetration
30
LDX
dose that kills X% of animals
31
LDH
lactate dehydrogenase
32
LOAEL
lowest-observed-adverse-effect level
33
MCP
monocyte chemoattractant protein
34
MDA
malodialdehyde
35
MMAD
mass median aerodynamic diameter
36
MT
metallothionein
37
NOAEL
no-ob served-adverse-effect level
38
NTP
National Toxicology Program
39
OCT
ornithine-carbamyl transferase
40
PAM
pulmonary alveolar macrophage
41
PBTK
physiologically based toxicokinetic
42
PCNA
proliferating cell nuclear antigen
43
PND
postnatal day
44
RDDR
regional deposited dose ratio
45
RfC
reference concentration
46
RfD
reference dose
May, 2008
vii
DRAFT - DO NOT CITE OR QUOTE
-------
1 SDH
2 SOD
3 UF
4
sorbitol dehydrogenase
superoxide dismutase
uncertainty factor
May, 2008
viii
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
FOREWORD
The purpose of this Toxicological Review is to provide scientific support and rationale
for the hazard and dose-response assessment in IRIS pertaining to chronic exposure to cerium
oxide and cerium compounds. It is not intended to be a comprehensive treatise on the chemical
or toxicological nature of cerium oxide and cerium compounds.
The intent of Section 6, Major Conclusions in the Characterization of Hazard and Dose
Response, is to present the major conclusions reached in the derivation of the reference dose,
reference concentration and cancer assessment, where applicable, and to characterize the overall
confidence in the quantitative and qualitative aspects of hazard and dose response by addressing
the quality of the data and related uncertainties. The discussion is intended to convey the
limitations of the assessment and to aid and guide the risk assessor in the ensuing steps of the
risk assessment process.
For other general information about this assessment or other questions relating to IRIS,
the reader is referred to EPA's IRIS Hotline at (202) 566-1676 (phone), (202) 566-1749 (fax), or
hotline.iris@epa.gov (email address).
May, 2008
IX
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
AUTHORS, CONTRIBUTORS, AND REVIEWERS
CHEMICAL MANAGER
Martin Gehlhaus
National Center for Environmental Assessment
U.S. Environmental Protection Agency
Washington, DC
AUTHORS
Martin Gehlhaus
National Center for Environmental Assessment
U.S. Environmental Protection Agency
Washington, DC
Mark Osier, Ph.D.
Environmental Science Center
Syracuse Research Corporation
Syracuse, NY
Fernando Llados, Ph.D.
Environmental Science Center
Syracuse Research Corporation
Syracuse, NY
Daniel Plewak
Environmental Science Center
Syracuse Research Corporation
Syracuse, NY
Michael Lumpkin, Ph.D.
Environmental Science Center
Syracuse Research Corporation
Syracuse, NY
Marc Odin, Ph.D.
Environmental Science Center
Syracuse Research Corporation
Syracuse, NY
Andrew Rooney, Ph.D.
National Center for Environmental Assessment
U.S. Environmental Protection Agency
Washington, DC
May, 2008
x
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
REVIEWERS
INTERNAL EPA REVIEWERS
Ted Berner
Office of Research and Development
National Center for Environmental Assessment
Kevin Dreyer, M.S, Ph.D.
Office of Research and Development
National Health and Environmental Effects Research Laboratory
Experimental Toxicology Division
Pulmonary Toxicology Branch
Lynn Flowers, Ph D, DABT
Office of Research and Development
National Center for Environmental Assessment
Mary Jane Selgrade, Ph.D.
Office of Research and Development
National Health and Environmental Effects Research Laboratory
Immunotoxicology Branch
Jamie Strong, Ph.D.
Office of Research and Development
National Center for Environmental Assessment
May, 2008
XI
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
1. INTRODUCTION
This document presents background information and justification for the Integrated Risk
Information System (IRIS) Summary of the hazard and dose-response assessment of cerium
oxide and cerium compounds. IRIS Summaries may include oral reference dose (RfD) and
inhalation reference concentration (RfC) values for chronic and less-than-lifetime exposure
durations, and a carcinogenicity assessment.
The RfD and RfC provide quantitative information for use in risk assessments for health
effects known or assumed to be produced through a nonlinear (presumed threshold) mode of
action. The RfD (expressed in units of mg/kg-day) is defined as an estimate (with uncertainty
spanning perhaps an order of magnitude) of a daily exposure to the human population (including
sensitive subgroups) that is likely to be without an appreciable risk of deleterious effects during a
lifetime. The inhalation RfC (expressed in units of mg/m3) is analogous to the oral RfD, but
provides a continuous inhalation exposure estimate. The inhalation RfC considers toxic effects
for both the respiratory system (portal of entry) and for effects peripheral to the respiratory
system (extrarespiratory or systemic effects). Reference values may also be derived for acute
(<24 hours), short-term (>24 hours up to 30 days), and subchronic (30 days up to 10% of
average lifetime) exposure durations, all of which are derived based on an assumption of
continuous exposure throughout the duration specified.
The carcinogenicity assessment provides information on the carcinogenic hazard
potential of the substance in question and quantitative estimates of risk from oral and inhalation
exposure. The information includes a weight-of-evidence judgment of the likelihood that the
agent is a human carcinogen and the conditions under which the carcinogenic effects may be
expressed. Quantitative risk estimates are derived from the application of a low-dose
extrapolation procedure. The oral slope factor is an upper bound on the estimate of risk per
mg/kg-day of oral exposure. Similarly, an inhalation unit risk is an upper bound on the estimate
of risk per (J,g/m3 air breathed.
Development of these hazard identification and dose-response assessments for cerium
oxide and cerium compounds has followed the general guidelines for risk assessment as set forth
by the National Research Council (1983). EPA guidelines and Risk Assessment Forum
Technical Panel Reports that were used in the development of this assessment include the
following: Guidelines for Mutagenicity Risk Assessment (U.S. EPA, 1986b), Guidelines for
Developmental Toxicity Risk Assessment (U.S. EPA, 1991), Guidelines for Reproductive Toxicity
Risk Assessment {U.S. EPA, 1996b), Guidelines for Neurotoxicity Risk Assessment (U.S. EPA,
1998a), Guidelines for Carcinogen Risk Assessment (U.S. EPA, 2005a), Supplemental Guidance
for Assessing Susceptibility from Early-Life Exposure to Carcinogens (U.S. EPA, 2005b),
May, 2008
1
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
Recommendations for and Documentation of Biological Values for Use in Risk Assessment (U.S.
EPA, 1988), (proposed) Interim Policy for Particle Size and Limit Concentration Issues in
Inhalation Toxicity (U.S. EPA, 1994a), Methods for Derivation of Inhalation Reference
Concentrations and Application of Inhalation Dosimetry (U.S. EPA, 1994b), Use of the
Benchmark Dose Approach in Health Risk Assessment (U.S. EPA, 1995), Science Policy Council
Handbook: Peer Review (U.S. EPA, 1998b, 2000a, 2005c), Science Policy Council Handbook.
Risk Characterization (U.S. EPA, 2000b), Benchmark Dose Technical Guidance Document
(U.S. EPA, 2000c), and A Review of the Reference Dose and Reference Concentration Processes
(U.S. EPA, 2002).
The literature search strategy employed for this compound was based on the Chemical
Abstract Service Registry Number (CASRN) and at least one common name. Any pertinent
scientific information submitted by the public to the IRIS Submission Desk was also considered
in the development of this document. The relevant literature was reviewed through June 2007.
May, 2008
2
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
2. CHEMICAL AND PHYSICAL INFORMATION
Cerium is a member of the lanthanide series of metals and is the most abundant of the
rare-earth elements in the earth's crust (average concentration of 50 ppm) (Hedrick, 2004).
Elemental cerium is an iron-gray, ductile, malleable metal (O'Neil, 2001). Cerium metal is very
reactive and is a strong oxidizing agent that is stabilized when associated with an oxygen ligand
(Kilbourn, 2003). When present in compounds, cerium exists in both the trivalent state (Ce3+,
cerous) and the tetravalent state (Ce4+, eerie) (Kilbourn, 2003; Reinhardt and Winkler, 2002).
Chemical structures and selected chemical and physical properties of cerium and cerium
compounds are listed in Table 2-1.
Cerium is found in nature along with other lanthanide elements in the minerals alanite,
bastanite, monazite, cerite, and samarskite; however, only bastanite and monazite are important
sources commercially (Lide, 2005; Kilbourn, 2003). Because of its unique stability in the
tetravalent state (other lanthanides are stable in only the trivalent state), cerium can be separated
out from the other rare-earth elements through oxidation (forming Ce02) followed by variable
solubility filtration (Reinhardt and Winkler, 2002). Cerium salts can be prepared by liquid-
liquid extraction from rare-earth cerium-containing solutions. Cerium metal is prepared by
reacting CeF3 with an excess of calcium at approximately 900°C (Kilbourn, 2003). It can also be
obtained by the fused-salt electrolysis of a mixture of cerium chlorides and fluorides (Reinhardt
and Winkler, 2002).
Cerium is most heavily used in the form of mischmetal for metallurgical purposes
(Kilbourn, 2003; Reinhardt and Winkler, 1996). Cerium is the major component of mischmetal
(50-75% by weight for the most common grades), a commercial mixture of metallic light
lanthanides prepared by the electrolysis of mixed lanthanide chlorides and fluorides obtained
from bastanite or monazite (Kilbourn, 2003; Reinhardt and Winkler, 2002). Mischmetal reacts
with the impurities found in metals to form solid compounds, thereby reducing the effect of these
impurities on the properties of the metal (Reinhardt and Winkler, 2002). Mischmetal has been
used in the manufacture of steel to improve shape control, reduce hot shortness, and increase
heat and oxidation resistance. It can be added to cast iron to improve ductility, toughness, and
microstructure. Mischmetal is also used in the manufacture of cerium-iron alloy lighter flints
(Kilbourn, 2003; Reinhardt and Winkler, 2002).
May, 2008
3
DRAFT - DO NOT CITE OR QUOTE
-------
1 Table 2-1. Physical properties of cerium and selected cerium compounds
Name
Cerium
Cerium oxide
Hydrated cerium oxide
CASRN
7440-45-1
1306-38-3
12014-56-1 (hydroxide)
23322-64-7 (hydrate)
Synonyms
Cerium dioxide; ceria;
cerium(IV) oxide
Hydrated eerie oxide; cerium
hydrate; eerie hydroxide;
cerium(IV) hydroxide; cerium
perhydroxide; cerium
tetrahydroxide
Structure
o
//
o
OH
HO—Ce-OH
OH
or
°* r i
*Ce • OH2
0 L ]2
Molecular weight
140.116
172.11
208.148
Molecular formula
Ce
Ce02
Ce02-2H20
Form
Iron-gray, ductile,
malleable metal
Pale-yellow, heavy powder
(white when pure);
commercial product is
brown
The hydroxide precipitate is
amorphous and on drying,
converts to hydrated eerie oxide;
whitish powder when pure.
Melting point
798°C; boiling point =
3443°C
2400°C
Not available
Density
6.770 g/cm3
7.65 g/cm3
Not available
Water solubility
Decomposes slowly
with cold water and
rapidly with hot water
Insoluble in water
Insoluble in water
Other solubility
Soluble in dilute
mineral acids
Insoluble in dilute acid
Soluble in concentrated mineral
acid
May, 2007
4
DRAFT - DO NOT CITE OR QUOTE
-------
1 Table 2-1 [continued]. Physical properties of cerium and selected cerium
2 compounds
Name
Cerium nitrate
Cerous chloride
Cerous fluoride
CASRN
13093-17-9
7790-86-5
7758-88-5
Synonyms
Cerium tetranitrate;
nitric acid,
cerium(4+) salt
Cerium(III) chloride;
cerium trichloride
Cerium(III) fluoride;
cerium trifluoride
Structure
0
- Ce
0" 0
L J4
Molecular weight
388.136 9
246.48
197.11
Molecular formula
Ce(N03)4
CeCl3
CeF3
Form
Not available
White crystals; fine
powder
Hexagonal crystals or
powder
Melting point
Not available
817°C
1430°C
Density
Not available
3.97 g/cm3
6.157 g/cm3
Water solubility
Not available
Soluble in water
Insoluble in water
Other solubility
Not available
Soluble in ethanol
Soluble in acids
(monohydrate)
3
May, 2007
5
DRAFT - DO NOT CITE OR QUOTE
-------
1 Table 2-1 [continued]. Physical properties of cerium and selected cerium
2 compounds
Name
Cerous acetate
Cerous citrate
CASRN
537-00-8
512-24-3
Synonyms
Cerium(III) acetate; cerium
triacetate; acetic acid;
cerium(3+) salt
Cerium citrate;
cerium(III) citrate;
cerium(3+)
2-hydroxypropane-
1,2,3 -tricarboxy late
Structure
r o i
_X Ce3*
0 CH3_ 3
P .OH
° \ 1 /°
\ ( o*
cAo°
Molecular weight
317.251
329.219
Molecular formula
Ce(C2H302)3
C6H807.Ce
Form
Not available
Not available
Melting point
Not available
Not available
Density
Not available
Not available
Water solubility
Not available
Not available
Other solubility
Not available
Not available
3
4 Sources: ChemlDplus (2006); Lide (2005); Lewis (2001); O'Neil (2001).
May, 2007
6
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
Exposure to commercially used cerium compounds is most likely through exposure to
cerium (eerie) oxide (Ce02). It is used either in the pure form or in a concentrate as a polishing
agent for glass mirrors, plate glass, television tubes, ophthalmic lenses, and precision optics
(Kilbourn, 2003; Reinhardt and Winkler, 2002). Cerium oxide is used as a glass constituent to
prevent solarization and discoloration (especially in the faceplates of television screens)
(Reinhardt and Winkler, 2002). Cerium oxide is also used in emission control systems in
gasoline engines and as a diesel fuel-born catalyst to reduce particulate matter emissions (Health
Effects Institute [HEI], 2001; Reinhardt and Winkler, 1996). Cerium nitrate has been used as a
topical treatment for burn wounds (Monafo et al., 1976). Major uses for selected cerium
compounds are listed in Table 2-2.
Table 2-2. Major uses of selected cerium compounds
Name
CASRN
Use
Cerium oxide
1306-38-3
Polishing and decolorizing glass; opacifier in vitreous enamels
and photochromic glasses; heat-resistant alloy coatings; as a
cracking catalyst; as a catalyst for automobile emission control; in
ceramic coatings; in phosphors; in cathodes; in capacitors; in
semiconductors; in refractory oxides; gemstone polishing
Hydrated cerium oxide
23322-64-7
Production of cerium salts and cerium oxide
Cerous chloride
7790-86-5
In the manufacturing of cerium metal and cerium salts; catalyst for
the polymerization of olefins
Cerous fluoride
7758-88-5
In the preparation of cerium metal; in arc carbons to increase
brilliance
Sources: Kilbourn (2003); HEI (2001); Lewis (2001); O'Neil (2001); Wells and Wells (2001).
Cerium is not expected to exist in elemental form in the environment since it is a very
reactive metal (Lewis, 2001). Cerium compounds are not expected to volatilize and will exist in
the particulate form if released into the air. For cerium compounds that are soluble in water,
Ce3+ would likely have a pKa close to La3+ (8.5) (Wulfsberg, 2000), which indicates that the
hydrated Ce3+ ion ([Ce(H20)n]3+) will remain in solution at environmental pHs (4-9). The
hydrated Ce4+ ion ([Ce(H20)n]4+) is expected to hydrolyze and polymerize at environmental pH
(Cotton et al., 1999) and may precipitate out of solution. In general, metal cations in solution are
attracted to the surfaces of soil particles, and the extent of adsorption to soils will depend on the
soil characteristics (e.g., pH, mineral content, organic content) (Evans, 1989).
May, 2007
7
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
3. TOXICOKINETICS
Many of the studies of the toxicokinetics of cerium were conducted using radioactive
cerium. Stable and radioactive cerium are expected to behave in a similar toxicokinetic manner
and possess the same chemical properties. Radioactive cerium is a beta-emitter. As such, data
from studies using either stable or radioactive cerium are presented below. Data characterizing
the toxicokinetics of cerium compounds, such as cerium oxide, are discussed in order to inform
the overall database.
3.1. ABSORPTION
3.1.1. Oral Exposure
Studies evaluating the absorption of cerium compounds following oral exposure in
humans are not available.
In adult animals, cerium compounds are very poorly absorbed following oral exposure,
while suckling animals exhibit higher absorption and retention of cerium in the gastrointestinal
(GI) tissues. Observed absorption of radioactive cerium salts from the GI tract of adult rats
ranged from 0.05% to less than 0.1% of the administered dose (Kostial et al., 1989b; Inaba and
Lengemann, 1972; Shiraishi and Ichikawa, 1972). Suckling rats, however, absorbed 40-98%) of
administered dose, with the youngest rats retaining the largest percentage of the dose (Kostial et
al., 1989a, b; Inaba and Lengemann, 1972).
Four litters of Sprague-Dawley rats (n = 7-9), age 0, 7, 14, or 26 days, were given a
single dose of [141Ce]-ceric nitrate of unreported concentration by intragastric dosing (Inaba and
Lengemann, 1972). Subsequent, periodic whole-body radioactivity measurements were taken
immediately after dosing and periodically thereafter. In another experiment in this study, two
litters of 1-day-old newborn rats were given single doses of [141Ce]-ceric nitrate, and one rat
from each litter was sacrificed at 1, 3, 5, 7, and 10 days after dosing, after which radioactivity in
the GI tract and whole body was measured and macro-autoradiographs of the GI tract were
produced.
It was observed that on or about day 16 of life, rats began consuming a solid, grain-based
diet and were completely weaned by day 26 (Inaba and Lengemann, 1972). In weanling rats
(rats dosed at 26 days of age), only 0.04%> of the administered radioactivity remained in the body
by 3 days after dosing. However, radioactivity in newborns diminished more slowly, dropping
from 98%o of administered dose on day 1 to 29%> on day 16. The GI tract accounted for nearly
100%o of whole-body retention on day 1 and 93%> on day 16. After the onset of weaning in the
suckling rats dosed as newborns, whole-body radioactivity fell to 3%> of administered dose by
day 24, of which only 17%> was measured in the GI tract. Autoradiographs of the GI tract from
the two litters dosed as 1-day-old newborns suggest rapid transit of radioactivity to the lower
May, 2007 8 DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
small intestine 1 day after exposure. Autoradiography of rats 5 days after dosing (6 days old)
suggests that the intestinal radioactivity is restricted to the upper two-thirds of the epithelial villi,
which the study authors associated with cerium concentration in the vacuoles.
Kostial et al. (1989b) administered single oral doses of an unreported concentration of
[141Ce]-cerous chloride by intragastric dosing to 6-day-old and 6-8-week-old rats (strain
unreported) to investigate whether distribution of cerium (and other metals) in the GI tract
differed in suckling versus mature rats. Six-day-old rats were measured for whole-body and gut
radioactivity 2, 4, 6, and 12 days after dosing, while adult rats were measured 2 days after
dosing. Orally administered cerium was more readily retained in the whole body, gut, and
carcass of suckling rats than in older rats. The ileum was the main site for cerium accumulation
in the suckling rats following oral administration, while the stomach and large intestine were the
main sites for cerium accumulation in the 6-8-week-old rats. Whole-body radioactivity 6 days
postexposure in 2-week-old suckling rats orally dosed with [141Ce] was 40% of the administered
dose, of which 95% was found in the gut. However, whole-body radioactivity 6 days
postexposure in adult (6-8-week-old) rats was only 0.05%, of which 66% was found in the gut,
mostly in the stomach and cecum.
Shiraishi and Ichikawa (1972) administered single oral doses of an unreported
concentration of [144Ce]-cerous chloride by intragastric dosing to 0-, 7-, 14-, and 21-day-old
juvenile and 100-day-old adult Wistar rats. Cortisone acetate, which alters the morphology of
the absorptive epithelium of the small intestine, was injected to a group of 7-day-old rats 4 days
prior to [144Ce]-cerous chloride exposure to observe its effect on whole-body retention.
Periodically (up to 70 days after dosing), small groups of rats from all age groups were sacrificed
and measured for radioactivity in the whole-body, gut, and various excised tissues. The decrease
in retention of whole-body radioactivity among suckling rats dosed at 0, 7, and 14 days after
birth was approximately 11, 6.5, and 1.5%, respectively, of the administered dose. Weanling and
adult rats exhibited a rapid decrease in whole-body radioactivity through 10 days following
dosing. At 2 weeks after dosing, the 21- and 100-day-old adults had whole-body radioactivity of
0.08 and 0.018%>, respectively, of the administered dose. The intestinal content accounted for
most of the retained radioactivity in neonates until the age of weaning. Cortisone acetate
treatment resulted in a more rapid loss of the oral dose from young rats, although whether this is
due to lower uptake by intestinal cells or a more rapid release to feces is not presently clear.
This investigation demonstrated that the whole-body retention of cerium by suckling rats was
greater than the retention by weanling and adult rats, and this increased retention by suckling rats
may be due to increased pinocytic activity in the absorptive epithelium of sucklings (Shiraishi
and Ichikawa, 1972).
Yorkshire piglets treated with [144Ce]-cerous chloride by gavage on the first or fourth day
after birth and sacrificed 4 or 18 or 3 or 21 days, respectively, after dosing absorbed 2.5—8% of
May, 2007
9
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
the administered dose (Mraz and Eisele, 1977). Absorption was threefold greater in piglets
treated at 1 day of age versus those treated at 4 days of age. Body content of cerium did not
differ significantly between piglets sacrificed at the earlier dates and those sacrificed later,
indicating that absorption was almost complete within 3-4 days.
Eisele et al. (1980) gave single gavage doses of either [144Ce]-cerous chloride or
[144Ce]-cerous citrate (concentration unreported) to 0-6- or 6-24-hour-old C3H mice and
Sprague-Dawley rats and to 6-24-hour-old Yorkshire piglets. Radioactivity levels were
measured in the GI tract and other tissues, including the remaining carcass, at days 1, 5, 7, 9, 12,
15, 17, 19, and 21. In the 0-6-hour-old mice, a high of 31% of the administered dose of cerium
chloride was retained in the body 9 days following exposure. At 21 days post administration, the
amount of pooled cerium citrate and chloride retained was approximately 25%. The mice dosed
0-6 hours after birth retained more cerium in the GI tract throughout the 21-day observation
period than the mice dosed 6-24 hours after birth. The 0-6- and 6-24-hour-old rats exhibited
absorption of 9—10% of administered cerium 21 days after exposure. In the Yorkshire piglets,
the absorbed dose did not differ significantly over the 21-day observation period.
Two studies in adult rats also reported low absorption of cerium following oral
exposures. Durbin et al. (1956) administered single [144Ce]-ceric nitrate (unreported
concentrations) intramuscular and intragastric doses to adult female Sprague-Dawley rats. The
rats dosed intramuscularly were sacrificed at post-administration days 1, 4, 64, and 256, while
the intragastric-dosed rats were sacrificed 4 days after administration. Less than 0.1% of the
intragastric-administered dose was absorbed from the GI tract.
Stineman et al. (1978) administered single intragastric [141Ce]-cerous chloride doses of
1000 mg/kg (lethal to 5% of the animals [LD5]) and 1163 (LD25) mg/kg to 6-8-week-old male
Swiss ICR mice and sacrificed them at 4 hours or at 1, 3, or 7 days later. No measurements were
made of whole-body radioactivity; however, 91-99% of radioactivity in the 12 sampled tissues
was found in the gut (stomach + duodenum).
In young (suckling) animals, cerium appears to be retained in intestinal cells, particularly
in the ileum, possibly resulting in a much greater absorption than in adult animals (Kostial et al.,
1989a, b; Inaba and Lengemann, 1972). However, cerium retained in intestinal cells has been
demonstrated to be unavailable systemically (Inaba and Lengemann, 1972). The high gut
retention of cerium in young animals may be associated with high pinocytotic activity of the
newborn intestinal cells (Kostial et al., 1989b). Glucocorticoids, such as methylprednisolone,
stimulate production of endogenous corticosteroids, which cause precocious gut closure
(decreased pinocytosis) and maturation of the metal absorptive process (Kargacin and Landeka,
1990). Administration of methylprednisolone in conjunction with artificial feeding of [141Ce]-
cerous chloride in cows' milk to 4-day-old suckling rats (strain not reported) resulted in a nearly
40-fold reduction in the amount of cerium detected in gut tissue (Kargacin and Landeka, 1990).
May, 2007
10
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
This finding suggests that the pinocytotic activity of the intestinal cells contributed to the
differences. Similarly, injection with cortisone acetate resulted in a more rapid loss of an oral
dose to feces of young rats (Shiraishi and Ichikawa, 1972), although whether this is due to lower
uptake by intestinal cells or a more rapid release is not presently clear.
3.1.2. Inhalation Exposure
Studies evaluating the deposition or absorption of cerium compounds following
inhalation exposure in humans are not available. However, cerium has been detected in the lung
tissue and alveolar macrophages of subjects believed to have been exposed to cerium
occupationally.
Case reports and a retrospective occupational investigation provide support for the
limited absorption of cerium deposited in the lung following inhalation exposure.
Transbronchial biopsies in a 60-year-old movie projectionist showed cerium concentrations of
11 p,g/g wet weight after 12 years of exposure (Porru et al., 2001). McDonald et al. (1995)
demonstrated particulate material (diameter range from <1 [j,m to 5-10 |iin) localized within lung
biopsy cells by using a scanning electron microscope. Analysis of bronchioalveolar lavage fluid
from a 58-year-old patient exposed to rare earth dusts and asbestos revealed cerium and
phosphorus in the alveolar macrophages (Pairon et al., 1995). The cerium particles accounted
for 70% of the particles observed in the lung tissue and were also identified in the interstitial
macrophages.
Microscopic examination of the tracheobronchial lymph nodes of a movie projectionist
of 25 years revealed grey granules in large macrophages, which were characterized as calcium
and rare earth elements (cerium, lanthanum, and neodymium) by energy-dispersive analysis of
X-rays (Waring and Watling, 1990). Energy-dispersive X-ray also characterized dark particles
(diameter of 1 to 6 (j,m) as cerium, unidentifiable by optical microscopy, in the bronchioalveolar
lavage of a 13-year photoengraver.
Lung tissues from a photoengraver exposed to smoke from cored carbon arc lamps for
46 years (Pietra et al., 1985) were found to have cerium concentrations 2,800-207,000 times
higher than those of urine, blood, or nails, suggesting that cerium particles in the lung are poorly
mobilized. The concentration of cerium in the lung and lymph nodes of a subject exposed to
cerium for 46 years as a photoengraver was 167 and 5 j_ig/g wet tissue weight, respectively, and
2,400- and 53-times higher, respectively, than the concentration in unexposed control subjects
(Vocaturo et al., 1983; Sabbioni et al., 1982).
Pairon et al. (1994) performed a retrospective evaluation of retention of cerium-
containing particles in the lungs of workers previously exposed to mineral dusts.
Bronchioalveolar lavage and lung tissue samples from mineral dust exposed workers and
controls were examined for cerium content. In the seven cases that were judged to have high
May, 2007
11
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
cerium particle retention (as defined by having lavage fluid or tissue cerium concentrations at
least 5 times higher than those of controls), time since last exposure ranged from present to 29
years, with one patient's exposure time not available.
Limited animal data are available regarding total deposition of cerium aerosols within the
respiratory tract. Thomas et al. (1972) exposed 4-month-old, mixed sex Holtzman rats to two
concentrations (unreported) of aerosolized 1.4 |iin median aerodynamic diameter (GSD of 2.0)
[144Ce]-ceric hydroxide for 10 minutes. Whole-body radioactivity measurements were used to
identify a 28% deposition rate of inhaled cerium aerosol. Boecker and Cuddihy (1974) reported
an average deposition of 71% in beagles exposed to [144Ce]-cerous chloride for 4-10 minutes. In
both studies, cerium was deposited in the lungs with evidence of absorption from the lung
provided by the subsequent detection of cerium in the skeleton, liver, and kidneys.
3.2. DISTRIBUTION
3.2.1. Oral Exposure
Although cerium appears to be poorly absorbed from the GI tract, the bone and liver were
the organs with the highest cerium levels in rats following oral gavage of cerium chloride
(Shiraishi and Ichikawa, 1972). The concentration of cerium in the kidney, liver, lung, and
spleen of male ICR mice was significantly elevated relative to controls following 6 and 12 weeks
of oral exposure to 20 or 200 ppm cerium chloride (Kawagoe et al., 2005). The lung and spleen
contained the highest cerium concentrations in male ICR mice.
Manoubi et al. (1998) gave a single intragastric dose of stable cerium nitrate (20 mg/mL)
to Wistar rats. Three hours after dosing, cerium was found in the lysosomes of the duodenal
villosity but not in the liver or spleen. In 1-day-old Sprague-Dawley rats given a single
intragastric dose of [141Ce]-ceric nitrate of unreported concentration, Inaba and Lengermann
(1972) found cerium to be localized centrally, likely in the vacuoles, within epithelial cells of the
small intestine.
Cerium is capable of crossing the placenta and entering the fetal circulation in mice, but
the amounts found in the uterus and placenta were generally less than 5% of the maternal body
burden and decreased rapidly with increased time after exposure (Naharin et al., 1969). Fetal
body burdens in rodents were generally less than 1% of the initial maternal body burden after
either injected or oral administration (Levack et al., 2002; Inaba et al., 1992; Naharin et al.,
1969). Small amounts of injected cerium were also found in the maternal milk of mice (Naharin
et al., 1974), although at a very small proportion (<0.01%) of the maternal body burden.
3.2.2. Inhalation Exposure
As poorly soluble particles, cerium particles behave like other airborne particles,
depositing within the respiratory tract based on aerodynamic character (Schulz et al., 2000).
May, 2007
12
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
Hamsters inhaling [144Ce]-cerium oxide aerosols with particle activity median diameters of 0.11
and 0.06 [j,m exhibited lung burdens of 3.6% at 5 hours and 50% at 3 hours after exposure,
respectively, of initial body burden (Kanapilly and Luna, 1975).
Once deposited in the lung, insoluble cerium compounds may dissolve slowly, as
evidenced by the low percentage of cerium found in other tissues. In an investigation of the
toxicity of insoluble cerium, Hahn et al. (2001) exposed beagles to aerosolized [144Ce]-fused
aluminosilicate particles for 2-48 minutes and collected a variety of tissues (tissues studied not
reported) at death (up to 6,205 days following exposure). Cerium was dissolved from the lung
into the systemic circulation and observed in the liver, skeleton, and tracheobronchial lymph
nodes. Hahn et al. (2001) found between 1.0 and 10% of the initial lung burden of [144Ce]-fused
aluminosilicate particle aerosol in the liver and skeleton of beagles observed for 800 days
following 2-48-minute inhalation exposures. Hahn et al. (1999) also observed [144Ce]-fused
aluminosilicate particle translocation to the tracheobronchial lymph nodes following acute
inhalation of the particles. Lundgren et al. (1992) exposed adult F344/Crl rats to [144Ce]-cerium
oxide for 5-50 minutes or to bimonthly exposures of 25 minutes for 1 year; rats were sacrificed
at 1 hour and 3, 7, 14, 28, 56, 112, 224, 448, 560, and 672 days after exposure. The lungs, heart,
liver, spleen, kidney, and skeleton (remaining carcass) were measured for cerium. Cerium was
detected in the liver and skeleton in increasing percentages of body burden with respect to time,
while cerium was not detected in the spleen and kidneys.
More soluble forms of cerium (e.g., cerium citrate) may be systemically absorbed more
easily from the lung due to the increased solubility of the compound. Morgan et al. (1970)
exposed Swiss mice to aerosols of [144Ce] in the form of chloride, citrate, or fused clay, with
activity median diameters from 1.3 to 2.75 [j,m, in unreported concentrations or durations. While
the initial body burden of all forms decreased rapidly during the first 2 weeks, likely due to
mucociliary elimination to the GI tract, the remaining lung burden for the relatively insoluble
fused clay remained higher than the chloride or citrate for the duration of the study (130 days).
Conversely, the liver burdens of the citrate and chloride forms remained higher than the fused
clay by about an order of magnitude. Further, as lung burdens of the chloride and citrate forms
decreased, the bone burdens of these forms increased. The bone burden for the fused clay form,
like the liver, was about an order of magnitude lower than the citrate or chloride forms.
Sturbaum et al. (1970) exposed 40 Chinese hamsters via the nose to [144Ce]-cerous
chloride aerosol, activity median diameter of 0.83 |im and a GSD of 1.7, for 20 minutes and
sacrificed small groups (n = 4) at 2, 8, 16, 28, 64, 128, and 256 days after exposure. Whole-body
and tissue (types unreported) measurements of cerium radioactivity were made. The liver and
skeleton exhibited between 1 and 10% of the initial body burden throughout the post-
administration measurements, while the lung portion of the initial body burden diminished from
approximately 20 at two hours to <1% by study's end.
May, 2007
13
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
Cerium has been observed to be localized in the cell, particularly in the lysosomes, where
it is concentrated and precipitated in an insoluble form in association with phosphorus. Wistar
rats were exposed to stable cerium chloride aerosol, mean diameter of 0.1 [j,m, 5 hours/day for
either 5 days or 4 days/week for 4 weeks (Galle et al., 1992; Berry et al., 1989, 1988). Several
hours after exposure, cerium was deposited in the lysosomes of alveolar macrophages. The
cerium deposits appeared to be in the form of aggregates of fine granules or fine needles that
varied in length from 30 to 60 nm, with the longer needles resulting from the 4-week exposure.
Cerium was found in the lysosomal fraction of liver centrifugate collected from rats (strain
unreported) given an intravenous (i.v.) injection of 1.3 mg/kg [141Ce]-cerous chloride (Wiener-
Schmuck et al., 1990). Cerium was also found in the lysosomes of the duodenal villosity, but not
in the liver or spleen of Wistar rats following intragastric dosing. Cerium was also localized
centrally, likely in the vacuoles, within epithelial cells of the small intestine of Sprague-Dawley
rats.
3.3. METABOLISM
As an element, cerium is neither created nor destroyed within the body. The particular
cerium compound (e.g., cerium chloride, cerium oxide) may be altered as a result of various
chemical reactions within the body, particularly dissolution, but data have not demonstrated a
change in the oxidation state of the cerium cation. Exposure to cerium has been shown to
change hepatic levels of some cytochrome (CYP) P450 isozymes in a species- and strain-
sensitive manner for mice. Salonpaa et al. (1992) gave i.v. cerous chloride injections of 2 mg/kg
to adult DBA/2 and C57BL/6 mice and observed increases in expression of CYP2A4 and
CYP2A5 in the livers (2 and 3 days after dosing) and in the kidneys (6 hours and 1 day after
dosing) of D2 mice but not in B6 mice. Arvela et al. (1991) gave i.v. cerous chloride injections
of 0.5, 1, and 2 mg/kg to adult male DBA/2 and C57BL/6 mice and found a greater sensitivity to
increased CYP450 expression (isoform not reported) in DBA/2 and C57BL/6 mice 24 hours and
3 days after exposure, respectively. Conversely, Arvela and Karki (1971) observed a 50%
reduction, compared to controls, in CYP450 activity in adult Sprague-Dawley rats 3 days after a
single i.v. injection of 2 mg/kg cerous chloride. The effect of changes in CYP450 levels on the
toxicokinetics or toxicity of cerium, if any, is not known. In addition, the relatively high
intravenous bolus doses used in the available studies may not be relevant to oral or inhaled
exposure to cerium oxide.
3.4. ELIMINATION
Following inhalation exposure, the initial rapid elimination of cerium from the body is
due primarily to transport up the respiratory tract by the mucociliary escalator and eventual
swallowing of the material, as with other poorly soluble particles (Boecker and Cuddihy, 1974).
May, 2007
14
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
Initial short-term clearance rates range from 35 to 95% of initial cerium body burden, depending
on the species tested and length of clearance time investigated. Lundgren et al. (1992) exposed
adult F344/Crl rats to [144Ce]-cerium oxide aerosol for 5-50 minutes, with clearance of
approximately 90% of the initial body burden by 7 days. Kanapilly and Luna (1975) exposed
hamsters to [144Ce]-cerium oxide aerosols with particle activity median aerodynamic diameters
of 0.11 and 0.06 [j,m and observed decreases in initial body burden of 95 and 60%, respectively,
4 days after exposure. Differences in clearance rates may have been dependent on particle size
differences, with the smaller particles taking more time for elimination; however, the authors
also stated that the difference may have resulted from a leak in the inhalation chamber used for
the first dose group. Boecker and Cuddihy (1974) observed an early clearance of initial body
burden from 35-80%> for individual dogs 4 days after exposure. Thomas et al. (1972) exposed
Holtzman rats to two concentrations (unreported) of aerosolized [144Ce]-ceric hydroxide for
10 minutes and observed approximately 75-95% clearance of initial body burden within 2 weeks
of exposure (Thomas et al, 1972). Sturbaum et al. (1970) reported clearance of 80%> of initial
cerium body burden by 7 days in Chinese hamsters exposed to [144Ce]-cerous chloride aerosol
for 20 minutes. After the initial clearance of cerium particles from the upper respiratory tract,
pulmonary clearance is slower, with reported slow-phase clearance half-times ranging from 100
to 190 days in rodents (Lundgren et al., 1974; Thomas et al., 1972; Morgan et al., 1970;
Sturbaum et al., 1970). The slow-phase clearance was slightly faster in beagles, with an
estimated half-time of 63 days (Boecker and Cuddihy, 1974). Slow-phase clearance from the
lung is a combination of cerium dissolution and absorption (Morgan et al., 1970) and mechanical
clearance from the respiratory tract (Sturbaum et al., 1970).
Elimination of orally administered cerium has been shown to be age dependent in
animals, with suckling animals absorbing cerium into the GI tissues (Inaba and Lengemann,
1972). This cerium remains in the intestinal cells, is not available systemically, and is eventually
eliminated in the feces.
Although quantitative estimates of cerium elimination are rare, it appears that the primary
route of elimination for cerium, whether inhaled, ingested, or injected, is through the feces, with
small (generally <10%>) amounts eliminated in the urine (Lustgarten et al., 1976; Durbin et al.,
1956). It has been suggested that the fecal excretion of systemically absorbed cerium is due to
elimination in the bile (Lustgarten et al., 1976), since hepatic clearance was due primarily to
biliary function.
3.5. PHYSIOLOGICALLY BASED TOXICOKINETIC MODELS
No physiologically based toxicokinetic (PBTK) models for cerium oxide or other cerium
compounds were located in the evaluated literature.
May, 2007
15
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
4. HAZARD IDENTIFICATION
4.1. STUDIES IN HUMANS—EPIDEMIOLOGY, CASE REPORTS, CLINICAL
CONTROLS
4.1.1. Oral Exposure
An association between exposure to rare earth elements, cerium in particular, in food and
the development of endomyocardial fibrosis has been suggested (Eapen, 1998; Kutty et al., 1996;
Valiathan et al., 1989). Cerium levels were elevated in the endomyocardial tissue samples from
patients who died of endomyocardial fibrosis compared with the tissues of controls who died of
accidents or congenital heart disease (Valiathan et al., 1989). Although causality has not been
conclusively demonstrated, a higher incidence of endomyocardial fibrosis has been reported
among a population consuming tubers grown in a region of India with high soil cerium
concentrations, compared to subjects consuming tubers grown in a soil with low cerium
concentrations. Analysis of the geographic distribution of endemic endomyocardial fibrosis in
India suggested a link to high cerium soil concentrations and possibly to magnesium deficiency
during childhood (Kutty et al., 1996).
A case-control study was conducted by Gomez-Aracena et al. (2006) to investigate the
role of chronic cerium exposure in coronary heart disease. Chronic cerium exposure was
represented by toenail cerium concentrations, and the occurrence of first myocardial infarction
was the characterization used for coronary heart disease. Odds ratios were calculated by
comparing the four exposure groups of 111, 142, 171, and 257 (J,g/kg toenail cerium
concentration to the control group. Gomez-Aracena et al. (2006) found an association between
increased toenail cerium concentrations and the risk of first myocardial infarction, when
controlling for confounding factors, such as smoking, body mass index history of hypertension,
diabetes, family history of coronary heart disease, P-carotene, lycopene, a-tocopherol, selenium,
mercury, and scandium. The odds ratio of first myocardial infarction in smokers with a toenail
cerium concentration of 257 (J,g/kg and without adjusting for additional risk factors of
myocardial infarction was 1.18 (95% confidence interval [CI]: 0.83-1.66), with ap value for
trend of 0.020. In nonsmokers, the odds ratio of first myocardial infarction with toenail cerium
concentrations of 169 and 227 (^g/kg were 2.09 (95% CI: 1.05-4.16) and 2.81 (95% CI: 1.21—
6.52), respectively, with ap value for trend of 0.011, when controlling for the confounding
factors listed above. The results of Gomez-Aracena et al. (2006) suggest a relationship between
chronic cerium exposure and increased risk of acute myocardial infarction, with the strongest
association observed in nonsmokers.
May, 2007
16
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
4.1.2. Inhalation Exposure
Reports have been published describing numerous cases of workers who developed
adverse lung effects, such as interstitial lung disease or pneumoconiosis, associated with
accumulation of cerium in the lungs after prolonged occupational exposure to cerium fumes or
dust (Yoon et al., 2005; Porru et al., 2001; McDonald et al., 1995; Pairon et al., 1995, 1994;
Sulotto et al., 1986; Vogt et al., 1986; Pietra et al., 1985; Vocaturo et al., 1983; Sabbioni et al.,
1982; Husain et al., 1980; Kappenberger and Biihlmann, 1975; Heuck and Hoschek, 1968).
The workers in the above reports had been exposed to cerium for periods of 10-46 years, with
exposures most commonly due to fumes from carbon arc lamps. These lamps, widely used in
the past in the fields of cinematography and photoengraving, have a central core consisting of
approximately 46% cerium oxide and smaller amounts of other rare earth oxides, including
lanthanum, neodymium, praseodymium, and samarium (Waring and Watling, 1990). As the
core burns, it emits oxide, and, to a lesser extent, fluoride dusts of cerium and the other rare
earth elements. Cases of cerium pneumoconiosis not associated with carbon arc lamps all
involved exposure to cerium oxide, either during processing or peripheral to its use as an
abrasive to grind and polish lenses (McDonald et al., 1995). Exposure concentrations of
cerium were not quantified in any of these studies.
Dendriform pulmonary ossification (DPO), a rare condition characterized by branching,
bony spicules often containing marrow, which are found in the lung parenchyma and are
associatied with pulmonary fibrosis, was observed in a 38-year-old man who worked as a
polisher in a crystal factory for 3 years (Yoon et al., 2005). Diffuse reticulonodular infiltrates in
the lung were observed in a chest radiograph, and a computer tomography (CT) scan showed
diffuse, tiny, circular, or beak-like densities with branching structures in the interlobular septum.
A CT scan with bone-setting showed branching, twig-like ossified masses in the right lower
lobe and a few dot-like ossifications in both lower lobes. The lung surface appeared irregular,
emphysematous, and mottled with anthracotic pigmentation during an open lung biopsy, with
several thorn-like hard materials in the lung parenchyma. Microscopic examination revealed
interstitial fibrosis, peripheral emphysema, multiple particles, and pneumonia. The particles
(0.1-0.3 (j,m) were determined to be cerium oxide and phosphates of cerium and lanthanum by
energy dispersive X-ray analysis, with particles of quartz, feldspar, mica, kaolinite, halloysite,
talc, and TiC>2 infrequently detected. Yoon et al. (2005) characterized this case study as the first
to present a case of DPO associated with pneumoconiosis caused by the inhalation of rare earth
metals.
A 60-year-old male who worked as a movie projectionist and was exposed to rare earth
dusts for 12 years presented with diffuse interstitial lung fibrosis, emphysema, and severe
obstructive impairment (Porru et al., 2001). An increase in the lung concentration of rare earth
elements was evident in the subject with the highest concentration for cerium, compared with the
May, 2007
17
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
five unexposed controls. Interstitial fibrosis accompanied by vascular thickening, reactive
alveolar macrophages, abundant macrophages in the air space, and moderate chronic interstitial
inflammation, along with small interstitial clumps of macrophages bearing scant deposits of
grayish-black pigment, was observed in a 68-year-old man who was employed as an optical lens
grinder for 35 years and smoked for 20 years (McDonald et al., 1995). Pairon et al. (1995)
identified particles containing cerium, lanthanum, and phosphorus in the alveolar macrophages
from a 58-year-old smoker with dyspnea who had been exposed to asbestos and rare earth dusts
as a crystal manufacturer, polisher, and movie projectionist from 1951-1967. Diffuse fibrosis of
interalveolar septa and perivascular hyalinized fibrosis was observed from the histologic analysis
(Pairon et al., 1995). Mild interstitial fibrosis, peribronchiolar fibrosis, and diffuse interstitial
fibrosis with emphysema were seen in a photoengraver or glass polisher, nonsmoking foundry
worker, and a movie projectionist and glass polisher, respectively (Pairon et al., 1994).
Deposits of carbon arc lamp fumes were evident in the marcrophages in the
tracheobronchial lymph nodes of a 66-year-old smoking movie projectionist of 25 years,
although pneumoconiosis was not considered because respiratory symptoms and radiographic
and histologic changes were not apparent (Waring and Watling, 1990). Dark particles, identified
as cerium by energy dispersive X-ray analysis, were observed in lung tissue from a 48-year-old
smoker employed as a photoengraver for 13 years (Sulotto et al., 1986). A chest X-ray showed a
micronodular pattern extending to all lung fields although lung examinations were normal and
the patient did not experience respiratory impairment. Vogt et al. (1986) observed five
reproduction photographers, exposed to carbon arc fumes for more than a decade, with slowly
progressive respiratory function restriction, as well as interstitial lung fibrosis and the
accumulation of fine, granular dusts, characterized as rare earth minerals and primarily cerium,
in the lung tissue.
Pulmonary hypertension with increased vascular resistance was observed in a 58-year-old
man who worked in the photoengraving industry for 46 years (Vocaturo et al., 1983; Sabbioni et
al., 1982). Rare earth elements, primarily cerium, were observed by neutron activation analysis
in the lung and lymph node biopsies, and the concentrations of the elements in these tissues were
greater than those in control subjects (Pietra et al., 1985).
Profuse discrete nodular shadowing was present in the chest X-ray of a 34-year-old
employee at a glass rubbing polish plant, although pulmonary function appeared normal (Husain
et al., 1980). The subject refused histologic examination, and an analysis of the occupational
dust concentrations revealed high levels of cerium oxide (50%) and other rare earth oxides.
A 65-year-old man working in the photographic department of a printing plant
demonstrated inactive right apical infiltrates in a chest roentgenogram in 1948; in 1951 diffuse
spotty infiltrates were noted; in 1953 and 1959 the infiltrates were more pronounced in the
middle and lower fields; and in 1965 slight fibrosis of the surrounding tissue was evident, along
May, 2007
18
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
with a perifocal emphysema (Heuck and Hoschek, 1968). Heuck and Hoschek (1968) also
documented fibrosis and small infiltrates in the lung of a 53-year-old male exposed to carbon arc
lamp smoke in printing industries and infiltrates in a 67-year-old man, with chemotherapy-
treated tuberculosis in both upper lobes of the lung, who worked with carbon arc lamps for 26
years.
Collectively, the available studies show that the defining characteristic of cerium
pneumoconiosis is accumulation of cerium particles, as well as other rare earth particles, in the
lungs and lymphoreticular system. In most cases, the initial indication of disease was the
presence of diffuse interstitial or reticulonodular opacities in chest X-rays. Pulmonary function
in the affected workers varied from normal to severe restrictive impairment. In several cases,
thorium, a common impurity in rare earth minerals, and the naturally occurring radioisotopes of
the rareearth elements were quantified, but, in each case, they were found to be present in
quantities too small to produce any effect due to radiation. Exposure to silica, which is
fibrogenic, may have contributed to the effects observed in the cerium oxide and glass workers
but is not a factor for the workers exposed to carbon arc lamp dust, which includes most of the
workers found to have fibrosis. Two reviews of the available case studies concluded that there is
convincing evidence that accumulation of cerium and other rare earth metals in the lungs is
causally related to the development of pulmonary interstitial fibrosis in workers (McDonald et
al., 1995; Waring and Watling, 1990). The human data were inadequate to identify potentially
sensitive subgroups.
4.2. SUBCHRONIC AND CHRONIC STUDIES AND CANCER BIOASSAYS IN
ANIMALS—ORAL AND INHALATION
4.2.1. Oral Exposure
Kawagoe et al. (2005) studied the possible association between cerium exposure and
oxidative stress in the mouse liver. Groups of male ICR mice (four per dose group) were
administered a diet containing 0, 20, or 200 ppm cerium chloride (CeCh) for 6 or 12 weeks.
This corresponds to doses of approximately 2.6 or 26 mg Ce/kg-day, assuming a reference food
consumption of 0.005 kg/day and reference body weight of 0.022 kg (U.S. EPA, 1988).
Evaluations conducted at termination included cerium levels in organs (liver, spleen, kidney, and
lung), levels of glutathione (GSH) and metallothionein (MT) in liver, kidney, and lung, levels of
lipoperoxide in liver and plasma, superoxide dismutase (SOD) activity in blood and liver, as well
as cholesterol levels, triglyceride levels, and aspartate aminotransferase (AST) and alanine
aminotransferase (ALT) activity in serum. Treatment with cerium did not affect food
consumption or body weight but statistically significantly decreased lipoperoxide levels in
hepatic tissues (33% at 20 and 38% at 200 ppm, 6 weeks; 29% at 20 ppm, 12 weeks), increased
liver GSH levels (200 ppm, 12 weeks) and liver MT activity (20 ppm, 12 weeks; 200 ppm,
May, 2007
19
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
6 weeks), and decreased plasma SOD activity (20 and 200 ppm, 6 weeks). Cerium in the kidney,
liver, lung, and spleen was statistically significantly elevated relative to controls in the organs of
mice in the 200 ppm group at 6 and 12 weeks, with the lung and spleen containing higher cerium
concentrations. Pathological alterations were not detected in the kidney, liver, lung, or spleen by
microscopic observation. According to Kawagoe et al. (2005), the increases in hepatic GSH and
MT activity represent a response to cerium-induced oxidative stress. It is unknown whether the
decrease in hepatic lipoperoxide is a consequence of the increase in GSH and MT. The study
authors suggest that the endpoints showing changes as a result of cerium exposure in this study
are indicators of reactive oxygen species generation in the liver.
A study (Cheng et al., 2000) in male Wistar rats (six per dose group) dosed orally (further
details not reported) with 0, 0.2, 2.0, or 20 mg cerium chloride/kg-day (0.1, 1.1, or 11.4 mg
Ce/kg-day) for up to 105 days investigated the effects of CeCh on the structure and oxygen
affinity of hemoglobin in vivo. The highest dose, 20 mg/kg-day, produced a slight increase of
hemoglobin content in the erythrocytes after 40 days of treatment, with an even greater increase
in hemoglobin content after 80 days. The effect on the oxygen affinity of the hemoglobin was
demonstrated by oxygen saturation curves for the dosed rats and control rats. Hemoglobin in
cerium-treated rats also exhibited altered oxygen affinity up to 80 days of exposure,
demonstrated by increased affinity up to 10 mm Hg and a double sigmoidal curve for rats treated
for 40 days and increased affinity above 20 mm Hg for rats treated for 80 days. A significant
change was not observed at 0.2 mg/kg-day for 105 days of exposure. These results suggest that
the oxygen affinity of hemoglobin increases following long-term oral exposure to CeCl3. When
the 80-day cerium feeding period was followed by 15 days of cerium-free exposure, the oxygen
affinity did not recover to normal levels. The investigators attributed the altered oxygen affinity
up to 80 days of exposure to conformational changes of hemoglobin, hydrolysis of hemoglobin-
bound diphosphoglyceric acid, and the partial oxidation of heme-Fe(II) to heme-Fe(III).
Kartha et al. (1998) examined the effects of cerium chloride, administered in the drinking
water with other rare earth chlorides, on the heart of New Zealand white rabbits (10/group) fed
diets with normal or restricted magnesium for 6 months. The rabbits were randomly distributed
into 4 groups with a male to female ratio of 1:1 within each group. Group 1 was exposed to a
magnesium-sufficient diet only, group 2 was exposed to a magnesium-sufficient diet and cerium,
group 3 was exposed to a magnesium-restricted diet, and group 4 was exposed to a magnesium-
restricted diet and cerium. The drinking water for the rare earth chloride-exposed rabbits was
adulterated with 1 g/L of rare earth chloride of which 56.6% was cerium (lanthanum 11.5%;
praseodymium and neodymium 14.5%; samarium 2.6%). Histologic evaluation of the heart at
the end of the study showed no cardiac lesions in the groups fed the normal magnesium diet,
regardless of whether they consumed water with or without cerium. Cardiac lesions were
evident in 6/10 rabbits from group 3, the magnesium-restricted diet, and in 9/10 rabbits from
May, 2007
20
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
group 4, the magnesium-restricted diet and cerium chloride-exposed group. Rabbits fed the
magnesium-restricted diets, treated with or without cerium, showed endocardial, subendocardial,
interstitial, and perivascular fibrosis. The lesions were more severe in those with cerium added
to the drinking water (group 4). The results suggest that cerium may intensify the effect of
magnesium deficiency on heart tissue. A no-observed-adverse-effect level (NOAEL) or lowest-
observed-adverse-effect level (LOAEL) for cerium cannot be established for cerium chloride in
this study because the cardiac lesions were observed in rabbits that were fed a magnesium-
restricted diet.
The only chronic oral exposure study to stable cerium was reported by Kumar et al.
(1996), who exposed mixed-sex (male to female ratio of 1:1) groups (n = 5-9) of Sprague-
Dawley rats to cerium chloride and a magnesium-sufficient or magnesium-deficient diet for
13 months. The rats were randomly distributed into four groups, with groups 1 and 2 fed a
magnesium-sufficient diet and groups 3 and 4 fed a magnesium-deficient diet. The rats in group
1 and 3 were exposed to 0 ppm cerium chloride, while the rats in groups 2 and 4 were exposed to
35 ppm cerium chloride in the drinking water. This cerium chloride exposure corresponds to a
concentration of 19 mg/L, which in turn, assuming a reference water consumption of 0.046 and
0.038 L/day and reference body weight of 0.523 and 0.338 kg (U.S. EPA, 1988) in males and
females, respectively, corresponds to an average daily dose in males of 1.7 mg/kg-day and in
females of 2.1 mg/kg-day. At 13 months, the animals were sacrificed and cardiac tissue was
collected for elemental analysis and histology. No statistically significant changes in serum or
cardiac levels of magnesium or calcium were reported. Cerium levels in cardiac tissue were
statistically significantly elevated in group 4. Cerium-treated animals had a significantly greater
level of collagen in cardiac tissue, relative to group 1, with an enhanced effect in animals fed a
magnesium-deficient diet (groups 3 and 4). No other endpoints were evaluated. This study was
not adequate to identify a NOAEL or LOAEL due to the limited number of evaluated endpoints
and the investigation of a single dose group.
4.2.2. Inhalation Exposure
No chronic inhalation studies on cerium toxicity are available. However, the National
Toxicology Program (NTP) is considering an evaluation of the chronic inhalation toxicity of
cerium oxide.
A subchronic inhalation study using cerium oxide (Ce02- eerie oxide) was conducted in
7-week-old Sprague-Dawley rats (BRL, 1994). Cerium oxide is the form of cerium typically
encountered in industrial exposures (Reinhardt and Winkler, 1986). This study is an
unpublished study; accordingly, it was externally peer reviewed by EPA in August 2006
(external peer review report available at www.epa.gov/iris).
Groups of 15 male and 15 female Sprague-Dawley CD rats were given nose-only
May, 2007
21
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
exposure to a dry powder aerosol (mass median aerodynamic diameter [MMAD] = 1.8-2.2 [j,m,
geometric standard deviation [GSD] = 1.8-1.9) of cerium oxide at concentrations of 0, 0.005,
0.0505, or 0.5075 mg/L (0, 5, 50.5, or 507.5 mg/m3) 6 hours/day, 5 days/week for 13 weeks.
The cerium oxide test material was 99% rare earth oxide with a maximum of 75 ppm Fe2C>3. Of
the 99% rare earth oxide, 99.95% was cerium oxide with a maximum of 25 ppm of both Pr6On
and Nd2C>3. Praseodymium and neodymium are also rare earth metals.
A functional observational battery was performed on all rats, as well as activity level
testing, hematology, clinical biochemistry, urinalysis, ophthalmological examination, and a gross
pathological examination of selected tissues weighed and retained for histopathologic
examination. No deaths or clinical signs related to cerium oxide were noted. Food consumption
and body weight gain were marginally, but statistically not significantly reduced in males at
507.5 mg/m3 and were considered the result of cerium oxide exposure.
A functional observational battery detected a statistically significant (p < 0.05) 17%
decrease in forelimb grip strength at week 13 in females exposed to 507.5 mg/m3. No other
changes were found in the functional observational battery. Motor activity, measured by
photocells in a figure-8 enclosure, was unaffected by cerium oxide exposure. The
ophthalmology examination was normal.
Hematological analysis revealed a statistically significant (p < 0.05) increase in absolute
neutrophil counts of 105% in 6-week males at 507.5 mg/m3, 130% in 6-week females at
50.5 mg/m3, 85% in 13-week males at 50.5 mg/m3, 75% in 13-week males at 507.5 mg/m3,
210% in 13-week females at 5 mg/m3, 177% in 13-week females at 50.5 mg/m3, and 233% in 13-
week females at 507.5 mg/m3. Differential white blood cell counts were largely restricted to
altered neutrophil counts (as shown in Table 4-1), with the exception being increased absolute
lymphocyte and eosinophil counts in 13-week males at 50.5 mg/m3 by 36% and 187%),
respectively. Differential white blood cell counts revealed changes in relative percentages of
neutrophils and lymphocytes. The relative percentage of neutrophils and lymphocytes were
significantly increased by 102 and 80% in 6-week and 13-week males, respectively, at
507.5 mg/m3. In 13-week males, there was a corresponding 19% decrease in the relative
percentage of lymphocytes. The 118% relative increase in neutrophils was accompanied by a
12%) decrease in lymphocytes in 6-week females at 50.5 mg/m3. In 13-week females, the 5, 50.5,
and 507.5 mg/m3 doses were associated with, respectively, 130, 130, and 164% relative increase
in the percentage of neutrophils and 12, 10, and 14% decreases in lymphocytes. Clinical
chemistry and urinalysis were normal.
May, 2007
22
DRAFT - DO NOT CITE OR QUOTE
-------
1 Table 4-1. Hematological changes in male and female Sprague-Dawley rats
2 following inhalation of cerium oxide aerosol 6 hours/day, 5 days/week for
3 13 weeks
Absolute
Absolute
Absolute
Relative
Relative
Dose
neutrophils
lymphocytes
eosinophils
neutrophils
lymphocytes
(mg/m3)
Mean
SD
Mean
SD
Mean
SD
Mean
SD
Mean
SD
Males, week 6
0
1,509
1,697
10,057
2,455
122
299
11.3
7.5
85.4
8.3
5
1,440
720
9,891
2,602
102
136
12.5
5.9
84.6
6.5
50.5
2,560
882
11,478
1,944
83
125
17.7
5.6
79
5.7
507.5
3,088a
1,162
10,296
3,670
110
175
22.8a
6.8
73.7a
6.9
Females, week 6
0
724
494
8,791
2,355
75
119
7.8
5.7
90.8
6.0
5
1,230
765
7,537
2,068
67
57
13.0
6.3
84.5
6.7
50.5
l,680b
651
8,083
1,737
92
88
17a
6.4
80. lb
7.5
507.5
1,328
562
8,916
3,678
87
104
13.3
4.3
84.9
4.5
Males, week 13
0
1,541
909
6,243
1,575
62
94
18.4
6.4
77.5
8.7
5
1,638
622
6,254
1,521
85
83
20.1
6.9
76.1
7.9
50.5
2,844a
1,038
8,481b
3,009
175b
173
25.4
10.9
70.7
11.0
507.5
2,698a
1,101
5,238
1,910
73
98
33.2a
9.5
62.T
11.0
Females, week 13
0
325
162
3,624
1,365
47
58
8.3
4.2
85.7
5.5
5
1,006°
797
3,794
1,200
61
55
19.la
10.0
75.3a
10.7
50.5
899°
414
3,802
1,228
36
44
19.la
7.8
76.9b
8.5
507.5
1,081°
399
3,722
1,137
32
42
21.9a
6.3
73.9a
7.1
4
5 aStatistically significantly different from control (Dunnett's test; p < 0.01).
6 Statistically significantly different from control (Dunnett's test; p < 0.05).
7 Statistically significantly different from control (Dunn's test; p < 0.01).
8 Source: BRL (1994).
9
10 At necropsy, there were treatment-related increases in the weight of the lungs and spleen
11 that correlated with gross and microscopic findings. Absolute and relative lung weights were
12 statistically significantly (p < 0.001) increased in both males and females at 50.5 and
13 507.5 mg/m3 (Tables 4-2 and 4-3). Lung weights, relative to brain weights, were also
14 statistically significantly increased in male and female rats at 50.5 and 507.5 mg/m3. Relative
15 spleen weight was statistically significantly (p < 0.05) increased in males at 507.5 mg/m3 (Table
16 4-4). A statistically significant increase in absolute (29%) and relative (28%) thymus weight in
17 male mice at 50.5 mg/m3 was not considered by the study authors to be related to cerium oxide
18 treatment. The increase in absolute and relative thymus weight was observed in only the mid-
19 dose male rats and a dose-response relationship was not observed in male or female rats. Thus,
20 the increase in thymus weight in male rats was not determined to be a biologically significant
21 effect of cerium oxide exposure.
May, 2007
23
DRAFT - DO NOT CITE OR QUOTE
-------
1 Table 4-2. Absolute lung weight in rats exposed to cerium oxide aerosol
2 6 hours/day, 5 days/week for 13 weeks
Dose
(mg/m3)
Males
Females
Mean weight (g)
SD
%
Change
Mean weight (g)
SD
%
Change
0
1.611
0.1088
-
1.145
0.0871
-
5
1.760
0.1398
9
1.311
0.1490
14
50.5
2.574a
0.3499
60
1.65 la
0.2900
44
507.5
4.662a
0.5161
189
3.1733
0.3235
177
3
4 Statistically significantly different from control (Dunn's test; p < 0.01).
5 Source: BRL (1994).
6
7
8 Table 4-3. Relative lung weight in rats exposed to cerium oxide aerosol
9 6 hours/day, 5 days/week for 13 weeks
Dose
(mg/m3)
Males
Females
Mean weight (g%)
SD
%
Change
Mean weight (g%)
SD
%
Change
0
0.334
0.0256
—
0.477
0.0443
—
5
0.371
0.0270
11
0.544
0.0557
14
50.5
0.5283
0.0775
58
0.6973
0.1124
46
507.5
1.0243
0.1621
207
1.3583
0.1432
185
10
11 "Statistically significantly different from control (Dunn's test; p < 0.01).
12 Source: BRL (1994).
13
14
15 Table 4-4. Relative spleen weight in rats exposed to cerium oxide aerosol
16 6 hours/day, 5 days/week for 13 weeks
Dose
(mg/m3)
Males
Females
Mean weight (g%)
SD
%
Change
Mean weight (g%)
SD
%
Change
0
0.162
0.0190
-
0.216
0.0281
-
5
0.169
0.0286
4
0.242
0.0259
12
50.5
0.178
0.0266
10
0.226
0.0445
5
507.5
0.1883
0.0298
16
0.222
0.0252
3
17
18 Statistically significantly different from control (Dunnett's test; p < 0.05)
19 Source: BRL (1994).
20
21
22 Gross examination found discoloration or pale areas, pale foci, and uncollapsed
23 parenchyma in the lungs of male and female rats (Table 4-5). Pale areas and discoloration in the
24 lung were evident at 50.5 and 507.5 mg/m3 in male and female rats, respectively, with
25 uncollapsed parenchyma evident at 50.5 and 507.5 mg/m3 in male rats and 507.5 mg/m3 in
26 female rats. Pale foci in the lungs were only seen in female rats exposed to 5 mg/m3. The
27 incidence of enlargement or pale discoloration of the mandibular, bronchial, mediastinal, and
28 pancreatic lymph nodes is shown in Table 4-6. Enlargement or pale discoloration of both lymph
29 nodes that drain the lungs (the bronchial and mediastinal lymph nodes) was evident in both
May, 2007 24 DRAFT - DO NOT CITE OR QUOTE
-------
1 males and females at >5 mg/m3 cerium oxide. The mandibular and pancreatic lymph nodes did
2 not display a dose-response trend of enlargement or discoloration. The study authors (BRL,
3 1994) judged the mandibular lymph node enlargement, which was observed in control and
4 cerium-exposed rats, not to be an effect of the cerium oxide exposure, while the pancreatic,
5 bronchial, and mediastinal lymph node enlargement and/or discoloration were considered to be
6 related to the cerium oxide treatment.
7
8 Table 4-5. Results of gross pathological examination of lungs of rats
9 exposed to cerium oxide 6 hours/day, 5 days/week for 13 weeks
Lung
Dose (mg/m3)
Male
Female
0
5
50.5
507.5
0
5
50.5
507.5
Pale foci
0/15
0/15
0/15
0/15
0/15
4/15
0/15
0/15
Pale areas or discoloration
0/15
0/15
15/15
15/15
0/15
0/15
15/15
15/15
Uncollapsed parenchyma
0/15
0/15
2/15
15/15
0/15
0/15
0/15
15/15
10 Source: BRL (1994).
11
12
13 Table 4-6. Results of gross pathological examination of bronchial,
14 mediastinal, and pancreatic lymph nodes of rats exposed to cerium oxide
15 6 hours/day, 5 days/week for 13 weeks
Lymph nodes
Dose (mg/m3)
Male
Female
0
5
50.5
507.5
0
5
50.5
507.5
Mandibular
enlargement
4/15
2/15
5/15
4/15
2/15
4/15
3/15
1/15
discoloration
-
-
-
-
-
-
-
-
Bronchial
enlargement
0/15
4/15
15/15
15/15
0/15
1/15
14/15
15/15
discoloration
0/15
13/15
15/15
15/15
0/15
15/15
15/15
15/15
Mediastinal
enlargement
0/15
2/15
9/15
9/15
0/15
1/15
8/15
8/15
discoloration
0/15
2/15
9/15
10/15
1/15
10/15
9/15
10/15
Pancreatic
enlargement
1/15
0/15
0/15
0/15
2/15
0/15
1/15
0/15
discoloration
-
-
-
-
0/15
0/15
0/15
1/15
16 Source: BRL (1994).
17
18 Histologic examination revealed dose-related alveolar epithelial and lymphoid
19 hyperplasia and pigment accumulation in the lungs, lymph nodes, and larynx of male and female
20 rats at >5 mg/m3. The incidence data are presented in Table 4-7. The pigment, which was also
21 found in other parts of the respiratory tract, including the nasal cavities and the trachea, and the
22 liver and spleen, was considered by the study authors to be the test compound or a product
23 thereof. The lymphoid hyperplasia in the lymph nodes following cerium oxide exposure was
24 characterized by the study pathologist as an increase in the number of lymphocytes, with lymph
May, 2007
25
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
node paracortices and cortices expansion. The authors reported that the severity of the
hyperplasia in a given tissue, lung or lymph node, was correlated with the amount of pigment
accumulated in the tissue but did not present any supporting data. BRL (1994) considered these
findings to be consistent with antigenic stimulation by cerium oxide; however, they did not
discuss the possibility of non-antigenic stimulation. The metaplasia evident in the larynx was
interpreted by the study pathologist as adaptive and reversible. Lesions were not observed in the
testes or ovaries of the high-dose group.
The NOAEL and LOAEL values for the toxicological effects observed are included in
Table 4-8. This study identified a LOAEL of 5 mg/m3 in rats, based on the increased incidence
of lymphoid hyperplasia in the bronchial lymph nodes of male and female rats. A NOAEL was
not identified.
May, 2007
26
DRAFT - DO NOT CITE OR QUOTE
-------
1 Table 4-7. Incidences of histopathologic effects in rats exposed to cerium
2 oxide aerosol 6 hours/day, 5 days/week for 13 weeks
Exposure (mg/m3)
Control
5
50.5
507.5
Males
Larynx
metaplasia
pigment accumulation
0/15
0/15
3/15
6/15a
9/15a
9/15a
13/15a
12/15a
Lung
lymphoid hyperplasia
alveolar epithelial hyperplasia
pigment accumulation
0/15
0/15
0/15
0/15
1/15
15/15a
0/15
11/153
15/153
12/15a
14/15a
15/15a
Bronchial lymph node
lymphoid hyperplasia
pigment accumulation
0/15
0/15
11/133
13/133
15/153
15/153
15/15a
15/15a
Mediastinal lymph node
lymphoid hyperplasia
pigment accumulation
0/0
0/0
2/2
2/2
9/10
8/10
9/9
9/9
Mandibular lymph node
lymphoid hyperplasia
pigment accumulation
0/15
0/15
0/3
0/3
0/5
0/5
2/15
6/15a
Pancreatic lymph node
lymphoid hyperplasia
pigment accumulation
-
-
-
-
Spleen
pigment accumulation
0/15
0/15
0/15
6/15a
Females
Larynx
metaplasia
pigment accumulation
0/15
0/15
3/15
0/15
6/15a
7/15a
9/15a
9/15a
Lung
lymphoid hyperplasia
alveolar epithelial hyperplasia
pigment accumulation
0/15
0/15
0/15
0/15
0/15
15/153
1/15
5/15a
15/153
7/15a
15/15a
15/15a
Bronchial lymph node
lymphoid hyperplasia
pigment accumulation
0/15
0/15
13/15a
14/15a
15/153
15/153
15/15a
15/15a
Mediastinal lymph node
lymphoid hyperplasia
pigment accumulation
0/1
0/1
10/10
10/10
9/9
9/9
9/10
9/10
Mandibular lymph node
lymphoid hyperplasia
pigment accumulation
0/15
0/15
0/5
0/5
0/3
0/3
0/15
6/15a
Pancreatic lymph node
lymphoid hyperplasia
pigment accumulation
0/2
0/2
0/0
0/0
1/1
1/1
0/1
1/1
Spleen
pigment accumulation
0/15
0/0
0/0
3/15
3
4 "Significantly different from vehicle control group (p < 0.05 by Fisher's exact test)
5 Source: BRL, 1994.
6
7 Table 4-8. NOAEL and LOAEL values for toxicological effects
May, 2007 27 DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
observed in BRL, 1994.
NOAEL
LOAEL
Toxicological effect
Sex
(mg/m3)
(mg/m3)
Organ weight changes
Absolute lung weight
M, F
5
50.5
Relative lung weight
M, F
5
50.5
Absolute spleen weight
M, F
507.5
-
Relative spleen weight
M
50.5
507.5
F
507.5
-
Gross pathological lesions in lung
Pale areas
M, F
5
50.5
Discoloration
M, F
50.5
507.5
Uncollapsed parenchyma
M
5
50.5
F
50.5
507.5
Gross pathological lesions in the lymph nodes
Mandibular enlargement
M, F
-
5
Bronchial enlargement
M, F
-
5
Bronchial discoloration
M, F
-
5
Mediastinal enlargement
M, F
-
5
Mediastinal discoloration
M, F
-
5
Histopathologic lesions
Metaplasia, larynx
M, F
-
5
Pigment accumulation, larynx
M
-
5
F
5
50.5
Lymphoid hyperplasia, lung
M
50.5
507.5
F
5
50.5
Alveolar epithelial hyperplasia, lung
M, F
5
50.5
Pigment accumulation, lung
M, F
-
5
Lymphoid hyperplasia, bronchial lymph node
M, F
-
5
Pigment accumulation, bronchial lymph node
M, F
-
5
Pigment accumulation, spleen
M
50.5
507.5
Pigment accumulation, mandibular lymph node
M
50.5
507.5
Lymphoid hyperplasia, mediastinal lymph node
M, F
5
Pigment accumulation, mediastinal lymph node
M, F
-
5
Intratracheal instillation of 50 mg of a dust suspension of cerium oxide produced mild
changes in the lung and spleen of white rats examined 8 months later (Mogilevskaya and
Raikhlin, 1967). There was a moderate proliferative response in the lungs and bronchi, with
occasional increased lymphocyte numbers and very slight development of connective tissue and
collagen fibers, but diffuse or nodular fibrotic changes were lacking. In the spleen, macrophages
and large multinucleated cells accumulated. No histologic changes were found in any of the
other major organs.
Lundgren et al. (1996) conducted an investigation into the pulmonary carcinogenicity of
beta-particle radiation from inhaled 144CeC>2 in F344/N rats, with stable cerium oxide serving as
the control group (n = 1,049). The radiation doses ranged from 3.6 to 37 Gy. The control rats
May, 2007
28
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
received stable cerium oxide in a single inhalation dose of comparable mass concentration to the
dose groups, although the concentration was not specified by the study authors. The control rats
were held for life-span observation and were evaluated histologically. The histologic evaluation
showed nonneoplastic lesions, such as inflammation (5.1%), fibrosis (5.6%), alveolar-epithelial
hyperplasia (4.5%), and alveolar macrophage hyperplasia (7.1%), in the lungs of control rats.
Seven primary lung neoplasms were also apparent in the control rats and included four alveolar
or papillary adenomas; one alveolar, papillary, or tubular adenocarcinoma; one squamous cell
carcinoma; and one fibro- or osteosarcoma. Control rats, unexposed to cerium oxide, in a
separate study of inhaled 144CeC>2 in F344/N rats demonstrated an incidence of lung tumors of
6/110 (Lundgren et al., 1992). The observed tumors included two papillary adenomas, two
papillary adenocarcinomas, one adenosquamous carcinoma, and one mesothelioma.
4.3. REPRODUCTIVE/DEVELOPMENTAL STUDIES—ORAL AND INHALATION
No studies were located regarding reproductive/developmental effects of cerium in
humans by any route of exposure. The only relevant information from a study in animals is the
administration of 0, 200, or 800 mg/kg-day cerium to male mice (five to eight per group) in the
diet for 30 or 45 days. This exposure increased the rate of unspecified sperm abnormalities but
did not statistically significantly affect testes weight or alter serum levels of testosterone (Yu et
al., 2001, English translation of abstract). The increased rate of the apparent sperm
abnormalities was dose and time dependent, with a 17 and 22% increase in abnormalities 30
days postexposure for 200 and 800 mg/kg-day, respectively, and a 28 and 32% increase in
abnormalities 45 days postexposure at the same dose levels, respectively.
4.4. OTHER DURATION- OR ENDPOINT-SPECIFIC STUDIES
As indicated in Section 3.1.1, cerium compounds are poorly absorbed from the GI tract of
animals; thus, parenteral administration of cerium compounds has been the preferred route of
exposure to study the systemic effects of these compounds.
4.4.1. Acute Toxicity Studies (Oral and Inhalation)
Single dose and/or acute oral data are limited but include a report of splenic lesions,
including hypertrophy, reticuloendothelial hyperplasia, and hyperactive lymphoid follicles, and
GI irritation, characterized by gastritis and enteritis with focal hemorrhage and necrosis of the
mucosa in the stomach and duodenum, in mice given single gavage doses of 1,000 or
1,163 mg/kg (1:3 cerium chloride-sodium citrate complex) (Stineman et al., 1978). Open field
behavior was not affected and the 7-day LD50 was 1,291 mg/kg cerium (95% CI: 1,198-1,440).
Bruce et al. (1963) identified an oral LD50 of 4,200 mg/kg for cerium nitrate in female Sprague-
Dawley rats (n=30).
May, 2007
29
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
There is a report of piloerection, hunched posture, unsteady gait, and dark coloring of
eyes in male and female rats, along with lethargy, abnormal respiration, and prostration in
females, given single gavage doses of 5,000 mg/kg cerium as cerium sulfide (Rhodia Inc., 1998).
Ji and Cui (1988) list an oral LD50 in female mice of 1,178 mg/kg (95% CI: 1,043-1,331) for
cerium nitrate (CefNChjs) and 622 mg/kg (95% CI: 550-702) for cerium oxides.
Data on the toxicity of single exposures to airborne cerium in animals are limited. A
review by the HEI (2001) reported that the LC50 for cerium oxide in rats was >50 mg/m3 in a
study by Rhone-Poulenc (1983), but the study was unavailable for examination and no further
details were reported.
4.4.2. Acute Studies (Injection)
4.4.2.1. Neurobehavioral andNeurodevelopmental Effects
D'Agostino et al. (1982, 1978a, b) studied the neurodevelopmental effects of cerium
following subcutaneous injection in mice. The three separate publications present the same
experimental design. Pregnant mice were administered a single dose of 80 mg/kg cerium
(sodium/citrate complex) or citrate (control) on gestation day (GD) 7 or 12 or postnatal day
(PND) 2. In order to differentiate the gestational effects from lactational effects or changes in
maternal behavior, a full cross-foster study design was employed. A cross-fostering design
distributed the offspring (3 males and 3 females per dam) of mothers receiving cerium or citrate
during gestation to lactating dams exposed to either cerium or citrate. Body weight and gross
activity of the neonates were assessed on PNDs 7 or 12 (D'Agostino et al., 1978a, b) or PNDs 8
or 13 (D'Agostino et al., 1982), whereas open-field behavior, accelerating rotarod performance,
and passive avoidance learning were assessed on PNDs 60-65. Maternal offspring retrieval
latency was measured on PND 3. The growth and behavioral data were analyzed via a
multivariate analysis.
Pups exposed to cerium in utero on GDs 7 and 12 demonstrated a statistically significant
decrease in body weight on PNDs 7 and 12 (D'Agostino et al., 1978a, b), whereas the body
weight of offspring of cerium-exposed dams on GD 7 did not differ significantly from that of
control offspring (D'Agostino et al., 1982). Offspring of dams exposed to cerium on GD 12
showed significantly decreased body weights on PNDs 8 and 13 (D'Agostino et al., 1982). Pups
exposed to cerium in utero were retrieved by dams and replaced in the nest significantly faster
than controls. When the dams were injected on PND 2, neonatal weights on PNDs 7 and 12 and
8 and 13 were statistically significantly reduced (D'Agostino et al., 1982, 1978a, b). The study
authors suggested that these effects may be due to altered maternal behavior (e.g., ineffective
suckling or lack of grooming) rather than cerium being transmitted to offspring in the milk. In
addition, offspring of rats exposed to cerium on GD 7 showed a higher frequency of rearings
than controls when evaluated on PNDs 60-70 (D'Agostino et al., 1982, 1978a, b). Rats exposed
May, 2007
30
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
to cerium on GD 7 demonstrated a decrease in activity, measured by the circular runway, on
PND 12 (D'Agostino et al., 1978a, b). Other behavioral differences were not reported. No
possible mechanism of action was discussed to explain the neurodevelopmental effects of cerium
administration. The lack of information on maternal health status following administration of
cerium limits the usefulness of these studies.
4.4.2.2. Neurological Effects
Morganti et al. (1978) examined the open-field and exploratory behavior of 6- to 8-week-
old male Swiss ICR mice following subcutaneous administration of cerium. Three days after
receiving a dose of 20 mg/kg cerium (from a 1:3 cerium chloride-cerium citrate complex),
10 mice were observed in the open-field and exploratory apparatuses and then sacrificed. This
procedure, including injection, was repeated every 3 days until the last group of mice had
received a total of 200 mg/kg cerium. Control mice were injected with sodium citrate. The
results showed that cerium exposure statistically significantly (p < 0.05) depressed ambulations,
marginally depressed explorations, but did not affect rearings. Behavioral measures were not
correlated with cerium levels in tissues. According to Morganti et al. (1978), the lack of
correlation between the behavioral measures and the cerium levels in tissue indicated that the
effect of cerium is not a one-step process depending directly on the level of cerium in a single
tissue but instead involves a biological chain of events.
In a different study by the same group of investigators (Stineman et al., 1978) in which
male Swiss mice received single, subcutaneous injections of cerium (136 or 173 mg/kg), there
was an inverse relationship between open field behavior (ambulations and rearings) and levels of
cerium in blood, brain, lung, stomach, intestine, and kidney. The levels of cerium in the brain
had the strongest association with decreased open field behavior, followed by the lung.
However, splenic levels of cerium were positively correlated with behavior, as mice with higher
levels of cerium in the spleen exhibited less altered behavior. Stineman et al. (1978) suggested
that the spleen may protect against the cerium-induced effects by sequestering cerium from the
circulation after the removal of damaged erythrocytes and leukocytes.
In a subsequent study (Morganti et al., 1980), 6- to 8-week-old adult male Swiss ICR
mice administered single, subcutaneous injections of 136 or 173 mg/kg cerium showed
depression of general activity, as measured in an activity wheel study and a passive avoidance
study. However, the subcutaneous injection of cerium did not significantly affect two-way
active avoidance learning or social behavior, although gross activity was depressed in the social
behavior study (Morganti et al., 1980). The study authors' interpretation of the study data was
that cerium exposure did not affect simple and complex learning, as measured in the above tests.
4.4.2.3. Hepatic Effects
May, 2007
31
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
Numerous studies have examined the effects of cerium on liver parameters following
parenteral dosing. The i.v administration of a dose of 3 mg/kg cerium (specific form not
specified) to rats resulted in increased serum levels of ornithine-carbamyl transferase (OCT),
which reached a peak 1-2 days after dosing (Magnusson, 1962). Blood glucose levels were
reduced markedly in females 2-3 days after dosing but recovered by day 4. No macroscopic
changes were seen in the liver of males, but fatty liver and fatty degeneration were evident in
females 1 day after treatment. Males showed hydropic changes and isolated necrotic cells 1-
4 days after dosing. In females, changes to the ultrastructure of the liver cells were seen 12
hours after administration of cerium and appeared to reach a maximum 2-3 days later, with only
slight ultrastructural changes observed in males.
Several additional reports have confirmed and expanded on the Magnusson (1962)
findings. Marciniak and Baltrukiewitcz (1981, 1977) showed that the increase in serum OCT
activity after cerium injection was linear in the range 1.5-4.5 mg/kg cerium. Lombardi and
Recknagel (1962) observed that i.v. administration of 5 mg/kg cerium (form not specified) to
female Sprague-Dawley rats produced a fourfold increase in liver triglyceride levels 24 hours
after treatment. Salas et al. (1976) showed that a single i.v. dose of 10 mg/kg cerium chloride
administered to female Sprague-Dawley rats decreased the total hepatic adenosine triphosphate
level by 12 hours postexposure and depleted liver glycogen levels and increased liver
triglyceride levels within 48 hours. Ultrastructurally, the rough endoplasmic reticulum was the
main target of cerium toxicity, with marked dilation and degranulation, as well as the appearance
of free ribosomes in the cytoplasm, 24 hours postexposure. The hepatic changes returned to
normal between the 5th and 8th day postexposure.
Arvela and Karki (1971) observed fatty degeneration on the first day following i.v.
administration of 2 mg/kg cerium chloride in Sprague-Dawley rats. On the third day after i.v.
administration, there was a 53% reduction in the level of CYP450 in the liver of male Sprague-
Dawley rats. Arvela et al. (1991) showed that C57BL/6N male mice were more resistant to the
hepatotoxic effects, including necrosis, cell membrane disintegration, and microsteatosis, of
cerium than were DBA/2 mice. However, the concentration of cerium in the livers of C57BL/6N
mice was about 50% higher at 72 hours than in the livers of DBA/2 mice. The authors also
showed that in C57BL/6N mice the slight to moderate liver injury 72 hours post administration
was associated with an increase in coumarin 7-hydroxylase (COH) activity; however, at doses
that caused severe damage in DBA/2 mice, COH activity 72 hours post administration was
drastically reduced. It was also reported that the total amount of CYP450 in the liver was
significantly increased in both strains after a dose of 1 mg/kg cerium, but higher doses tended to
decrease CYP450 content, producing a biphasic relationship. A follow-up study found that
cerium increased the amount of hepatic CYP2A5 mRNA only in DBA/2 mice (Salonpaa et al.,
1992), which have been shown to be more sensitive to cerium exposure than the C57BL/6N
May, 2007
32
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
strain. Since the CYP2A5 gene encodes P450 isozymes catalyzing COH activity in mice, the
study authors suggested that some association exists between the development of liver damage
and COH induction.
Strubelt et al. (1980) reported dose-dependent increases in AST (reported using the older
name of glutamic-oxaloacetic transaminase) and ALT (reported using the older name of
glutamic-pyruvic transaminase) and sorbitol dehydrogenase (SDH) in male Wistar rats after i.v.
administration of 3, 5, 7, and 10 mg/kg cerium nitrate.
Sex differences in sensitivity were also reported by Wiener-Schmuck et al. (1990).
Intravenous injection of 1.3 mg/kg of cerium chloride to rats caused lipid deposition, damage to
mitochondria, and invaginations of the nuclear membrane in hepatocytes 48 hours after dosing.
These changes, which were accompanied by increased activities of serum transaminases, were
reversible and occurred only in females. However, when isolated hepatocytes of female rats
were incubated in medium with cerium chloride for 20 hours, there was no sign of cell damage.
The lack of toxicity of cerium to isolated hepatocytes in vitro suggests that hepatocellular lesions
and related changes observed following injection of cerium result from an indirect cause.
Wiener-Schmuck et al. (1990) suggested that deposition of injected rare earths in Kupffer cells
leads to blockage of the reticuloendothelial system in the liver, inducing damage to
macrophages, which release mediators that in turn damage the hepatocytes.
A more recent paper showed that administration of the metal chelator ethylene glycol
bis(2-aminoethylether)tetraacetic acid (EGTA) to mice after dosing with cerous sulfate for 7
days decreased the severity of the histologic effects of cerium on the liver (Shrivastava and
Mathur, 2004). The histologic effects that resulted from this 7-day subcutaneous exposure of 0.5
mL cerous sulfate included lymphocytic infiltration, hepatocyte hypertrophy, cytoplasmic
vacuolation, and stumpy Kupffer cells. In addition to the histologic effects, the activity of ALT
and AST were statistically significantly increased, with maximum increases of 350 and 98%,
respectively, and hepatic acid, alkaline phosphatases, and succinic dehydrogenase were
statistically significantly decreased, 42, 69, and 46%, respectively. The decrease in severity of
the histologic effects would indicate that the presence of circulating cerium is necessary for
hepatotoxicity but does not provide information on a possible mechanism of hepatotoxicity.
In summary, administration of cerium to rodents caused lipid deposition, mitochondrial
damage, and invaginations of the nuclear membrane in hepatocytes, as well as adverse
morphological effects in the liver, characterized by fatty liver, fatty degeneration, and necrosis.
In addition, histologic effects included lymphocytic infiltration, hepatocyte hypertrophy,
cytoplasmic vacuolation, and stumpy Kupffer cells. Arvela and Karki (1971) suggested that
liver toxicity may result indirectly from induction of CYP2A5 and COH.
4.4.2.4. Cardiovascular Effects
May, 2007
33
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
Information regarding effects of cerium on the human heart derives mainly from a series
of studies (see Section 4.1.1) in which naturally high levels of cerium in the soil in certain
geographical regions appear to be correlated with higher levels of cerium in serum and cardiac
tissue of individuals with endomyocardial fibrosis (Eapen, 1998; Kutty et al., 1996; Valiathan et
al., 1989).
The i.v. administration of a single dose of 1.3 mg/kg cerium as cerium chloride to female
Sprague-Dawley rats resulted in a statistically significant twofold increase in protein synthesis
(p < 0.001) and transcription (p < 0.01) in cardiac muscle relative to controls 24 hours after
injection (Kumar et al., 1995). This was consistent with findings by the same group of
investigators who reported that incubation of cardiac fibroblasts in vitro with 100 nM cerium
increased RNA synthesis approximately 64%, but the rate of DNA synthesis was unaffected
(Shivakumar et al., 1992). However, it should be noted that higher concentrations of cerium in
the medium were inhibitory. This was taken as evidence suggesting that cerium at very low
levels may act at the level of transcription to stimulate collagen and non-collagen protein
synthesis. This, in turn, may contribute to the accumulation of collagen in endocardial fibrosis.
In a follow-up study, Kumar and Shivakumar (1998) reported that a single i.v. dose of 1.3 mg/kg
cerium increased lipid peroxidation by 30% in cardiac tissue of Sprague-Dawley rats and
increased proliferation of cardiac fibroblasts by 23%. Treatment with cerium also statistically
significantly decreased collagen degradation by 7% and increased the rate of deposition of newly
synthesized collagen by 27% in cardiac tissue 48 hours after cerium administration (Kumar and
Shivakumar, 1998).
In summary, the limited data from oral exposure experiments in rats and rabbits and i.v.
administration studies in rats suggest that cerium increases collagen accumulation in cardiac
muscle by increasing synthesis and decreasing degradation by unknown mechanisms.
4.4.2.5. Hematological Effects
The i.v. administration of a single dose of 10 mg/kg cerium as the chloride, citrate
complex, or ethylenediamine tetraacetic acid (EDTA) complex to anesthetized dogs every
10 minutes for 10 total doses, significantly increased prothrombin levels and coagulation time
within minutes of the injection (Graca et al., 1964). The magnitude of the increased prothrombin
levels and coagulation time was greatest for cerium chloride, followed by the citrate complex,
and finally the EDTA complex. No other hematological endpoint was significantly affected by
cerium exposure. Talbot et al. (1965) implanted a pellet of cerium metal under the skin of
C57BL mice and collected blood samples from five male and five female mice every 6 months
for hematological determinations. They stated that there were no significant differences between
coagulation times of cerium-implanted mice and controls; however, a table shows that after
6 months, coagulation time in implanted male mice was approximately twice that in controls. In
May, 2007
34
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
this study, the only statistically significant difference (p < 0.05) between cerium implanted and
nonimplanted mice was a decrease in total leukocyte counts in males and females at 6, 12, and
18 months relative to controls. However, analysis of differential counts showed no significant
differences between treated and control mice. In another study, Shrivastava and Mathur (2004)
injected subcutaneous doses of cerous sulfate (Ce2[SO/t]3) daily for 7 days into male mice and
reported 18 and 37% decreases in hemoglobin and red blood cell counts, respectively, and 93
and 39% increases in sedimentation rate and hematocrit, respectively, all of which appeared to
reach a maximum on day 14 (7 days after the last injection) and appeared to return to control
levels approximately 60 days posttreatment.
The information available from these few studies is insufficient to determine a possible
mechanism of action by which cerium might be causing hematological alterations. Alterations in
prothrombin and coagulation times could be secondary to liver dysfunction, but more
information is necessary to confirm this hypothesis.
4.4.2.6. Renal Effects
The i.v. administration of a single dose of 2 mg/kg of cerium chloride to adult male
DBA/2 and C57BL/6 mice resulted in a more than fourfold increase in COH activity in the
kidneys of DBA/2 mice, but less than a twofold increase in C57BL/6 mice (Salonpaa et al.,
1992). The effect was maximized in DBA/2 mice 4 hours after dosing, and the enzyme activity
returned to predosing levels 6 hours after treatment. Cerium also increased CYP2A5 mRNA in
the kidneys of DBA/2 mice sevenfold 6 hours after dosing and sixfold 1 day after the injection,
but no such increase occurred in C57BL/6 mice.
Injection of subcutaneous doses of 1 mM cerous sulfate to male mice for 7 days produced
statistically significant (p < 0.05) decreases up to 62 and 42% in the activities of renal acid and
alkaline phosphatases, respectively, and up to 31% in succinic dehydrogenase activity
(Shrivastava and Mathur, 2004). Cerium also induced histologic damage to the kidneys,
consisting of hypertrophy in the epithelial cells, deformed Bowman's capsules, exfoliated nuclei
in tubular lumen, and leukocyte infiltration. Maximum injury was observed on day 21 (14 days
after the last injection). As with the findings regarding liver toxicity, mice administered EGTA
after cerium exhibited less severe necrotic effects in the kidney than mice not receiving EGTA.
The mechanism of kidney toxicity of cerium is unknown.
In summary, cerium exposure initiated an increase in COH activity and CYP2A5
expression and decreased acid and alkaline phosphatases and succinic dehydrogenase activities,
as well as kidney epithelial hypertrophy, exfoliated nuclei in the tubular lumen, and leukocyte
infiltration.
4.4.3. Genotoxicity
May, 2007
35
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
No information was located regarding genotoxic effects of cerium or cerium compounds
in humans and only three studies were identified with pertinent information in mammals and in
lower organisms (Sharma and Taluker, 1987; Shimizu et al., 1985; Nishioka, 1975). The
available information is insufficient to ascertain the genotoxicity of cerium.
Shimizu et al. (1985) examined the potential mutagenicity of cerium oxide in five strains
of Salmonella typhimurium (TA98, TA100, TA1535, TA1537, and TA1538) and in Escherichia
coli WP2uvrA. Tests were conducted with and without metabolic activation (S9 fraction from
male Sprague-Dawley rats). Eight different concentrations were tested, ranging from 1 to
5,000 |ig/plate. No increase in the number of revertant colonies per plate was observed at any
dose level; in some cases the highest concentration produced growth inhibition. In a short
communication, Nishioka (1975) reported that cerium chloride did not induce DNA damage in
two strains of Bacillus subtilis using the rec-assay. Cerium nitrate was reported to induce
chromosomal breaks and reduce the mitotic index in rat bone marrow in vivo (Sharma and
Talukder, 1987).
4.5. MECHANISTIC DATA AND OTHER STUDIES IN SUPPORT OF THE MODE OF
ACTION
4.5.1. In Vitro Studies
The cytotoxicity of soluble cerium chloride and insoluble cerium oxide was assayed in
Sprague-Dawley rat pulmonary alveolar macrophages (PAMs) and compared to the cytotoxic
and fibrogenic cadmium chloride and oxide (Palmer et al., 1987). Cell viability, effect on
lysosomal enzyme release, and cell morphology were investigated. Cerium chloride was
cytotoxic to rat PAMs with an LC50 (concentration inducing 50% cell death) of 29 |iM, while
cerium oxide was less toxic, with an LC50 of approximately 4,700 |iM; the LC50 values in this
assay for cadmium chloride and oxide were 28 and 15 |iM, respectively. Cerium chloride and
oxide did not affect lysosomal enzyme release, although the sensitivity of this assay was
questioned by the study authors. Cerium oxide induced an increase in cells with a featureless
surface and a decrease in cells consistent with the control population. Cells typified by blebs on
the cell surface and/or surface structure were absent. The induction of cells with a featureless
surface was considered by the study authors to be minimal. Cerium chloride was not evaluated
for effect on cell morphology due to an experimental error during cell culture preparation.
Cerium chloride was more cytotoxic than cerium oxide and of similar cytotoxicity to cadmium
chloride, which had an LC50 of 28 |iM,
Shivakumar and Nair (1991) conducted an in vitro study to examine the effect of cerium
on protein synthesis in cultured rat heart cells and human lung fibroblasts exposed to normal and
reduced levels of magnesium in the growth medium. Cerium exposure resulted in the inhibition
of protein synthesis in rat heart cells and human lung fibroblasts, evident by the decreased
May, 2007
36
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
amount of [3H]-tyrosine incorporated into heart cell and fibroblast proteins, 73 and 76%,
respectively; the cell cultures grown on low-magnesium medium displayed a more pronounced
decrease in protein synthesis, with [3H]-tyrosine incorporation decreased 51 and 40% in heart
cell and fibroblast proteins, respectively. However, the mechanism of protein synthesis
inhibition is unknown.
Another in vitro study was conducted to investigate the toxicity of lanthanides, which
includes cerium, towards cultured rat alveolar macrophages (Lizon and Fritsch, 1999). Rat
alveolar macrophages were acquired by pulmonary lavage of 2-month-old male Sprague-Dawley
rats and were cultured for 1 day on culture medium and then for 3 days on medium containing
soluble cerium concentrations of 1 x 10~4 and 5/10 6 M. The fraction of apoptotic cells
increased with concentration and as a function of time, with observations at 24, 48, and 78 hours.
As cerium concentration increased from 10 6 to 5 x 10 5 M, the fraction of normal cells
decreased and the fraction of post-apototic and unstained cells increased. The LC50 for cerium
was approximately 5 x 10 5 M. Cerium exposure to cultured rat alveolar macrophages resulted
in significant alveolar macrophage death.
Studies with cardiac fibroblasts from neonatal rats in vitro showed that the stimulation of
fibroblast proliferation was accompanied by an increase in the generation of free radicals. Preeta
and Nair (1999) isolated cells from the hearts of 3-4-day-old neonatal Wistar rats. Fibroblasts
were selected from the cultured isolated heart cells via selective adhesion and reincubated in
fresh medium. To assess fibroblast growth, the selected fibroblasts were exposed to cerium
concentrations of 0.1, 0.5, 1, 10, and 100 [xM and were harvested 96 hours postexposure.
Growth dynamics were evaluated at 24, 48, and 96 hours postexposure. Immunohistochemical
labeling for proliferating cell nuclear antigen (PCNA) was also conducted to determine if an
increase in cell number was due to cell proliferation. Intracellular free-radical generation was
determined by spectrophotometric assay with reduced nitroblue tetrazolium, with the cardiac
fibroblasts exposed to the same cerium concentrations as in the growth assessment. In addition,
the role of free radicals in cell proliferation was investigated using cardiac fibroblasts exposed to
0.5 |iM cerium, SOD (100 U/mL), and catalase (120 U/mL) with cell counts measured after
96 hours.
The results of this investigation showed that cardiac fibroblast proliferation followed a
concentration-dependent response to cerium exposure, with increased proliferation at low
concentrations and decreased proliferation at high concentrations (Preeta and Nair, 1999).
Increased proliferation was evident from 0.1 to 1 [xM, with a statistically significant (p < 0.01)
peak at 0.5 |iM, while decreased proliferation was evident from 10-100 [xM. A statistically
significant increase in the proliferation of PCNA immunoreactive cells was evident at low
cerium concentrations (0.1-1.0 [xM), while a decrease was evident at higher concentrations. The
addition of SOD inhibited the increased PCNA expression. The reduction of nitroblue
May, 2007
37
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
tetrazolium to formazan, which peaked at 0.5 [xM, showed the increase in free radicals from the
fibroblasts exposed to cerium. An increase in free radical production resulted from the fibroblast
exposure to cerium at 0.5 [xM, and the cerium-induced stimulatory response was inhibited by the
addition of SOD. This in vitro study demonstrates that low concentrations of cerium stimulate
cardiac fibroblast proliferation in association with an increase in intracellular generation of free
radicals.
A comparative study of cardiac and pulmonary fibroblasts in vitro by Nair et al. (2003)
was conducted to investigate the mitogenic effect of cerium. Fibroblasts were isolated from
heart and lung tissue of 3- to 4-day-old neonatal Wistar rats through selective adhesion. The
cardiac and pulmonary fibroblasts were exposed to fetal bovine serum, a known nonspecific
mitogen, which resulted in cell proliferation in association with intracellular free radical
generation in both types of fibroblasts. Exposure to SOD resulted in significant reductions in
intracellular superoxide anion content and cell density for both types of fibroblasts. The cardiac
fibroblast proliferation was stimulated by exposure to cerium at 0.5 [xM for 96 hours, whereas
lung fibroblast proliferation was not stimulated by 0.5 [xM cerium exposure for 96 hours. These
data indicate that cardiac fibroblasts may be more sensitive to cerium exposure than lung
fibroblasts.
Du et al. (2001) exposed packed human erythrocytes to cerium chloride at concentrations
ranging from 4.9 x 10 5 to 3.9 x 10 3 M Ce3+ to investigate the aggregation of membrane
proteins after exposure to cerium. At the highest concentration tested, 3.9 x 10 3 M, aggregation
of membrane proteins was clearly evident, with aggregation increasing gradually with increasing
concentration from 4.9 x 10 5 to 3.9 x 10 3 M. Further analysis, using SDS-PAGE of the
membrane proteins and light scattering measurements, showed that the membrane protein
aggregation was mainly due to non-covalent cross-linking and to a lesser extent oxidative cross-
linking through disulfide bond formation.
Nanoparticle studies
In addition to the above in vitro studies, data on nano-sized cerium particles provide
information on absorption and cytotoxicity relevant to the mode of action. Limbach et al. (2005)
measured the uptake of cerium oxide nanoparticles at four different sizes, 20-50, 40-80, 80-150,
and 250-500 nm, into cultured human lung fibroblasts. The cultured fibroblasts absorbed the
nanoparticle cerium linearly with exposure time, with absorption occurring at concentrations as
low as 100 ng/g. Particles were not found outside of the vesicles or flowing freely in the
cytoplasm and were present exclusively in the form of agglomerates. The size of the cerium
nanoparticle greatly affected the amount of cerium incorporated into the cell, with better
absorption of larger nanoparticles. Particle size was a more important factor in absorption than
particle number and total surface area (Limbach et al., 2005).
May, 2007
38
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
Brunner et al. (2006) evaluated the cytotoxicity of nanoparticle Ce02 to human
mesothelioma and rodent fibroblast cell lines by measuring metabolic activity and cell
proliferation. Cerium oxide was tested at exposures of 0, 3.75, 7.5, and 15 ppm for 6 days and 0,
7.5, 15, and 30 ppm for 3 days. The cerium oxide particles had a specific surface area derived
particle size of 6 nm and a hydrodynamic particle size of 19 nm, a specific surface area of
124 ± 3% m2/g, and a GSD of 1.49. Metabolic activity was spectroscopically measured as the
total mitochondrial activity through the conversion of the leuko form of a formazan-type dye to
the active dye, and cell proliferation was measured by the total DNA content of the cells
measured spectroscopically after converting DNA with an intercalating dye into a highly
fluorescent complex (Brunner et al., 2006). The human mesothelioma cells were more sensitive
to Ce02 than the rodent fibroblast cells, with cell activity and DNA content decreased
approximately 50% after 3 days. The mesothelioma and fibroblasts were, however, not
completely killed at 30 ppm. After 6 days, the cell activity was not significantly altered and
DNA content was increased slightly in the mesothelioma cells; in the rat fibroblasts, the DNA
content increased slightly and the cell activity was not significantly affected.
The impact of a model water dispersion of 7 nm Ce02 nanoparticles (specific surface
area of 400 m2/g) on E. coli and cytotoxicity, assessed by counting colony-forming units (CFUs),
was investigated at concentrations ranging from 0 to 730 mg/L by Thill et al. (2006). Cerium
was almost completely adsorbed to the bacteria cell surface at a concentration of approximately
30 mg/L, and above this concentration an increasing amount of cerium was found in the
supernatant with a maximum adsorbed concentration of approximately 48 mg/L. The adsorption
appeared to be due to electrostatic attraction between the cerium oxide nanoparticles and the cell
membrane. The percentage of CFUs was strongly affected as the Ce02 nanoparticle
concentration increased, with a 50% survival rate at around 5 mg/L and no survival above
230 mg/L. Thill et al. (2006) also showed that the speciation of the adsorbed cerium was
modified and the nanoparticles reduced. The study authors concluded that direct contact
between the E. coli and the Ce02 nanoparticles needs to be established for Ce02 cytotoxicity to
occur and that the reduction of the nanoparticles occurs at or close to the surface of the bacteria
and may be associated with cytotoxicity.
The toxicity of cerium nanoparticles was also investigated using the human
bronchioalveolar carcinoma-derived cell line A549 (Lin et al., 2006). The A549 cell line was
exposed to 20 ± 3 nm cerium oxide nanoparticles at 0, 3.5, 10.5, or 23.3 [j.g/mL for 24, 48, or
72 hours. The cells were evaluated for cytotoxicity, as well as for intracellular reactive oxygen
species generation, lactate dehydrogenase (LDH) activity, dichlorodihydrofluorescein diacetate
(DCF) fluorescence, GSH levels, a-tocopherol levels, and malondialdehyde (MDA) levels. Cell
viability was decreased at all three dose levels and exposure durations and followed a dose- and
time-dependent decrease, with viability at 3.5, 10.5, and 23.3 [j,g/mL at 72 hours decreased 12,
May, 2007
39
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
22, and 46%, respectively. LDH levels, indicative of cell membrane damage, increased 15, 32,
and 71% at 24, 48, and 72 hours, respectively, and were greatest at 23.3 [j,g/mL. DCF
fluorescence, indicative of oxidative stress, increased 70, 139, and 181% after exposure to 3.5,
10.5, and 23.3 [j,g/mL cerium oxide, respectively. Antioxidant levels were decreased, with
cellular GSH levels reaching a maximal decrease of approximately 70% at 48 hours, with a
possible recovery of GSH levels at 72 hours, and a-tocopherol levels decreased 38, 76, and 88%
at 3.5, 10.5, and 23.3 (J,g/mL, respectively. MDA levels, indicative of lipid peroxidation, were
significantly increased in a dose- and time-dependent manner in the 3.5, 10.5, and 23.3 [j,g/mL
dose groups at 48 and 72 hours. Lin et al. (2006) demonstrated the induction of significant
oxidative stress at levels of 3.5, 10.5, and 23.3 [j,g/mL of 20 nm cerium oxide particles. The
elevated reactive oxygen species levels, increased lipid peroxidation, increased membrane
damage, and reduced antioxidant levels are evidence of the increased oxidative stress from
cerium oxide nanoparticle exposure.
In addition to the evidence of cytotoxicity, there is evidence of neuroprotective and
cardioprotective effects of nanoparticle cerium exposure. Schubert et al. (2006) exposed the
HT22 hippocampal nerve cell line, derived from the rodent nervous system, to CeC>2
nanoparticles and monitored the intracellular generation of reactive oxygen species with a
nonfluorescent compound that fluoresces when in contact with reactive oxygen species. The
cerium oxide nanoparticles were characterized as single, monodisperse crystals. The Ce02
nanoparticles were, for the most part, nontoxic to the HT22 cell line, with the exception of the
1 ^im CeC>2 nanoparticles at concentrations >20 [j,g/mL. The 6 and 12 nm, as well as 1 [j,m,
cerium particles were protective of oxidative stress to HT22 cells, and a difference in level of
protection offered by the 6 nm, 12 nm, and 1 [j,m sized particles could not be produced. It was
also shown that 12 nm CeC>2 particles at 20 and 200 [j,g/mL were able to rapidly reduce pools of
reactive oxygen species formed by the cells after 8 hours of exposure to glutamate. Cerium
nitrate and cerium chloride, when tested at the same concentrations as cerium oxide, displayed
no protective effects. Schubert et al. (2006) were able to provide evidence that cerium oxide
nanoparticles have antioxidant properties that promote nerve cell survival under oxidative stress
conditions.
The cardioprotective effects of nano-sized CeC>2 were evident in cultured cells from MCP
mice administered 15 nmol of 7 nm Ce02 intravenously twice a week for two weeks (Niu et al.,
2007). The cardiac-specific expression of monocyte chemoattractant protein (MCP-1) in mice
causes ischemic cardiomyopathy. The treatment of MCP mice with nanoparticle CeC>2 inhibited
monocyte/macrophage infiltration into the myocardial interstitial space, suppressed
proinflammatory cytokine production in the myocardium, and limited myocardial oxidative
stress.
May, 2007
40
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
4.5.2. Ex Vivo Studies
Manju et al. (2003) utilized isolated papillary muscle in an effort to assess the mechanical
response of the myocardium to varying levels of cerium. Cerium chloride concentrations of 0,
0.1, 0.5, 1, 5, 10, 20, 50, and 100 [xM were applied to papillary muscle isolated from Sprague-
Dawley rats and the force of contraction was recorded using a force transducer. The role of SOD
was also investigated by superfusing the muscle with SOD prior to cerium chloride exposure and
recording of contractile force. A statistically significant (p < 0.01) reduction in contractile force
was evident at cerium chloride exposures as low as 0.1 [xM, with the lowest dose reducing the
force of contraction approximately 15% (Manju et al., 2003). Complete recovery of contractile
force was apparent at or below 0.5 |iM when cerium exposure was removed. The addition of
SOD completely inhibited the effect of cerium exposure, up to 5 [xM, on the contractile force of
the isolated papillary muscle. These results provide further evidence for the involvement of
reactive oxygen species in the effects of cerium on the heart.
4.6. SYNTHESIS OF MAJOR NONCANCER EFFECTS
4.6.1. Oral
No studies evaluating the oral toxicity of cerium in humans were located, but an
association between exposure to cerium in food and the development of endomyocardial fibrosis
has been suggested (Eapen et al., 1998; Kutty et al., 1996; Valiathan et al., 1989). Long-term
studies in animals are limited to a 13-month drinking water study in rats, which investigated the
effects of a single dose group (Kumar et al., 1996), a 6-month drinking water study in rabbits in
which the administered dose consisted of a mixture of rare earth chlorides (Kartha et al., 1998), a
12-week dietary study in mice (Kawagoe et al., 2005), and a 105-day gavage study in rats
(Cheng et al., 2000). The study of Kumar et al. (1998) suggested that cerium may increase the
levels of collagen in the heart, while the findings of Kartha et al. (1998) suggested that cerium
may intensify the adverse cardiac effects of magnesium deficiency. Kawagoe et al. (2005)
suggested that cerium may increase oxidative stress in tissues of mice, and Cheng et al. (2000)
reported that cerium increased the oxygen affinity of hemoglobin in rats.
A single dose study in mice observed splenic lesions and GI irritation in treated mice
(Stineman et al., 1978). Other acute oral data consist of a report of piloerection, hunched
posture, unsteady gait, and dark coloring of eyes in male and female rats (along with lethargy,
abnormal respiration, and prostration in females only) given single gavage doses of 5,000 mg/kg
cerium as cerium sulfide (Rhodia Inc., 1998); a 7-day LD50 in mice of 1,291 mg/kg cerium as the
chloride/citrate complex (Stineman et al., 1978); and an LD50 of 1,178 mg/kg for cerium nitrate
and 622 mg/kg for cerium oxide, also in mice (Ji and Cui, 1988).
The few long-term studies available in animals identify cardiac tissue and hemoglobin
oxygen affinity as possible adverse health effects, but the animal studies are of limited scope and
May, 2007
41
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
insufficient duration and experimental design. Toxicokinetics data have shown that orally
administered cerium compounds are poorly absorbed, and this may be reflected in the results of
the studies conducted with these compounds by the oral route of exposure.
4.6.2. Inhalation
Inhalation data in humans consist of reports describing numerous cases of workers who
developed pneumoconiosis associated with accumulation of cerium in the lungs after prolonged
occupational exposure to cerium fumes or dust (Yoon et al., 2005; Porru et al., 2001; McDonald
et al., 1995; Pairon et al., 1995, 1994; Sulotto et al., 1986; Vogt et al., 1986; Pietra et al., 1985;
Vocaturo et al., 1983; Sabbioni et al., 1982; Husain et al., 1980; Kappenberger and Biihlmann,
1975; Heuck and Hoschek, 1968). In these cases, the exposure was to cerium oxide, and cerium-
induced pneumoconiosis was characterized by accumulation of cerium particles (and other rare
earth particles) in the lungs and lymphoreticular system. Exposure was not quantified in any of
these cases. The available data in humans are inadequate to identify potentially sensitive
subgroups.
Information regarding long-term inhalation exposure in animals is derived from a single
subchronic study in rats (BRL, 1994). Sprague-Dawley rats were exposed nose only to cerium
oxide aerosol 6 hours/day, 5 days/week for 13 weeks. Endpoints evaluated included a functional
observational battery, hematology and clinical chemistry, urinalysis, and gross and microscopic
morphology of tissues. The results revealed statistically significant increases in absolute and
differential neutrophil counts in the blood, treatment-related increases in the absolute and
relative weight of the lungs in both males and females dosed at 50.5 and 507.5 mg/m3 and in the
relative spleen weight of male rats at 507.5 mg/m3, discoloration or pale areas and uncollapsed
parenchyma in the lungs of male and female rats at >50 mg/m3 and pale foci in female rats at
5 mg/m3, and dose-related alveolar epithelial and lymphoid hyperplasia and pigment
accumulation in the lungs, lymph nodes, and larynx of males and females at >5 mg/m3. The
lowest exposure level, 5 mg/m3, was a LOAEL for lymphoid hyperplasia of the lymph nodes. A
NOAEL was not identified.
Acute inhalation data are limited to the determination of an LC50 greater than 50 mg/m3
for cerium oxide in rats (Rhone-Poulenc, 1983) and a report of mild histologic alterations,
including slight connective tissue and collagen fiber development and increased lymphocyte
numbers, in the lung, as well as the accumulation of macrophages and large, multinucleated cells
in the spleen, of rats that received an intratracheal instillation of cerium oxide 8 months prior to
examination (Mogilevskaya and Raikhlin, 1967).
4.6.3. Mode-of-Action Information
4.6.3.1. Respiratory Tissues
May, 2007
42
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
The mode of action for cerium toxicity following chronic human inhalation exposures is
uncertain, since limited pathological data are available from the human case reports. In animals,
the observed pathology has been attributed to immune responses to cerium dust loads, which
overwhelmed innate pulmonary clearance mechanisms, namely clearance by pulmonary
macrophages (BRL, 1994). The accumulation of insoluble cerium particles in the respiratory
tract of humans and animals following chronic and subchronic inhalation exposures,
respectively, suggests that impaired clearance may influence pulmonary toxicity for both
species. In animals, correspondence of pulmonary and lymphoid hyperplasia with the
accumulation of cerium treatment-related pigmentation in the same tissues suggests that the
mode of action for cerium inhalation toxicity may be mediated by cytokine and fibrogenic
effects resulting from pulmonary macrophage activation followed by macrophage
immobilization.
The concept of dust overloading of the lungs refers to inhalation exposures that are
sufficiently intense as to overwhelm the pulmonary clearance mechanisms, specifically,
macrophagic phagocytosis in the alveolar spaces with mucociliary escalation in the bronchial
airways (Morrow, 1988). The concept of particle overload applies, specifically, to particles of
relatively low cytotoxicity that secondarily induce pulmonary toxicity via chronic activation of
immune-responsive cells (Oberdorster, 1995). In addition to immune cellular activation, the
uncleared particles may traverse the pulmonary epithelial boundary to the interstitial spaces,
where fibrogenesis may occur (Oberdorster, 1995).
Cullen et al. (2000) demonstrated overwhelmed pulmonary clearance in male Wistar rats
following inhalation exposure to another poorly soluble metal oxide (titanium dioxide; Ti02) in
which the particles were of similar size and concentration to that used for cerium oxide in the
BRL (1994) study. Cullen et al. (2000) investigated pulmonary effects in male Wistar rats
following the 4- and 7-month inhalation exposure of TiC>2 particles with a MMAD of 2.1 |j,m
administered at 25 mg/m3 for 209 calendar days and 50 mg/m3 for 118 calendar days. The
lymph node burdens, measured in mg/day, for TiC>2 demonstrated the initiation of overload
retardation after approximately 50 days at 50 mg/m3 and 150 days at 25 mg/m3. In addition,
inhalation of TiC>2 led to an accumulation of pulmonary macrophages around dust deposition
sites, and most of the macrophages contained phagocytized dust particles. The results observed
in Cullen et al. (2000) for TiC>2 support the proposed overload of pulmonary clearance during the
13-week inhalation exposure to cerium oxide particles of similar size and solubility observed in
BRL (1994). Cullen et al. (2000) did not include exposure concentrations as low as the BRL
cerium oxide study (i.e., 5 mg/m3); however at the lowest concentration of TiC>2 examined,
overload was also observed, although time to overload was longer at the lower concentration
(140 days at 25 mg/m3). The demonstration of pulmonary overload by Cullen et al. (2000) in an
exposure time frame and at a concentration of TiC>2 particles with a similar MMAD to the cerium
May, 2007
43
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
oxide particles in the BRL (1994) study supports the proposed mode of action of overwhelmed
pulmonary clearance for the relatively insoluble cerium oxide.
Overwhelmed pulmonary clearance, as marked by increased translocation of dust to
lymph nodes, was observed for Ti02 but not for barium sulfate (BaSO/t) under similar conditions
in the study by Cullen et al. (2000). Although pulmonary overload is a mode of action that may
apply to other relatively insoluble particles of low cytotoxicity, it is clear from the Cullen et al.
(2000) study that there are strong differences between exposure to the dust of different
chemicals, such that exposure to HO2 leads to overloading of the lung, whereas overloading was
not observed for BaSC>4.
Data were unavailable to definitively identify toxic mechanisms of cerium-induced nasal
and bronchial metaplasia and hyperplasia. In a review of lanthanide toxicity, Haley (1991)
proposed that rare earth metals, including cerium, may exert toxicity both from innate chemical
characteristics as well as pulmonary dust burden. A suggested mechanism explaining both
inflammatory and fibrogenic responses involves the activation of PAMs following dust overload.
Tissue-destructive release products of activated PAMs include acid hydrolases, elastases,
collagenases, and several reactive oxidant species (Haley, 1991). Additionally, the release of
fibroblast growth factor and fibronectin may stimulate the proliferation of fibroblasts, and PAMs
may release neutrophil attractant factors (Haley, 1991). The subsequently enlisted neutrophils
can release several oxidants and proteases that are known to result in connective tissue damage,
possibly stimulating fibrogenesis (Hunninghake et al., 1984). Although BRL (1994) did not
collect data on macrophage activation or the release of chemokines, the significant increase in
mature neutrophils in rats exposed to high levels of cerium observed in this study is consistent
with stimulation of neutrophils by PAMs. The exposure-related increase in pigment
accumulation and lymphoid hyperplasia in lymph nodes draining the lungs (i.e., bronchial and
mediastinal lymph nodes) and the absence of significant effects in pancreatic and mandibular
lymph nodes further supports the role of pulmonary macrophages.
The immobilization of PAMs by excessive cerium dust loads, as hypothesized by
Morrow (1992, 1988), results in macrophages carrying a heavy cerium load to lose the ability to
move either toward the mucociliary escalator system, for subsequent clearance to the gut, or to
the lymphoid vasculature. A dense tissue population of activated, yet immobile, macrophages
may serve to induce significant cell damage by effectively increasing the concentration of
inflammatory cytokines and fibrogenic growth factors within the pulmonary epithelium.
Reduction in the ability to clear insoluble cerium from the lung spaces due to macrophage
immobilization is consistent with reports of extracellular presence of pigment in mid- and high-
dose rats relative to low-dose animals (BRL, 1994). The presence of inflammatory cytokines
and growth factors was not examined.
May, 2007
44
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
The ability of immobilized macrophages to induce toxicity via concentration of immune
signaling requires minimal cytotoxicity of the macrophages themselves. In vitro experiments
have shown that cytotoxicity of rat pulmonary macrophages is high for soluble cerium (cerium
chloride) (Lizon and Fritsch, 1999; Palmer et al., 1987) but quite low for the insoluble salt
(cerium oxide) (Palmer et al., 1987). This suggests that the pulmonary responses to inhaled
cerium reported in human cases and animal bioassays (which predominantly involved insoluble
cerium oxide) may not have caused significant macrophage cell death. This finding is consistent
with pathological findings in human case reports, animal studies, and in vivo assays of PAMs in
which cerium-related granule pigmentation was visible in the lung and lymphoid tissues,
suggesting that the cerium sequestration and concentration occurred in viable PAMs.
The precipitation and concentration of both soluble and insoluble cerium within cytosolic
lysosomes has been demonstrated (Berry et al., 1997; Berry, 1996). The high phosphate
concentration and extensive enzymatic hydrolysis activity (acid phosphatase) within lysosomes
was shown to effectively precipitate cerium as cerium phosphate. Rats exposed to 5 jag insoluble
cerium oxide via intratracheal dosage for 30 days displayed rat alveolar macrophages that
contained very fine needles or granules in the lysosomes (Berry et al., 1997). The needle or
granule inclusions contained both phosphorus and cerium. After 2 days of intratracheal dosage,
the rat alveolar macrophage lysosomes contained cerium particles approximately 1 [j,m in length
and displayed a crystalline structure. Rats exposed to soluble cerous chloride displayed fine
granules and needles in the lysosomes (Berry et al., 1997). Berry (1996) showed that cerium
precipitated with phosphorus in the lysosomes of hepatocytes and of bone marrow and splenic
macrophages of rats after intraperitoneal injections of cerium. Granular or needle-like
precipitate deposits were observed in the lysosomes of alveolar macrophages of rats exposed to
an aerosol solution of cerium chloride (Galle et al., 1992). The authors (Berry et al., 1997; Galle
et al., 1992) suggest that the precipitation and concentration of cerium within the lysosomes may
serve to inhibit diffusion of the metal to other tissues.
The presence of insoluble cerium particles in the alveolar macrophages of rats exposed
via the intratracheal route is consistent with the appearance of cerium-related particles within the
alveolar macrophages of a worker in an occupation associated with cerium oxide exposure
(McDonald et al., 1995). The particles in the human alveolar macrophages ranged from small,
pinpoint particles to oblong or needle-shaped. Pairon et al. (1995) revealed particles of
phosphorus, calcium, lanthanum, and cerium in human alveolar macrophages following
occupational exposure and identified interstitial macrophages that contained both phosphorus
and rare earth elements. Cerium was essentially always associated with phosphorus in the
biological samples studied by Pairon et al (1995).
The presence of the insoluble cerium particles in the rat alveolar macrophages (Berry et
al., 1997) and in the lysosomes of hepatocytes and splenic macrophages (Berry, 1996) is also
May, 2007
45
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
consistent with the involvement of macrophages in the appearance of cerium-related
pigmentation in the rat lung, liver, and spleen following inhalation exposures (BRL, 1994).
Shivakumar and Nair (1991) have shown that cerium exposure resulted in the inhibition
of protein synthesis in human lung fibroblasts, while lung fibroblast proliferation was not
stimulated by 0.5 |iM cerium exposure (Nair et al., 2003). However, cell types other than
fibroblasts may be involved in the inflammatory response associated with cytokine and growth
factor release that leads to a fibrotic response (Nair et al., 2003).
The hypothesized mode of action for the pulmonary effects observed following cerium
oxide exposure is the overloading of the PAMs by cerium oxide particles, leading to the release
of inflammatory cytokines and fibrogenic growth factors and subsequent cell damage.
4.6.3.2. Other Tissues
No unifying mode of action for cerium in non-respiratory tissues was identified in the
studies available. Studies conducted utilized primarily parenteral routes of exposure and suggest
that the production of free radicals may be involved in cerium toxicity, as well as actions at the
RNA transcription level. However, details of potential mode(s) of action in non-respiratory
tissues are lacking.
Cerium administered subcutaneously was found to have minor effects on some tests of
spontaneous motor behavior in young adult mice (Morganti et al., 1980, 1978; Stineman et al.,
1978) and in mice exposed in utero (D'Agostino et al., 1982, 1978a, b); however, a mode of
action was not discussed or apparent in these studies. Interestingly, behavioral measures were
not correlated with cerium levels in tissues, which suggested an unknown, indirect mode of
action involving a biological chain of events (Morganti et al., 1978).
Liver effects have been reported in several studies in rats and mice following i.v.
administration of cerium compounds. The effect is characterized by fatty liver, fatty
degeneration, and necrosis (Magnusson, 1962; Lombardi and Recknagel, 1962). Female rats
appeared more sensitive than male rats (Wiener-Schmuck et al., 1990; Magnusson, 1962).
Incubation of hepatocytes from female rats with cerium did not result in damage to the cells
(Wiener-Schmuck et al., 1990), suggesting that the damage is not a result of direct contact of
cerium with the hepatocytes but that cerium exposure may trigger a series of events that
ultimately result in liver damage. This seems consistent with the results of a study in two strains
of mice having different sensitivities to cerium toxicity that showed that cerium levels in the
liver of the more resistant strain were 50% higher than those in the more sensitive strain (Arvela
et al., 1991). The latter study and an additional study by Salonpaa et al. (1992) presented
evidence suggesting that some association exists between the development of liver damage and
COH induction. Enzyme induction was accompanied by increased expression of CYP2A5
mRNA, but the mechanism by which cerium increases CYP2A5 expression is unknown.
May, 2007
46
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
Cerium exposure has been found to increase the levels of collagen in the heart of animals
in an oral study (Kumar et al., 1996) and following i.v. dosing (Kumar and Shivakumar, 1998).
Cerium exposure decreased collagen degradation and increased the rate of deposition of newly
synthesized collagen in cardiac tissue of rats (Kumar and Shivakumar, 1998). The increase in
collagen synthesis is consistent with a marked increase in mRNA in cardiac fibroblasts incubated
with cerium (Shivakumar et al., 1992), pointing to an action at the level of transcription.
Cerium-induced production of free radicals was also associated with stimulation of fibroblast
proliferation in studies in vitro (Nair et al., 2003; Preeta and Nair, 1999). While cerium-induced
collagen proliferation can explain the occurrence of cardiac fibrosis, the mechanism by which
cerium triggers this event at the molecular level remains unknown.
In vitro studies demonstrated that low cardiac fibroblast proliferation is stimulated by
exposure to cerium (0.5 |iM) (Nair et al., 2003), that low concentrations of cerium stimulate
cardiac fibroblast proliferation in association with an increase in intracellular generation of free
radicals (Preeta and Nair, 1999), and that cerium exposure results in the inhibition of protein
synthesis in rat heart cells (Shivakumar and Nair, 1991). Kuruvilla and Kartha (2006)
demonstrated that cerium reduced the incorporation of [H3]-thymidine into DNA of cardiac
fibroblasts grown in endocardial endothelial cells conditioned medium. These results suggest
that the cardiac lesions in endomyocardial fibrosis may result from the direct stimulation, and
not through toxic effects on the endocardial endothelium, of subendomyocardial fibroblasts by
cerium (Kuruvilla and Kartha, 2006).
Cerium inhibited contraction of isolated rat ventricular papillary muscle, an effect that
could be partially prevented by the free radical scavenger SOD (Manju et al., 2003), which adds
support for the involvement of free radicals in the cerium-induced cardiac effects.
4.7. EVALUATION OF CARCINOGENICITY
4.7.1. Summary of Overall Weight of Evidence
Data were unavailable regarding the carcinogenicity of cerium compounds in humans or
experimental animals. In accordance with U.S. EPA (2005a) Guidelines for Carcinogen Risk
Assessment, there is "inadequate information to assess the carcinogenic potential" of cerium in
humans.
4.7.2. Synthesis of Human, Animal, and Other Supporting Evidence
No relevant human or animal data are available. In addition, the available information is
insufficient to ascertain the mutagenicity of cerium compounds. The NTP has recently begun an
evaluation of the chronic inhalation toxicity of cerium oxide, including a cancer bioassay. The
date of completion and public availability of the results is unknown but may be expected in
2009.
May, 2007
47
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
A study in various strains of S. typhimurium demonstrated negative evidence of
mutagenicity under the conditions of the assay (Shimizu et al., 1985). Cerium chloride did not
induce DNA damage in two strains of B. subtilis by using the rec-assay (Nishioka, 1975), but
cerium nitrate was reported to induce chromosomal breaks and reduce the mitotic index in rat
bone marrow in vivo, and cerium sulfate was reported to cause differential destaining of
chromosomal segments in plants (Sharma and Talukder, 1987).
4.8. SUSCEPTIBLE POPULATIONS AND LIFE STAGES
4.8.1. Possible Childhood Susceptibility
No studies were located regarding possible childhood susceptibility to cerium
compounds. Kutty et al. (1996) stated that high cerium soil concentrations and magnesium
deficiency during childhood may lead to endomyocardial fibrosis, although causation has not
been established. Increased GI uptake of cerium in preweaning, suckling animals compared with
adults is apparent, due primarily to pinocytosis in intestinal cells, but it is unclear if this has any
toxic consequences, since the cerium remains in the intestinal cells, may be minimally available
systemically, and is eventually eliminated in the feces as the intestinal cells die and are replaced
(Kargacin and Landeka, 1990; Kostial et al., 1989a, b; Inaba and Lengemann, 1972).
4.8.2. Possible Gender Differences
No information was located regarding gender differences in humans in response to
exposure to cerium compounds. Information addressing gender differences in animals is from a
study by Magnusson (1962) in which i.v. administration of cerium to rats resulted in noticeably
more severe adverse liver effects in females than in males. This was confirmed in another study
in rats also treated intravenously with cerium (Wiener-Schmuck et al., 1990). There is no
explanation for this gender-related difference in susceptibility, and it is unknown whether it also
applies to endpoints other than the liver. Additionally, it appears that female rats may be more
susceptible to the hematological changes (Table 4-1) observed as the result of cerium oxide
inhalation (BRL, 1994).
4.8.3. Possible Susceptible Populations
A variety of health effects could result from the accumulation of a sufficient amount of
persistently retained particles in the lung (Morrow, 1988). Smoking has suppressive effects on
pulmonary clearance in humans (Morrow et al., 1992), and the co-exposure of cerium with
cigarette smoke, or particulates that accumulate in the lungs, may potentially lead to more severe
adverse pulmonary effects in exposed populations. Chen et al. (2006) found that pulmonary
inflammation, in this case from lipopolysaccharide treatment, may play an integral role in
enhancing the extrapulmonary translocation of particles.
May, 2007
48
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Particle size is another factor that influences overload and toxicity that may be
particularly relevant for human exposure if human exposure is associated with fumes from
carbon arc lamps, as fumes include smaller particles from the low nm range to <1 |im
aggregates. Impaired lung clearance and lung effects in rats exposed to ultrafine particles (<100
nm) occur at lower mass concentrations than in rats exposed to fine particles (<10 |im) (Baan et
al., 2006). Translocation of particles to the insterstitium is a function of number of particles, and
appears to be dependent on the dose and particle size (Ferin et al., 1992). Excessive
translocation into the insterstitium may cause damage to epithelial cells, pulmonary edema, and
eventual fibrosis (Ferin 1994, Oberdorster et al., 1990). Populations exposed to ultra-fine cerium
oxide may have a higher than expected toxicity when compared to cerium oxide particles of a
larger size, as ultrafine particles have higher than expected toxicity when compared to similar
particles of a larger size (Ferin, 1994).
May, 2007
49
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
5. DOSE-RESPONSE ASSESSMENTS
5.1. ORAL REFERENCE DOSE (RfD)
5.1.1. Choice of Principal Study and Critical Effect—with Rationale and Justification
The available human and animal studies demonstrate that cerium may have an effect on
cardiac tissue and hemoglobin oxygen affinity. However, an RfD for cerium was not derived
because the available studies were not suitable for quantitation of effects for various reasons,
including unknown exposure concentrations in the available human studies, lack of a dose-
response, uncertain biological significance (e.g., increased oxygen affinity for hemoglobin,
changes in measures of oxidative stress), and inadequate study design (e.g., effects noted only
under conditions of a restricted diet).
An association between exposure to cerium in food and the development of
endomyocardial fibrosis has been suggested (Eapen et al., 1998; Kutty et al., 1996; Valiathan et
al., 1989). In addition, Gomez-Aracena et al. (2006) suggest a relationship between chronic
cerium exposures, characterized by cerium toenail concentrations, and increased risk of acute
myocardial infarction. The data set for long-term animal studies consists of a 13-month drinking
water study in rats (Kumar et al., 1996), a 6-month drinking water study in rabbits (Kartha et al.,
1998), a 12-week dietary study in mice (Kawagoe et al., 2005), and a 105-day gavage study in
rats (Cheng et al., 2000).
Kumar et al. (1996) demonstrated increased, highly variable cerium levels in cardiac
tissue. Cerium chloride-treated rats, relative to untreated controls, had an increased level of
collagen in the cardiac tissue, with an enhanced effect in rats fed a magnesium-deficient diet.
This study suggested that cerium might increase the levels of collagen in the heart. This study
was conducted on a small number of rats at one dose level (control and dosed rats) and evaluated
few endpoints, and the observed effects were highly variable and not statistically significant.
Kartha et al. (1998) suggested that cerium chloride may intensify the adverse cardiac
effects of magnesium deficiency. Cardiac lesions were apparent in 6/10 rabbits fed a
magnesium-deficient diet with no cerium exposure and 9/10 rabbits fed a magnesium-deficient
diet with cerium exposure. Rabbits fed magnesium-restricted diets, treated with or without
cerium, showed endocardial, subendocardial, interstitial, and perivascular fibrosis and the lesions
were more severe in those with cerium added to the drinking water. Cardiac lesions were absent
from the groups fed the normal magnesium diet regardless of whether they consumed water with
or without cerium. This study was conducted in an adequate number of animals but used only
one dose group. The authors reported that cerium may intensify the cardiotoxicity associated
with a magnesium-deficient or restricted diet but did not elicit a cardiac effect when tested under
conditions of a normal magnesium diet.
May, 2007
50
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
Cheng et al. (2000) reported that cerium chloride exposure in rats produced a slight
increase of hemoglobin content in erythrocytes after 40 days of treatment with an even greater
increase in hemoglobin content after 80 days of exposure. The effect on the oxygen affinity of
hemoglobin was demonstrated by altered oxygen saturation curves for the dosed rats compared
to control rats. Hemoglobin in cerium-treated rats exhibited altered oxygen affinity up to 80
days of exposure, demonstrated by increased affinity up to 10 mm Hg and a double sigmoidal
curve for 40-day rats and increased affinity above 20 mm Hg for 80-day rats.
Kawagoe et al. (2005) demonstrated that cerium chloride statistically significantly
decreased liver lipoperoxide levels, increased liver GSH levels and liver MT activity, and
decreased plasma SOD activity in mice. Cerium concentrations in the kidney, liver, lung, and
spleen were statistically significantly elevated relative to controls, with the lung and spleen
containing the highest levels. The study authors suggested that the endpoints showing changes
as a result of cerium exposure in this study are indicators of reactive oxygen species generation
and resultant oxidative stress but indicated that their toxicological significance was uncertain.
The authors did not report any other effects in the liver.
5.2. INHALATION REFERENCE CONCENTRATION (RfC)
5.2.1. Choice of Principal Study and Critical Effect—with Rationale and Justification
Exposure to cerium compounds in the environment is most likely through cerium (eerie)
oxide. There are numerous case reports of workers who developed pneumoconiosis or
interstitial lung disease associated with the accumulation of cerium particles in the lungs after
prolonged occupational exposure to cerium fumes or dust (Yoon et al., 2005; Porru et al., 2001;
McDonald et al., 1995; Pairon et al., 1995, 1994; Sulotto et al., 1986; Vogt et al., 1986; Pietra et
al., 1985; Vocaturo et al., 1983; Sabbioni et al., 1982; Husain et al., 1980; Kappenberger and
Biihlmann, 1975; Heuck and Hoschek, 1968). In these cases, the inhalation exposure was
primarily to cerium oxide, and cerium-induced pneumoconiosis was characterized by
accumulation of cerium particles (and other rare earth particles) in the lungs and lymphoreticular
system. Cerium exposure was associated with interstitial, peribronchial, and perivascular
fibrosis, a restriction of respiratory function and/or pulmonary hypertension (Porru et al., 2001;
Pairon et al., 1995, 1994; Vogt et al., 1986; Vocaturo et al., 1983; Sabbioni et al., 1982). The
available human studies were not selected for the derivation of an inhalation RfC because the
cerium exposures were not available in any of the case reports.
Information regarding long-term inhalation exposure in animals is derived from a single
subchronic study in rats (BRL, 1994), which was chosen as the principal study. Sprague-Dawley
rats (n = 30) were exposed nose only to cerium oxide aerosol 6 hours/day, 5 days/week for
13 weeks. Endpoints evaluated included a functional observational battery, hematology and
clinical chemistry, urinalysis, and gross and microscopic morphology of tissues, as discussed in
May, 2007
51
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
Section 4.2.2. The following effects were observed: (1) a statistically significant increase in
absolute and differential neutrophil counts in the blood in both males and females at 50.5 and
507.5 mg/m3 at both 6 and 13 weeks; (2) a statistically significant increase in absolute and
relative lung weight in both male and female rats at 50.5 and 507.5 mg/m3; (3) relative spleen
weight was statistically significantly increased in male rats at 507.5 mg/m3; (4) discoloration or
pale areas and uncollapsed parenchyma in the lungs in male and females rats at 50.5 and
507.5 mg/m3, with pale foci in female rats at 5 mg/m3; (5) an increased incidence of lymphoid
hyperplasia, alveolar epithelial hyperplasia, and pigment accumulation in the lungs at
507.5 mg/m3, 50.5 mg/m3, and 5 mg/m3, respectively, in both males and females; (6) an
increased incidence of lymphoid hyperplasia and pigment accumulation in the bronchial lymph
nodes of both males and females at 5 mg/m3; and (7) an increased incidence of metaplasia and
pigment accumulation in the larynx in males at 50.5 mg/m3 and 5 mg/m3, respectively, and in
females at 50.5 mg/m3.
The BRL (1994) study is an unpublished study; accordingly, it was externally peer
reviewed by EPA in August 2006 to establish its suitability for a dose-response evaluation of
cerium toxicity.
Based on the results of the human case reports, the BRL (1994) study, and the mode of
action analysis (Section 4.6.3.1), cerium toxicity may be the result of nonspecific stimulation of
PAMs that are activated and immobilized by the accumulation of insoluble cerium particles. The
subchronic BRL (1994) study in rats displayed increased lung weight; discoloration or pale
areas, pale foci, and uncollapsed parenchyma in the lungs; enlargement or pale discoloration of
the bronchial, mediastinal, and pancreatic lymph nodes; and dose-related alveolar epithelial and
lymphoid hyperplasia and pigment accumulation in the lungs and lymph nodes. These effects
are similar to the pneumoconiosis described in the human case reports, which were characterized
by the accumulation of cerium particles in the lungs and lymphoreticular system and histologic
effects throughout the lung. In addition, Hahn et al. (2001, 1999) demonstrated the retention of
cerium-alumniosilicate particles in the tracheobronchial lymph nodes of dogs for several years
following a single inhalation exposure.
Clearance of foreign substances from the lung by macrophages involves removal to the
stomach and GI tract, the lymphatics and lymph nodes, and the pulmonary vasculature (Witschi
and Last, 2001). As part of that process, macrophages that have phagocytized cerium particles
and have retained their mobility, would be removed from the lung and may accumulate in the
lymph nodes. The point in the neutrophilic response when normal clearance is overwhelmed
represents a shift to PAMs that have been activated and immobilized by the absorption of cerium
particles. This shift is reflected by the alveolar epithelial and lymphoid hyperplasia and pigment
accumulation in the lungs and lymph nodes of male and female rats (BRL, 1994). The
overloading of the PAMs by cerium, leading to the release of inflammatory cytokines and
May, 2007
52
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
fibrogenic growth factors, and the subsequent cell damage are the hypothesized mode of action
presented in Section 4.6.3.1. Therefore, the critical effect of cerium oxide exposure is increased
incidence of lymphoid hyperplasia in the bronchial lymph nodes of male and female rats, which
represents the most sensitive effect following cerium oxide exposure.
5.2.2. Methods of Analysis—Including Models (PBTK, BMD, etc.)
A NOAEL/LOAEL approach was used to derive the RfC for cerium oxide. Benchmark
dose (BMD) modeling was not utilized because the incidences of lymphoid hyperplasia in the
bronchial lymph nodes of male and female rats approached 100% at the lowest dose tested
(5 mg/m3) and were not amenable to modeling. Thus, the RfC is based on the LOAEL of
5 mg/m3 as the point of departure. Additionally, there was an increase in the incidence of
lymphoid and alveolar epithelial hyperplasia in the lung at 50.5 mg/m3. The selected point of
departure is considered to be protective of the pulmonary effects.
The human equivalent concentration (HEC) was calculated from the point of departure
by adjusting to a continuous exposure (24 hours a day, 7 days a week) and multiplying by a
dosimetric adjustment factor (DAF), which, in this case, was the regional deposited dose ratio
(RDDR) for the pulmonary region of the lung. Adjustment to a continuous exposure was
calculated as follows:
LOAELadj = LOAEL x (6 hours)/(24 hours) x (5 days)/(7 days)
= 5 mg/m3 x 0.25 x 0.71
= 0.89 mg/m3
The RDDR was calculated using the RDDR v.2.3 program (Table 5-1), as described in
Methods for Derivation of Inhalation Reference Concentrations and Application of Inhalation
Dosimetry (U.S. EPA, 1994b). The pulmonary region of the lung was selected as the deposition
site because the critical effect and the mode-of-action data indicate that the pulmonary region of
the lung is the initial location of cerium oxide toxicity.
The human parameters used were body weight, 70 kg; minute volume, 13.8 L;
extrathoracic surface area, 200 cm2; tracheobronchial surface area, 3,200 cm2; and pulmonary
surface area, 54.0 m2. The parameters used for the rat were body weight, 345 g (calculated by
averaging the mean body weights for each dose group for the course of the experiment); minute
volume, 234.18 mL; extrathoracic surface area, 15 cm2; tracheobronchial surface area,
22.50 cm2; and pulmonary surface area, 0.34 m2. The GSD for the cerium oxide particles in the
BRL (1994) study ranged from 1.8 to 1.9 and the MMAD for the particles was approximately
2.0 [j,m. The RDDRs calculated from the above model parameters vary depending upon the
May, 2007
53
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
deposition site in the lung (Table 5-1). The RDDR for pulmonary depositon is 0.536. The
calculation is as follows:
LOAELhec = LOAELadj x RDDR
LOAELrec = 0.89 mg/m3 x 0.536
LOAELhec = 0.48 mg/m3
Table 5-1. Output from RDDR v.2.3 used in the analysis in Section 5.2.2
Species
Body
weight (g)
VE (ml)
Extrathoracic
T racheobronchial
Pulmonary
SA (cm2)
dep
SA (cm2)
dep
SA (m2)
dep
Rat
345
234.2
15.000
0.542
22.500
0.042
0.340
0.049
Human
70000
13800.0
200.000
0.352
3200.000
0.085
54.000
0.245
Ratio
0.005
0.017
0.075
1.540
0.007
0.490
0.006
0.199
RDDR
0.348
1.183
0.536
Thoracic
Total RT
Extrares
>iratory
SA (cm2)
dep
SA (cm2)
dep
SA (m2)
dep
Rat
0.342
0.091
0.344
0.632
345
0.632
Human
54.320
0.125
54.340
0.682
70000
0.682
Ratio
0.006
0.724
0.006
0.927
0.005
0.927
RDDR
0.738
2.487
3.191
MMAD = 2.00; Sigma g = 1.85.
5.2.3. RfC Derivation—Including Application of Uncertainty Factors (UFs)
The LOAELhec value of 0.48 mg/m3 for lymphoid hyperplasia in the bronchial lymph
nodes of male and female CD rats (BRL, 1994) was used to derive the RfC for cerium oxide. A
total UF of 3,000 was applied to this point of departure: 3 for extrapolation from animals to
humans (UFA: animal to human), 10 for consideration of interindividual variability (UFH: human
variability), 10 for extrapolation from a subchronic study (UFS), 3 for LOAEL-to-NOAEL
extrapolation (UFL), and 3 for database deficiencies (UFD). The rationale for the application of
the UFs is described below.
A factor of 3 was selected to account for uncertainties in extrapolating from rats to
humans (UFA). This value is adopted by convention where an adjustment from an animal-
specific LOAELadj to a LOAELhec has been incorporated. Application of a full UF of 10 would
depend on two areas of uncertainty (i.e., toxicokinetic and toxicodynamic uncertainties). In this
assessment, the toxicokinetic component is mostly addressed by the determination of a HEC as
described in the RfC methodology (U.S. EPA, 1994b). The toxicodynamic uncertainty is also
accounted for to a certain degree by the use of the applied dosimetry method.
A factor of 10 was used to account for variation in susceptibility among members of the
human population (UFH). Insufficient information is available to predict potential variability in
susceptibility among the population to inhaled cerium oxide and cerium compounds.
May, 2007
54
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
A factor of 10 was used to account for uncertainty in extrapolating from a subchronic to
chronic (UFS) exposure duration, since the BRL (1994) study, which was selected as the
principal study, is a subchronic study. The critical effect, increased incidence of lymphoid
hyperplasia in the bronchial lymph nodes, may be more pronounced at longer durations.
A factor of 3 was used to account for uncertainty in extrapolating from a LOAEL to a
NOAEL (UFl) because the critical effect selected to determine the point of departure for this
analysis, lymphoid hyperplasia in the bronchial lymph nodes, represents a sensitive, precursor
effect that occurs early in the series of critical events leading to more severe effects in the lung.
Specifically, lymphoid hyperplasia in the bronchial lymph nodes represents the point at which
normal clearance of particles from the lung by alveolar macrophages becomes overwhelmed and
particles are no longer cleared effectively. This delayed clearance leads to increased
accumulation of cerium oxide particles in the respiratory tract, an inflammatory response, and
subsequent cell proliferation.
The involvement of additional lymph nodes (e.g., enlargement and discoloration of the
mediastinal lymph nodes) and other tissues (e.g., enlargement and discoloration of the pancreatic
lymph nodes), along with alveolar epithelial hyperplasia in the lung at higher exposure
concentrations, supports the hypothesis that the clearance process may be increasingly
overwhelmed at higher concentrations and that the point of departure for the derivation of the
RfC (i.e., the LOAEL) is based on a sensitive effect. The ability of cerium-oxide-exposed rats to
recover after exposure has not been studied, but would demonstrate the level of persistence of
the bronchial lymphoid hyperplasia observed in BRL (1994) study. Case studies in workers
exposed to cerium oxide (Yoon et al., 2005; Porru et al., 2001; McDonald et al., 1995; Pairon et
al., 1995, 1994; Sulotto et al., 1986; Vogt et al., 1986; Pietra et al., 1985; Vocaturo et al., 1983;
Sabbioni et al., 1982; Husain et al., 1980; Kappenberger and Biihlmann, 1975; Heuck and
Hoschek, 1968) have identified interstitial lung disease as a possible outcome of exposure to
cerium oxide in the workplace. The BRL (1994) study and proposed mode of action for cerium
oxide-induced lung effects supports the conclusion that lymphoid hyperplasia in the bronchial
lymph nodes of humans may be an early manifestation of the nonspecific response to cerium
oxide.
A UF of 3 was used to account for deficiencies in the cerium oxide database. The
database includes multiple case reports of inhalation exposure to workers and a single 13-week
subchronic inhalation study in rats. The effects from the subchronic rat inhalation study that are
used for the derivation for the RfC (i.e., bronchiolar lymph node hyperplasia) may be early
indicators of the more overt toxicity that is found in humans (i.e., interstitial lung disease)
exposed to cerium oxide in the workplace. The database does not include an exposure and
recovery study that could demonstrate the persistence or, conversely, the adaptive nature of the
lymphoid hyperplasia in the bronchial lymph nodes.
May, 2007
55
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
Toxicity via the inhalation route is expected to be a portal-of-entry effect. Cerium oxide
is a relatively insoluble metal oxide and absorption or translocation from the lung to the
circulation is expected to be minimal at low doses. The pulmonary effects observed in the
human case reports and in the BRL (1994) study are likely due to the physical deposition of
cerium oxide particles in the lung and the immunological reaction to the particles, and are not
due to a chemical reaction of cerium oxide with lung tissues. The lymphoid hyperplasia in the
bronchial lymph nodes is an immunological response to the cerium oxide particles and is not due
to cytotoxicity with regenerative cell growth. The observed immunological response is a portal-
of-entry effect and systemic circulation and effects are not expected because of the insoluble
nature of the cerium oxide particles.
In considering the impact of database deficiencies on the derivation of the RfC,
substantial weight was given to the available data demonstrating similarities between the effects
observed in humans following prolonged exposure to cerium oxide and the effects observed in
rats in the subchronic principal study, along with the data on deposition and absorption of cerium
oxide in the lung. Thus, these data support the assumption that the respiratory system may be
the most sensitive target of toxicity following inhalation exposure to cerium and that inhalation
exposure to cerium may primarily involve portal-of-entry effects.
The database for cerium oxide lacks both a two-generation reproductive toxicity bioassay
and a developmental toxicity bioassay. Systemic effects following the inhalation of cerium
oxide, with an MMAD of approximately 2.0 |iin and a GSD from 1.8 to 1.9, are not likely to be
observed outside the lung. While it is recognized that the investigation of systemic effects
following cerium oxide exposure has not been the focus of existing studies, there is no reason to
expect that reproductive, developmental, or other systemic effects would occur, and a UF of 3 is
sufficient for the absence of data on these effects. The analysis of the scientific information
available for cerium as a whole supports the utilization of a database UF of 3.
The chronic RfC for cerium oxide was calculated as follows:
RfC = LOAELrec - UF
= 0.48 mg/m3 - 3000
= 2 x 10 4 mg/m3 (rounded to one significant figure)
Note that the RfC was quantified for cerium oxide particles with an MMAD of
approximately 2.0 [j,m and a GSD from 1.8 to 1.9, and may not appropriately characterize the
potential toxicity from exposures to cerium oxide particles with smaller MMADs and GSDs,
including nano-sized cerium particles. The use of the RfC for cerium compounds other than
cerium oxide is not recommended as the similarity between this form of cerium and other cerium
compounds is unknown.
May, 2007
56
DRAFT - DO NOT CITE OR QUOTE
-------
1 5.2.4. RfC Comparison Information
2 Figure 5-1 is an exposure-response array, which presents NOAELs, LOAELs, and the
3 dose range tested corresponding to selected health effects from the BRL (1994) study, some of
4 which were considered candidates for chronic RfC derivation. Figure 5-2 presents the point of
5 departure, calculated as a human equivalent dose, applied uncertainty factors, and derived
6 chronic inhalation reference values for selected endpoints from Figure 5-1. This comparison is
7 intended to provide information for additional endpoints associated with cerium oxide inhalation
May, 2007
57
DRAFT - DO NOT CITE OR QUOTE
-------
1
1000
100
O)
E
10
2
3
4
5
V?* v<<^ vf^ ^ ^ O 0^ ^ <<> ^
# .# J> J? Jff <*? ^ y J? J? -V -V
p* ^ £ tf <$?' >®
c& o* ...o° **
,o<>' ^ ^ ^ .«&* .J?
\On \0X
^ ^ *f *f ~ /* ^ ^ ^r ^ ^ ^ ^ _
-------
1 Figure 5-2. Points of Departure for selected endpoints from Figure 5-1 with corresponding applied uncertainty factors and
2 derived chronic inhalation RfVs.
10
1
0.1
0.01
0.001
0.0001
Lymphoid
hyperplasia
- bronchial
lymph
node 0,$)
Pigment
accumulation
- bronchial
lymphoid
Pigment
accumulation
-lung 0,$)
Increased
absolute lung
weight 0,$)
Lymphoid
hyperplasia
-lung (?)
Alveolar
epithelial
hyperplasia -
lung 0,$)
Lymphoid
hyperplasia
- lung 0)
'SSSSSi
'SSSSSi
UFa
UFh
UFsc
UFl
UFd
~ POD
• RfV
May, 2007
59
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
5.2.5. Previous RfC Assessment
A previous IRIS assessment was not available for cerium oxide and cerium compounds.
5.3. UNCERTAINTIES IN THE INHALATION REFERENCE CONCENTRATION
(RfC)
Risk assessments need to portray associated uncertainty. The following discussion
identifies uncertainties associated with the RfC for cerium oxide. As presented earlier in this
chapter (Sections 5.2.2 and 5.2.3), UFs were applied to the point of departure for the RfC.
Factors accounting for uncertainties associated with a number of steps in the analyses were
adopted to account for extrapolating from an animal bioassay to human exposure, a diverse
population of varying susceptibilities, extrapolating from a subchronic to chronic exposure
duration, extrapolating from a LOAEL to a NOAEL, and database deficiencies.
A limited range of animal toxicology data is available for the hazard assessment of
cerium oxide, as described throughout the previous sections (see Sections 4 and 5). For the oral
route, human studies showing an association between exposure to cerium in food and the
development of endomyocardial fibrosis are available. Long-term studies in animals are limited
to a 13-month drinking water study in rats, which investigated the effects of a single dose group;
a 6-month drinking water study in rabbits in which the administered dose consisted of a mixture
of rare earth chlorides; a 12-week dietary study in mice; and a 105-day gavage study in rats. An
RfD for cerium was not derived since the available studies were not suitable for quantitation of
effects for various reasons, including unknown exposure concentrations in the available human
studies, lack of a dose response, uncertain biological significance (e.g., increased oxygen affinity
for hemoglobin, changes in measures of oxidative stress), and inadequate study design (e.g.,
effects noted only under conditions of a restricted diet).
The inhalation database includes a single, 13-week subchronic inhalation bioassay in rats,
with numerous case reports of workers who developed pneumoconiosis associated with
accumulation of cerium particles in the lungs after prolonged occupational exposure to cerium
fumes or dust. Critical data gaps have been identified and uncertainties associated with data
deficiencies are more fully discussed below.
Consideration of the available dose-response data to determine an estimate of inhalation
exposure that is likely to be without an appreciable risk of adverse health effects over a lifetime
led to the selection of the 13-week subchronic inhalation bioassay in Sprague-Dawley rats (BRL,
1994) and increased incidence of lymphoid hyperplasia in the bronchial lymph nodes of male
and female rats as the principal study and critical effect for deriving the RfC for cerium oxide.
The presence of lymphoid hyperplasia in the bronchial lymph nodes following cerium oxide
exposure may be part of a clearance process in which macrophages that have phagocytosed
cerium particles are removed to the lymphatic drainage. Foreign substances that are introduced
May, 2007
60
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
to the lung are cleared to the stomach and GI tract, the lymphatics and lymph nodes, and the
pulmonary vasculature (Witschi and Last, 2001). However, the database of cerium studies does
not include an exposure and recovery study that may demonstrate the persistence or, conversely,
the adaptive nature of the lymphoid hyperplasia in the bronchial lymph nodes. In the absence of
evidence indicating otherwise, these effects are assumed to represent cerium-induced toxicity.
The derived RfC was quantified using a LOAEL for the point of departure. A point of
departure based on a LOAEL or NOAEL is, in part, a reflection of the particular exposure
concentration or dose at which a study was conducted. It lacks characterization of the dose-
response curve and for this reason is less informative than a point of departure defined as an
effect level concentration (i.e., benchmark concentration [BMC]) obtained from benchmark
dose-response modeling. In this assessment, the exposure-related increase in pigment
accumulation and lymphoid hyperplasia in lymph nodes draining lungs, the bronchial and
mediastinal lymph nodes, in combination with the hypothesized mode of action, the overloading
of the PAMs by cerium oxide particles, further supports the role of pulmonary macrophages in
the nonspecific response. Thus, the lymphoid hyperplasia in the bronchial lymph nodes, which
was observed in 80% of the exposed rats, may be an early manifestation of the nonspecific
response to cerium oxide.
Extrapolating from animals to humans embodies further issues and uncertainties. The
effect and its magnitude associated with the concentration at the point of departure in rodents are
extrapolated to human response. Pharmacokinetic models are useful to examine species
differences in pharmacokinetic processing; however, dosimetric adjustment using
pharmacokinetic modeling was not possible for the toxicity observed following inhalation
exposure to cerium oxide. For the RfC, HECs were calculated from the point of departure by
multiplying the LOAELadj by a DAF. The calculated DAF in this assessment is an RDDR. The
RDDR was calculated using the RDDR v.2.3 program, as described in Methods for Derivation of
Inhalation Reference Concentrations and Application of Inhalation Dosimetry (U. S. EPA,
1994b).
Heterogeneity among humans is another uncertainty associated with extrapolating doses
from animals to humans. Uncertainty related to human variation needs consideration, also, in
extrapolating dose from a subset or smaller sized population, say of one sex or a narrow range of
life stages typical of occupational epidemiologic studies, to a larger, more diverse population.
Human variation may be larger or smaller; however, cerium-specific data to examine the
potential magnitude of over- or underestimation is unavailable.
Critical data gaps have been identified with uncertainties associated with database
deficiencies with regards to chronic toxicity, especially evidence demonstrating persistence of
lymphoid hyperplasia, and reproductive and developmental toxicity associated with inhalation
exposure to cerium. The available oral cerium exposure information in both humans and rats
May, 2007
61
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
identifies cardiac tissue and hemoglobin oxygen affinity as possible adverse health effects, but
the animal studies are of insufficient duration and experimental design and the human studies do
not provide an adequate exposure characterization. The lack of a sufficient study to derive an
RfD given the possible adverse effects demonstrated represents a critical data gap in the oral
database. A chronic, cerium oxide inhalation exposure study in animals is unavailable; however,
the workers in the human case reports were exposed to cerium oxide for periods of 10 to
46 years, and, while the case reports do not contain the necessary exposure analysis for dose
response assessment, they do provide evidence of adverse respiratory effects following long-
term exposure to cerium oxide. There are limited data available addressing possible
reproductive or neurodevelopmental toxicity following exposure to cerium. The accumulation of
insoluble cerium particles in the respiratory tract of humans and animals following chronic and
subchronic inhalation exposures, respectively, suggests that impaired clearance may influence
pulmonary toxicity in both rats and humans and limit systemic availability; therefore, possible
reproductive or developmental toxicity may be expected to occur at doses higher than those at
which portal-of-entry effects occurred.
5.4. CANCER ASSESSMENT
Studies addressing the carcinogenic effects of cerium or cerium compounds upon which
to base a cancer assessment are unavailable. Lundgren et al. (1996) observed lung neoplasms
(7/1,049) in cerium oxide exposed control F344/N rats in an investigation of the carcinogenicity
of the beta-particle emitter, 144Ce. The incidence of lung neoplasms in the stable cerium oxide
exposed rats was not compared to a treatment-free group or to historical background levels in
F344/N rats.
As discussed in Section 4.7, data were unavailable regarding the carcinogenicity of stable
cerium in humans or experimental animals. In accordance with U.S. EPA (2005a) guidelines for
carcinogen risk assessment, there is "inadequate information to assess the carcinogenic
potential" of cerium oxide and cerium compounds. Lack of carcinogenicity data precludes
derivation of an oral slope factor or inhalation unit risk. Genotoxicity evidence was insufficient
to assess the genotoxic potential. This overall lack of information represents a data gap and does
not allow for a quantitative assessment of the carcinogenicity of cerium oxide and cerium
compounds. A previous cancer assessment was not available for cerium oxide and cerium
compounds.
May, 2007
62
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
6. MAJOR CONCLUSIONS IN THE CHARACTERIZATION OF HAZARD
AND DOSE RESPONSE
6.1. HUMAN HAZARD POTENTIAL
Cerium (CAS No. 7440-45-1) exposure is mostly in the form of mischmetal for
metallurgical purposes (Kilbourn, 2003). Cerium is the major component of mischmetal (50-
75% by weight for the most common grades), a commercial mixture of metallic light lanthanides
prepared by the electrolysis of mixed lanthanide chlorides and fluorides obtained from bastanite
or monazite (Kilbourn, 2003; Reinhardt and Winkler, 2002). Mischmetal reacts with the
impurities found in metals to form solid compounds, thereby reducing the effect of these
impurities on the properties of the metal (Reinhardt and Winkler, 2002). Cerium oxide (Ce02;
eerie oxide) is the most important of the commercial cerium compounds. It is used either in the
pure form or in a concentrate as a polishing agent for glass mirrors, plate glass, television tubes,
ophthalmic lenses, and precision optics (Kilbourn, 2003; Reinhardt and Winkler, 2002). Cerium
oxide is used as a glass constituent to prevent solarization and discoloration (especially in the
faceplates of television screens) (Reinhardt and Winkler, 2002). Cerium oxide is also used as a
diesel fuel-born catalyst to reduce particulate matter emissions (HEI, 2001). Cerium is not
expected to exist in elemental form in the environment since it is a very reactive metal (Lewis,
2001). Cerium compounds are not expected to volatilize and will exist in the particulate form if
released into the air.
Toxicokinetic studies in rodents have examined the absorption, distribution, metabolism,
and elimination of cerium. In adult rats, cerium compounds are very poorly absorbed following
oral exposure, while suckling animals exhibit higher absorption and retention of cerium in the GI
tissues (Kostial et al., 1989a, b; Inaba and Lengemann, 1972). Following inhalation exposure,
cerium, a poorly-soluble particle, behaves like other airborne particles, depositing within the
respiratory tract based on aerodynamic character (for review, see Schulz et al., 2000). Cerium
has been detected in lung tissues and in alveolar macrophages of subjects believed to have been
exposed occupationally (Vocaturo et al., 1983; Sabbioni et al., 1982), with cerium concentrations
in the lung tissues 2,800-207,000 times higher than those found in the urine, blood, or nails
(Pietra et al., 1985). The early clearance of the radioactive cerium administered dose suggests
that the majority of cerium aerosol is deposited in the airways, where it is subject to removal via
the mucociliary escalator, swallowing, and elimination in the feces.
Once absorbed into the body, cerium tends to accumulate primarily in the bone, liver,
heart, and lung. Cerium has been observed to be localized in the cell, particularly in the
lysosomes, where it is concentrated and precipitated in an insoluble form in association with
phosphorus. As an element, cerium is neither created nor destroyed within the body. The
particular cerium compound (e.g., cerium chloride, cerium oxide) may be altered as a result of
May, 2007 63 DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
various chemical reactions within the body, particularly dissolution, but data have not
demonstrated a change in the oxidation state of the cerium molecule (Berry et al., 1989, 1988).
Following inhalation exposure, the initial rapid elimination of cerium from the body is
due primarily to transport up the respiratory tract by the mucociliary escalator and eventual
swallowing of the material, as with other poorly soluble particles (Boecker and Cuddihy, 1974).
After the initial clearance of cerium particles from the upper respiratory tract, pulmonary
clearance is slower, with reported slow-phase clearance half-times ranging from 100 to 190 days
in rodents (Lundgren et al., 1974; Thomas et al., 1972; Morgan et al., 1970; Sturbaum et al.,
1970). Elimination of orally administered cerium has been shown to be age dependent in
animals, with suckling animals absorbing cerium into the GI tissues (Inaba and Lengemann,
1972).
An epidemiological study reports a higher incidence of endomyocardial fibrosis among a
population consuming tubers grown in a high cerium soil in India (Eapen, 1998; Kutty et al.,
1996; Valiathan et al., 1989), and a case control study found an association between increased
toenail cerium concentrations and the risk of first myocardial infarction (Gomez-Aracena et al.,
2006). The few long-term animal studies available included dietary, gavage, and drinking water
administrations, although clear indicators of adverse effects were not evident.
Numerous case reports have been published describing cases of workers who developed
adverse lung effects, such as interstitial lung disease or pneumoconiosis, associated with
accumulation of cerium in the lungs after prolonged occupational exposure to cerium fumes or
dust (Yoon et al., 2005; Porru et al., 2001; McDonald et al., 1995; Pairon et al., 1995, 1994;
Sulotto et al., 1986; Vogt et al., 1986; Pietra et al., 1985; Vocaturo et al., 1983; Sabbioni et al.,
1982; Husain et al., 1980; Kappenberger and Biihlmann, 1975; Heuck and Hoschek, 1968). The
human cases of cerium exposure demonstrate the accumulation of cerium particles in the lungs
and lymphoreticular system, with pulmonary function varying from normal to severe restriction
and interstitial fibrosis in one case and granulomas in another. Interstitial fibrosis accompanied
by vascular thickening, reactive alveolar macrophages, abundant macrophages in the airspace,
and moderate chronic interstitial inflammation, along with small interstitial clumps of
macrophages bearing scant deposits of grayish-black pigment, was observed in a 68-year-old
man who was employed as an optical lens grinder for 35 years (McDonald et al., 1995).
Additionally, particles characterized as cerium were identified in alveolar macrophages,
macrophages in the tracheobronchial lymph nodes, and lung and lymph node tissue (Porru et al.,
2001; Pairon et al., 1995; Waring and Watling, 1990; Sulotto et al., 1986; Vogt et al., 1986;
Pietra et al., 1985). Cerium exposure was associated with interstitial, peribronchial, and
perivascular fibrosis, a restriction of respiratory function, and/or pulmonary hypertension (Porru
et al., 2001; Pairon et al., 1995, 1994; Vogt et al., 1986; Vocaturo et al., 1983; Sabbioni et al.,
1982).
May, 2007
64
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
Data from a subchronic toxicity test in Sprague-Dawley rats (BRL, 1994) identified an
increased incidence of lymphoid hyperplasia in the bronchial lymph nodes as the critical effect
for noncancer effects. Histologic examination revealed dose-related alveolar epithelial and
lymphoid hyperplasia and pigment accumulation in the lungs, lymph nodes, and larynx of male
and female rats at >5 mg/m3.
The mode of action for cerium oxide inhalation toxicity may be mediated by cytokine
and fibrogenic effects resulting from pulmonary macrophage activation followed by macrophage
immobilization. The accumulation of insoluble cerium oxide particles in the respiratory tract of
humans and rodents following chronic and subchronic exposure, respectively, suggests that
impaired clearance may influence pulmonary toxicity for rats and humans. A population of
immobilized, activated macrophages may serve to induce significant cell damage by effectively
increasing the concentration of inflammatory cytokines and fibrogenic growth factors within the
pulmonary epithelium.
6.2. DOSE RESPONSE
6.2.1. Noncancer/Oral
The database for oral exposure to cerium is limited to geographical distribution studies
evaluating the possible association between exposure to cerium in food and the development of
endomyocardial fibrosis (Eapen et al., 1998; Kutty et al., 1996; Valiathan et al., 1989), and long-
term animal studies, including a 13-month drinking water study in rats (Kumar et al., 1996), a
6-month drinking water study in rabbits (Kartha et al., 1998), a 12-week dietary study in mice
(Kawagoe et al., 2005), and a 105-day gavage study in rats (Cheng et al., 2000).
The geographical distribution studies suggest that there is an association between
exposure to cerium in food and the development of endomyocardial fibrosis but are unsuitable
for RfD derivation. The above animal studies are unsuitable for derivation of an RfD for various
reasons. The 13-month study by Kumar et al. (1996) was conducted on a small number of rats at
only one dose level (control and dosed rats) and evaluated few endpoints, and the health effects
were highly variable and not statistically significant. Kartha et al. (1998) conducted a 6-month
drinking water study in rabbits that utilized a rare earth chloride mixture in the drinking water,
which suggests that cerium may enhance the effect of magnesium deficiency in heart tissue. The
Cheng et al. (2000) study was limited in scope, since it investigated the effects of cerium
chloride exposure on the structure and affinity of hemoglobin in rats. The evaluations conducted
in Kawagoe et al. (2005), showing statistically significant changes as a result of cerium exposure
(e.g., decreased lipoperoxide levels), increased GSH levels and MT activity, and decreased SOD
activity, are of unknown biological significance and may represent oxidative stress in response to
cerium exposure.
May, 2007
65
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
The available oral cerium exposure information in both humans and rats identifies
cardiac abnormalities and changes in hemoglobin oxygen affinity as possible effects, but the
animal studies are of insufficient duration and experimental design and the human studies do not
provide an adequate exposure characterization. The lack of a sufficient study to derive an RfD
given the effects demonstrated represents a critical data gap in the oral database.
6.2.2. Noncancer/Inhalation
There are numerous case reports of workers who developed pneumoconiosis, associated
with accumulation of cerium particles in the lungs, after prolonged occupational exposure to
cerium fumes or dust (Yoon et al., 2005; Porru et al., 2001; McDonald et al., 1995; Pairon et al.,
1995, 1994; Sulotto et al., 1986; Vogt et al., 1986; Pietra et al., 1985; Vocaturo et al., 1983;
Sabbioni et al., 1982; Husain et al., 1980; Kappenberger and Biihlmann, 1975; Heuck and
Hoschek, 1968). Information regarding long-term inhalation exposure in animals is derived
from a single subchronic study in rats (BRL, 1994). Sprague-Dawley rats were exposed nose
only to cerium oxide aerosol 6 hours/day, 5 days/week for 13 weeks. Endpoints evaluated
included a functional observational battery, hematology and clinical chemistry, urinalysis, and
gross and microscopic morphology of tissues. The critical effect of cerium oxide exposure is
increased incidence of alveolar lymphoid hyperplasia in the bronchial lymph nodes of male and
female rats with a LOAEL of 5 mg/m3.
Consideration of the available dose-response data to determine an estimate of inhalation
exposure that is likely to be without an appreciable risk of adverse health effects over a lifetime
led to the selection of the 13-week subchronic inhalation bioassay in Sprague-Dawley rats (BRL,
1994) and increased incidence of lymphoid hyperplasia in the bronchial lymph nodes of male
and female rats as the principal study and critical effect for deriving the RfC for cerium oxide.
The presence of lymphoid hyperplasia in the bronchial lymph nodes following cerium oxide
exposure may be part of a clearance process in which macrophages that have phagocytosed
cerium particles are removed to the lymphatic drainage. Foreign substances that are introduced
to the lung are cleared to the stomach and GI tract, lymphatics and lymph nodes, and pulmonary
vasculature (Witschi and Last, 2001). However, the database of cerium studies does not include
an exposure and recovery study that may demonstrate the persistence of the lymphoid
hyperplasia in the bronchial lymph nodes. Thus, in the absence of evidence demonstrating
otherwise, these effects are believed to represent cerium-induced toxicity.
The RfC was derived using a LOAEL for the point of departure. A point of departure
based on a LOAEL or NOAEL is, in part, a reflection of the particular exposure concentration or
dose at which a study was conducted. It lacks characterization of the dose-response curve and
for this reason is less informative than a point of departure defined as an effect level
concentration (i.e., BMC) obtained from benchmark dose-response modeling. In this
May, 2007
66
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
assessment, the exposure-related increase in pigment accumulation and lymphoid hyperplasia in
lymph nodes draining lungs (the bronchial and mediastinal lymph nodes), in combination with
the hypothesized mode of action (the overloading of the PAMs by cerium oxide particles),
further supports the role of pulmonary macrophages in the nonspecific response. Thus, the
lymphoid hyperplasia in the bronchial lymph nodes is an early manifestation of the nonspecific
response to cerium oxide.
Extrapolating from animals to humans embodies further issues and uncertainties. The
effect and its magnitude associated with the concentration at the point of departure in rodents are
extrapolated to human response. Pharmacokinetic models are useful to examine species
differences in pharmacokinetic processing; however, dosimetric adjustment using
pharmacokinetic modeling was not possible for the toxicity observed following inhalation
exposure to cerium oxide. For the RfC, an HEC was calculated from the point of departure by
multiplying the LOAELadj by a DAF. The calculated DAF in this assessment is an RDDR. The
RDDR was calculated using the RDDR v.2.3 program, as described in Methods for Derivation of
Inhalation Reference Concentrations and Application of Inhalation Dosimetry (U. S. EPA,
1994b).
Heterogeneity among humans is another uncertainty associated with extrapolating doses
from animals to humans. Uncertainty related to human variation needs consideration, also, in
extrapolating dose from a subset or smaller-sized population, say of one sex or a narrow range of
life stages typical of occupational epidemiologic studies, to a larger, more diverse population.
Human variation may be larger or smaller; however, cerium-specific data to examine the
potential magnitude of over- or underestimation are unavailable.
The RfC of 2 x 10 4 mg/m3 was calculated from a LOAELHec of 0.48 mg/m3 for
increased incidence of lymphoid hyperplasia in the bronchial lymph nodes in male and female
rats following subchronic cerium oxide inhalation exposure (BRL, 1994). A total UF of 3,000
was used: 3 for interspecies extrapolation, 10 for intraspecies variability, 10 for subchronic to
chronic extrapolation, 3 for extrapolating from a LOAEL to a NOAEL, and 3 for database
deficiencies.
A factor of 3 was selected to account for uncertainties in extrapolating from rats to
humans, which is adopted by convention where an adjustment from an animal-specific
NOAELadj to a NOAELHec has been incorporated. Insufficient information is available to
predict potential variability in susceptibility among the population, thus a human variability UF
of 10 was applied. A 10-fold UF was used to account for uncertainty in extrapolating from a
subchronic to chronic exposure duration. A 3-fold UF was applied to account for the
extrapolation from a LOAEL to NOAEL. The critical effect for this analysis, lymphoid
hyperplasia in the bronchial lymph nodes, may represent the point at which normal clearance of
particles from the lung by alveolar macrophages becomes overwhelmed and particles are no
May, 2007
67
DRAFT - DO NOT CITE OR QUOTE
-------
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
longer cleared effectively, leading to an increasing accumulation of particles in the lung and
airways, an inflammatory response, and subsequent cell proliferation. However, due to the
absence of evidence demonstrating the persistence of lymphoid hyperplasia, these effects are
believed to represent cerium-induced toxicity. Data gaps have been identified with uncertainties
associated with database deficiencies with regards to reproductive and developmental toxicity
associated with cerium inhalation exposure. A database UF of 3 was applied with special
consideration of the information pertaining to the deposition and absorption of cerium oxide, the
effects observed in humans following prolonged exposure, the mode-of-action data, and the
effects observed in animals in BRL (1994), in addition to the lack of reproductive and
developmental studies.
The overall confidence in this RfC assessment is low. Confidence in the principal study
(BRL, 1994) is medium. EPA conducted an external peer review to evaluate the accuracy of the
experimental procedures, results, and interpretation and discussion of the results presented in this
study. The peer reviewers considered the BRL (1994) study to be adequate and the study
conclusions to be supported by the data. The peer reviewers were not specifically asked to
comment on their confidence in the study. In addition, the results observed in the BRL (1994)
study were consistent with the observed effects in the human case reports (Yoon et al., 2005;
Porru et al., 2001; McDonald et al., 1995; Pairon et al., 1995, 1994; Sulotto et al., 1986; Vogt et
al., 1986; Pietra et al., 1985; Vocaturo et al., 1983; Sabbioni et al., 1982; Husain et al., 1980;
Kappenberger and Biihlmann, 1975; Heuck and Hoschek, 1968). Confidence in the database is
low. The database lacks chronic exposure information on cerium via any route of exposure and
multigenerational developmental and reproductive toxicity studies. However, there is evidence
of cerium pneumoconiosis in humans exposed to cerium compounds, and the anticipated critical
effects observed are point-of-entry effects that would be expected in humans. Reflecting
medium confidence in the principal study and low confidence in the database, confidence in the
RfC is low.
6.2.3. Cancer/Oral and Inhalation
Under the Guidelines for Carcinogen Risk Assessment (U.S. EPA, 2005a), the database
for cerium oxide and cerium compounds is inadequate to assess human carcinogenic potential
and to calculate quantitative cancer risk estimates.
May, 2007
68
DRAFT - DO NOT CITE OR QUOTE
-------
1 7. REFERENCES
2
3 Arvela, P; Karki, NT. (1971) Effect of cerium on drug metabolism activity in rat liver. Experientia 27(10): 1189—
4 1190.
5 Arvela, P; Kraul, H; Stenback, F; et al. (1991) The cerium-induced liver injury and oxidative drug metabolism in
6 Dba/2 and C57bl/6 mice (Finland). Toxicology 69(1): 1-9.
7 Berry, JP. (1996) The role of lysosomes in the selective concentration of mineral elements. A microanalytical study.
8 Cell Mol Biol 42(3):395-411.
9 Berry, JP; Meignan, M; Escaig, F; et al. (1988) Inhaled soluble aerosols insolubilised by lysosomes of alveolar cells.
10 Application to some toxic compounds; electron microprobe and ion microprobe studies. Toxicology 52(1-2): 127-
11 139.
12 Berry, JP; Masse, R; Escaig, F; et al. (1989) Intracellular localization of cerium. A microanalytical study using an
13 electron microprobe and ionic microanalysis. Hum Toxicol 8(6):511-520.
14 Berry, JP; Zhang, L; Galle, P; et al. (1997) Role of alveolar macrophage lysosomes in metal detoxification.
15 Microscop Res Technol 36:313-323.
16 Boecker, BB; Cuddihy, RG. (1974) Toxicity of 144Ce Inhaled as 144CeCl3 by the beagle: metabolism and dosimetry.
17 Radiat Res 60(1): 133-154.
18 BRL (Bio-Research Laboratories). (1994) Final report for a 90-day inhalation neurotoxicity and toxicity study by
19 exposure to a dry powder aerosol of eerie oxide in the albino rat with cover letter dated 013095. Produced by Bio-
20 Research Laboratories, Montreal, Canada for Rhone-Poulenc Inc. Submitted under TSCA Section 8E; EPA Doc. No.
21 89-950000107; NTIS No. OTS0556254. [An external peer review was conducted by EPA in August 2006 to
22 evaluate the accuracy of experimental procedures, results, and interpretation and discussion of the findings
23 presented. A report of this peer review is available through the EPA's IRIS Hotline, at (202) 566-1676 (phone),
24 (202) 566-1749 (fax), or hotline.iris@epa.gov (e-mail address) and on the IRIS website (www.epa.gov/iris).]
25 Bruce, DW; Hietbrink, BE; DuBois, KP. (1963) The acute mammalian toxicity of rare earth nitrates and oxides.
26 Toxicol and App Pharmacol 5:750-759.
27 Brunner, TJ; Wick, P; Manser, P; et al. (2006) In vitro cytotoxicity of oxide nanoparticles: comparison to asbestos,
28 silica, and the effect of particle solubility. Environ Sci Technol 40(14)4374-4381
29 Chen, J; Tan, M; Nemmar, A; et al. (2006) Quantification of extrapulmonary translocation of intratracheal-instilled
30 particles in vivo in rats: effect of lipopolysaccharide. Toxicology 222:195-201.
31 Cheng, Y; Li, Y; Li, R; et al. (2000) Orally administered cerium chloride induces the conformational changes of rat
32 hemoglobin, the hydrolysis of 2,3-DPG and the oxidation of heme-Fe(II), leading to changes of oxygen affinity.
33 ChemBiol Interact 125(3): 191-208.
34 Cotton, FA; Wilkinson G; Murillo CA. (1999) Advanced inorganic chemistry. 6th edition. New York, NY: John
35 Wiley and Sons, Inc.; p. 1125.
36 Cullen, RT; Tran, CL; Buchanan, D; et al. (2000) Inhalation of poorly soluble particles. I. differences in
37 inflammatory response and clearance during exposure. Inhal Tox 12:1089-1111.
3 8 D'Agostino, R; Olson, FC; Massaro, EJ. (1978a) Growth and behavioral abnormalities in offspring of mice treated
39 with cerium (citrate) or platinum (hexachloroplatinate) during pregnancy or lactation. Proc Int Congr Toxicol
40 1:608-609.
May, 2007
69
DRAFT - DO NOT CITE OR QUOTE
-------
1 D' Agostino, R; Olson, FC; Massaro, EJ; et al. (1978b) Alterations of growth and behavioral parameters of offspring
2 of female mice exposed to cerium (citrate) or platinum (hexachloroplatinate) during pregnancy or lactation. In:
3 Hemphill, DD; ed. Trace substances in environmental health, XI: proceedings of the University of Missouri's 11th
4 annual conference on trace substances in environmental health. Columbia, MO: University of Missouri; pp. 257-
5 263.
6 D'Agostino, RB; Lown, BA; Morganti, JB; et al. (1982) Effects of in utero or suckling exposure to cerium (citrate)
7 on the postnatal development of the mouse. J Toxicol Environ Health 10:449-458.
8 Du, X; Zhang, T; Li, R; et al. (2001) Nature of cerium(III)- and lanthanum(III)-induced aggregation of human
9 erythrocyte membrane proteins. J Inorg Biochem 84(l-2):67-75.
10 Durbin, PW; Williams, MH; Gee, M; et al. (1956) Metabolism of the lanthanons in the rat. Proc Soc Exper Biol
11 Med 91:78-85.
12 Eapen, JT; Kartha, CC; Rathinam, K; et al. (1996) Levels of cerium in the tissues of rats fed a magnesium-restricted
13 and cerium-adulterated diet. Bull Environ Contam Toxicol 56(2): 178-182.
14 Eapen, JT. (1998) Elevated levels of cerium in tubers from regions endemic for endomyocardial fibrosis (EMF).
15 Bull Environ Contam Toxicol 60:168-170.
16 Eisele, GR; Mraz, FR; Woody, MC. (1980) Gastrointestinal uptake of 144Ce in the neonatal mouse, rat and pig.
17 Health Phys 39:185-192.
18 Evans, LJ. (1989) Chemistry of metal retention by soils. Environ Sci Technol 23:1046-1054.
19 Fedulov AV; Leme, A; Yang, Z; et al. (2008) Pulmonary exposure to particles during pregnancy causes increased
20 neonatal asthma susceptibility. Amer Journ Resp Cell Molec Biol 38:57-67.
21 Ferin, J. (1994) Pulmonary retention and clearance of particles. Toxicol Lett 72(1-3):121-125.
22 Galle, P; Berry, JP; Galle, C. (1992) Role of alveolar macrophages in precipitation of mineral elements inhaled as
23 soluble aerosols. Environ Health Perspect 97:145-147.
24 Gomez-Aracena, J; Riemersma, RA; Guitierrez-Bedmar, M; et al. (2006) Toenail cerium levels and risk of a first
25 acute myocardial infarction: The EURAMIC and heavy metals study. Chemosphere 64: 112-120.
26 Graca, JG; Davison, FC; Feavel, JB. (1964) Comparative toxicity of stable rare earth compounds. III. Acute toxicity
27 of intravenous injections of chlorides and chelates in dogs. Arch Environ Health 89:555-564.
28 Hahn, FF; Muggenberg, BA; Guilmette, RA; et al. (1999) Comparative stochastic effects of inhaled alpha- and beta-
29 particle-emitting radionuclides in beagle dogs. Radiat Res 152:S19-S22
30 Hahn, FF; Muggenburg, BA; Snipes, MB; et al. (2001) The toxicity of insoluble cerium-144 inhaled by beagle dogs:
31 non-neoplastic effects. Radiat Res 155(1):95—112.
32 Haley, PJ. (1991) Pulmonary toxicity of stable and radioactive lanthanides. Health Phys 61(6):809-820.
33 Hedrick, JB. (2004) Rare earths. Minerals yearbook. Vol. I. Metals and minerals. U.S. Geological Survey, U.S.
34 Department of the Interior, Reston, VA. Available online at
3 5 http://minerals.usgs.gov/minerals/pubs/commodity/myb/.
3 6 HEI (Health Effects Institute). (2001) Evaluation of human health risk from cerium added to diesel fuel. HEI
37 communication 9. North Andover, MA; Flagship Press. Available online at http://pubs.healtheffects.org/.
38 Heuck, F; Hoschek, R. (1968) Cer-pneumoconiosis. Am J Roentgenol Radium Ther Nucl Med 104(4):777-783.
May, 2007
70
DRAFT - DO NOT CITE OR QUOTE
-------
1 Hunninghake, GW; Hemken, C; Brady, M; et al. (1984) Mediators released by human lung cells are potent growth
2 factors for human lung fibroblasts. Trans Assoc Am Physicians 97:41-48. (as cited in Haley, 1991).
3 Husain, MH; Dick, JA; Kaplan, YS. (1980) Rare earth pneumoconiosis. J Soc Occup Med 30:15-19.
4 Inaba, J; Lengemann, FW. (1972) Intestinal uptake and whole-body retention of 141Ce by suckling rats. Health Phys
5 22:169-175.
6 Inaba, J; Nishimura, Y; Takeda, H; et al. (1992) Placental transfer of cerium in the rat with special reference to route
7 of administration. RadiatProtDosimet41(2/4):119-122.
8 Ji, YJ; Cui, MZ. (1988) Toxicological studies on safety of rare earths used in agriculture. Biomed Environ Sci 1:
9 270-276.
10 Kanapilly, GM; Luna, RJ. (1975) Deposition and retention of inhaled condensation aerosols of 144Ce02 in Syrian
11 hamsters. In: Boecker, BB; Rupprecht, FC; eds. Annual report of the Inhalation Toxicology Research Institute.
12 Lovelace Foundation for Medical Education and Research, Albuqueque, NM; Report No. LF-52; pp. 79-83.
13 Kappenberger, L; Biihlmann, AA. (1975) [Lung lesions caused by "rare earths"]. Schweiz Med Wochenscher
14 105:1799-1801. (German with English summary).
15 Kargacin, B; Landeka, M. (1990) Effect of glucocorticoids on metal retention in rats. Bull Environ Contam Toxicol
16 45(5):655-661.
17 Kartha, CC; Eapen, JT; Radhakumary, C; et al. (1998) Pattern of cardiac fibrosis in rabbits periodically fed a
18 magnesium-restricted diet and administered rare earth chloride through drinking water. Biol Trace Elem Res
19 63(1): 19—30.
20 Kawagoe, M; Hirasawa, F; Wang, SC; et al. (2005) Orally administered rare earth element cerium induces
21 metallothionein synthesis and increases glutathione in the mouse liver. Life Sci 77:922-937.
22 Kilbourn, BT. (2003) Cerium and cerium compounds. In: Kirk-Othmer encyclopedia of chemical technology. New
23 York, NY: John Wiley and Sons, Inc. Available online with subscription at
24 http://www.mrw.interscience.wiley.com/kirk/kirk_search_fs.html.
25 Kostial, K; Kargacin, B; Landeka, M. (1989a) Gut retention of metals in rats. Biol Trace ElemRes 21:213-218.
26 Kostial, K; Kargacin, B; Blanusa, M; et al. (1989b) Location of mercury, cerium, and cadmium in the gut of suckling
27 and weaned rats. Period Biol 91(3):321-326.
28 Kumar, BP; Shivakumar, K. (1998) Alterations in collagen metabolism and increased fibroproliferation in the heart
29 in cerium-treated rats. Biol Trace ElemRes 63:73-79.
30 Kumar, BP; D'Souza, SL; Shivakumar, K; etal. (1995) Cerium stimulates protein biosynthesis in rat heart in vivo.
31 Biol Trace Elem Res 50(3):237-242.
32 Kumar, BP; Shivakumar, K; Kartha, CC; et al. (1996) Magnesium deficiency and cerium promote fibrogenesis in rat
33 heart. Bull Environ Contam Toxicol 57(4):517-524.
34 Kuruvilla, L; Kartha, CC. (2006) Cerium depresses endocardial endothelial cell-mediated proliferation of cardiac
3 5 fibroblasts. Biol Trace Elem Res 114:85-92.
36 Kutty, VR; Abraham, S; Kartha, CC. (1996) Geographical distribution of endomyocardial fibrosis in South Kerala.
37 Int J Epi 25(6): 1202-1207.
38 Levack, VM; Hone, PA; Phipps, AW; et al. (2002) The placental transfer of cerium: experimental studies and
39 estimates of doses to the human fetus from 141Ce and 144Ce. Int J Radiat Biol 78:227-235.
May, 2007
71
DRAFT - DO NOT CITE OR QUOTE
-------
1 Lewis, RJ; ed. (2001) Hawley's condensed chemical dictionary. New York, NY: John Wiley and Sons, Inc.; pp.
2 229-231.
3 Lide, DR; ed. (2005) CRC handbook of chemistry and physics. 86th edition. Boca Raton, FL: CRC Press, Taylor &
4 Francis; pp. 4-9, 4-47, 4-48.
5 Limbach, LK; Li, Y; Grass, RN; et al. (2005) Oxide nanoparticle uptake in human lung fibroblast: effects of particle
6 size, agglomeration, and diffusion at low concentrations. Environ Sci Technol 39(23):9370-9376.
7 Lin, W; Huang, Y; Zhou, X; et al. (2006) Toxicity of cerium oxide nanoparticles in human lung cancer cells. Int J
8 Toxicol 25:451-457.
9 Lizon, C; Fritsch, P. (1999) Chemical toxicity of some actinides and lanthanides towards alveolar macrophages: an
10 in vitro study. Int J Radiat Biol 75(11): 1459-1471.
11 Lombardi, B; Recknagel, RO. (1962) Interference with secretion of triglycerides by the liver as a common factor in
12 toxic liver injury. AmJPathol40(5):571-586.
13 Lundgren, DL; McClellan, RO; Thomas, RL; et al. (1974) Toxicity of inhaled 144Ce02 in mice. Radiat Res 58:448-
14 461.
15 Lundgren, DL; Hahn, FF; Diel, JH; et al. (1992) Repeated inhalation exposure of rats to aerosols of 144Ce02.1. Lung,
16 liver, and skeletal dosimetry. Radiat Res 132(3):312-24.
17 Lundgren, DL; Hahn, FF; Griffith, WC; et al., (1996) Pulmonary carcinogenicity of relatively low doses of beta-
18 particle radiation from inhaled 144Ce02 in rats. Radiat Res 146(5):525-535.
19 Lustgarten, CS; Boecker, BB; Cuddihy, RG; et al. (1976) Biliary excretion of 144Ce after inhalation of 144Ce citrate in
20 rats and Syrian hamsters. III. Annual report of the Inhalation Toxicology Research Institute. Lovelace Foundation
21 for Medical Education and Research, Albuqueque, NM; pp. 84-87.
22 Magnusson, G. (1962) Acute toxic effects of yttrium, cerium, terbium, holmium and ytterbium on the liver. Acta
23 Pharmacol Toxicol 20(Suppl.):53-74.
24 Manju, L; Remani, K; Nair, RR. (2003) Negative inotropic response to cerium in ventricular papillary muscle is
25 mediated by reactive oxygen species. Biol Trace Elem Res 96(l-3):203-208.
26 Manoubi, L; Hocine, N; Jaafoura, H; et al. (1998) Subcellular localization of cerium in intestinal mucose, liver,
27 kidney, suprarenal and testicle glands, after cerium administration in the rat. J Trace Microprobe Techniques
28 16(2):209-219.
29 Marciniak, M; Baltrukiewicz, Z. (1977) Serum ornithine carbamoyltransferase (OCT) in rats poisoned with
30 lanthanum, cerium and praseodymium. Acta Physiol Pol 28(6):589-594.
31 Marciniak, M; Baltrukiewicz, Z. (1981) Effect of various doses of cerium on the serum level of ornithine-
3 2 carbamyltransferase in rats. Acta Physiol Pol 32(2) :205-211.
33 McDonald, JW; Ghio, AJ; Sheehan, CE; et al. (1995) Rare earth (cerium oxide) pneumoconiosis—analytical
34 scanning electron microscopy and literature review. Mod Pathol 8(8):859-865.
35 Mogilevskaya, OY; Raikhlin, NT. (1967) Rare earth metals. In: Izrael'son, ZI; ed. Toxicology of the rare metals.
36 Jerusalem, Israel: Israel Program for Scientific Translations; pp. 132-141.
37 Monafo, WW; Tandon, SN; Ayvazian, VH; et al. (1976) Cerium nitrate: a new topical antiseptic for extensive burns.
38 Surgery 80(4):465-473.
39 Morgan, BN; Thomas, RG; McClellan, RO. (1970) Influence of chemical state of cerium-144 on its metabolism
40 following inhalation by mice. Am Ind Hyg Assoc J 31(4):479-484.
May, 2007 72 DRAFT - DO NOT CITE OR QUOTE
-------
1 Morganti, JB; Lown, BA; Stineman, CH; et al. (1978) Cerium tissue/organ distribution and alterations in open field
2 and exploratory behavior following repeated exposure of the mouse to citrate complexed cerium. Gen Pharmacol
3 9(4):257-261.
4 Morganti, JB; Lown, BA; Chapin, E; et al. (1980) Effects of acute exposure to cerium (citrate) on selected measures
5 of activity, learning and social behavior of the mouse. Gen Pharmacol 11(4):369-373.
6 Morrow, PE. (1988) Possible mechanisms to explain dust overloading of the lungs. Fundam Appl Toxicol 10:369-
7 384.
8 Morrow, PE. (1992) Dust overloading of the lungs: update and appraisal. Toxicol Appl Pharmacol 113(1): 1-12.
9 Mraz, FR; Eisele, GR. (1977) Gastrointestinal absorption and distribution of 144Ce in the suckling pig. Health Phys
10 33:494-495.
11 Naharin, A; Lubin, E; Feige, Y. (1969) Transfer of 144Ce to mouse embryos and offspring via placenta and lactation.
12 Health Phys 17:717-722.
13 Naharin, A; Feige, Y; Lubin, E; et al. (1974) Internal deposition of ingested cerium in suckling mice. Health Phys
14 27(2):207-211.
15 Nair, R; Preeta, R; Smitha, G; et al. (2003) Variation in mitogenic response of cardiac and pulmonary fibroblasts to
16 cerium. Biol Trace Elem Res 94(3):237-246.
17 Nishioka, H. (1975) Mutagenic activities of metal compounds in bacteria. Mutat Res 31:185-190.
18 Niu et al., 2007, p. 43—could this be Niu, J; Azfer, A; Rogers, LM. (2007) Cardioprotective effects of cerium oxide
19 nanoparticles in a transgenic murine model of cardiomyopathy. Cardiovasc Res 73(3):549-559.
20 NLM (National Library of Medicine) (2006) Cerium (last modified in September, 2004). ChemlDplus. National
21 Institutes of Health, U.S. Department of Health and Human Services, Bethesda, MD. Available online at
22 http://toxnet.nlm.nih.gov.
23 NRC (National Research Council). (1983) Risk assessment in the federal government: managing the process.
24 Washington, DC: National Academy Press.
25 Oberdorster, G. (1995) Lung particle overload: implications for occupational exposures to particles. Reg Toxicol
26 Pharmacol 27:123-135.
27 O'Neil, MJ; ed. (2001) The Merck index: an encyclopedia of chemicals, drugs, and biologicals. 13th edition.
28 Whitehouse Station, NJ: Merck and Co., Inc.; pp. 89, 342-344.
29 Pairon, JC; Roos, F; Iwatsubo, Y; et al. (1994) Lung retention of cerium in humans. Occup EnvironMed 51(3): 195—
30 199.
31 Pairon, JC; Roos, F; Sebastien, P; et al. (1995) Biopersistence of cerium in the human respiratory tract and
32 ultrastructural findings. AmJIndusMed27(3):349-358.
33 Palmer, RJ; Butenhoff, JL; Stevens, JB. (1987) Cytotoxicity of the rare earth metals cerium, lanthanum, and
34 neodymium in vitro: comparisons with cadmium in a pulmonary macrophage primary culture system. Environ Res
35 43(1): 142—156.
36 Pietra, R; Pietra, R; Sabbioni, E; et al. (1985) Trace elements in tissues of a worker affected by rare-earth
37 pneumoconiosis. A study carried out by neutron activation analysis. J Radioanal Nucl Chem 92(2):247-269.
38 Porru, S; Placidi, D; Quarta, C; et al. (2001) The potential role of rare earths in the pathogenesis of interstitial lung
39 disease: a case report of movie projectionist as investigated by neutron activation analysis. J Trace Elem Med Biol
40 14(4):232-236.
May, 2007
73
DRAFT - DO NOT CITE OR QUOTE
-------
1 Preeta, R; Nair, R. (1999) Stimulation of cardiac fibroblast proliferation by cerium: a superoxide anion-mediated
2 response. J Mol Cell Cardiol 31(8): 1573-1580.
3 Reinhardt, K; Winkler, H. (1996) Cerium mischmetal, cerium alloys, and cerium compounds. In: Ullmann's
4 encyclopedia of industrial chemistry. Vol. A6. Weinheim, Germany: Wiley-VCH; pp. 139-152.
5 Reinhardt K; Winkler H. (2002) Cerium mischmetal, cerium alloys, and cerium compounds. In: Ullmann's
6 encyclopedia of industrial chemistry. Vol. 7. Weinheim, Germany: Wiley-VCH, pp. 285-300. Available online with
7 subscription at http://www.mrw.interscience.wiley.com/ueic/ueic_search_fs.html.
8 Rhodia, Inc. (1998) Initial submission: letter from Rhodia, Inc. to US EPA re acute oral toxicity study in rats of
9 lithium sulfide, sodium sulfide, and cerium sulfide, with attachments and dated 11/23/1998. Produced by
10 Huntingdon Life Sciences Ltd., Cambridgeshire, England, for Rhodia Inc., Cranbury, NJ. Submitted under TSCA
11 Section 8E; EPA Doc. No. 88990000042; NTIS No. OTS0559644.
12 Rhone-Poulenc. (1983) Ceric oxide: acute toxicity studies via the respiratory tract in the rat. Rhone-Poulenc French
13 Technology Institute, Courbevoie Ceuex, France; Report 302217. (as cited in HEI, 2001).
14 Sabbioni, E; Pietra, R; Gaglione, P; et al. (1982) Long-term occupational risk of rare-earth pneumoconiosis: a case
15 report as investigated by neutron activation analysis. Sci Total Environ 26:19-32.
16 Salas, M; Tuchweber, B; Kovacs, K; et al. (1976) Effect of cerium on the rat liver: an ultrastructural and biochemical
17 study. Beitr Pathol 157(l):23-44.
18 Salonpaa, P; Iscan, M; Pasanen, M; et al. (1992) Cerium-induced strain-dependent increase in Cyp2a-4/5
19 (cytochrome P4502a-4/5) expression in the liver and kidneys of inbred mice. Biochem Pharmacol 44(7): 1269-1274.
20 Schubert, D; Dargusch, R; Raitano, J; et al. (2006) Cerium and yttrium oxide nanoparticles are neuroprotective.
21 Biochem Biophys Res Commun 342:86-91
22 Schulz, H; Brand, P; Heyder, J. (2000) Particle deposition in the respiratory tract. In: Gehr, P; Heyder, J; eds.
23 Particle-lung interactions. New York, NY: Marcel Dekker; pp. 229-290.
24 Sharma, A; Talukder, G. (1987) Effects of metals on chromosomes of higher organisms. EnvironMutat 9:191-226.
25 Shimizu, H; Suzuki, Y; Takemura, N; et al. (1985) The results of microbial mutation test for forty-three industrial
26 chemicals. Sangyo Igaku (Jpn J Ind Health) 27:400-419.
27 Shiraishi, Y; Ichikawa, R. (1972) Absorption and retention of 144Ce and 95Zr-95Nb in newborn, juvenile and adult
28 rats. Health Phys 22:373-378.
29 Shivakumar, K; Nair, RR. (1991) Cerium depresses protein synthesis in cultured cardiac myocytes and lung
30 fibroblasts. Mol Cell Biochem 100:91-96.
31 Shivakumar, K; Nair, RR; Valiathan, MS. (1992) Paradoxical effect of cerium on collagen synthesis in cardiac
32 fibroblasts. J Mol Cell Cardiol. 24:775-780.
33 Shrivastava, S; Mathur, R. (2004) Effectiveness of ethylene glycol bis (2-aminoethyl ether) tetraacetic acid (EGTA)
34 against cerium toxicity. Indian J Exp Biol 42:876-883.
3 5 Stineman, CH; Massaro, EJ; Lown, BA; et al. (1978) Cerium tissue/organ distribution and alterations in open field
36 and exploratory behavior following acute exposure of the mouse to cerium (citrate). J Environ Pathol Toxicol
37 2(2):553-570.
3 8 Strubelt, O; Siegers, CP; Younes, M. (1980) The influence of silybin on the hepatotoxic and hypoglycemic effects of
39 praseodymium and other lanthanides. Drug Res 30(10): 1690-1694.
May, 2007
74
DRAFT - DO NOT CITE OR QUOTE
-------
1 Sturbaum, B; Brooks, AL; McClellan, RO. (1970) Tissue distribution and dosimetry of 144Ce in Chinese hamsters.
2 Radiat Res 44:359-367.
3 Sulotto, F; Romano, C; Berra, A; et al. (1986) Rare-earth pneumoconiosis: a new case. Am J Ind Med 9(6):567-
4 575.
5 Talbot, RB; Davison, FG; Green, JW; et al. (1965) Effects of subcutaneous implantation of rare earth metals.
6 Produced by the Iowa State University of Science and Technology for the U.S. Atomic Energy Commission;
7 USAEC Report 1170. Available from the National Technical Information Service, Springfield, VA; COOl 1701.
8 Thill, A; Zeyons, O; Spalla, O; et al. (2006) Cytotoxicity of Ce02 nanoparticles for Escherichia coli. Physico-
9 chemical insight of the cytotoxicity mechanism. Environ Sci Technol 40(19):6151-6156.
10 Thomas, RL; Scott, JK; Chiffelle, TL. (1972) Metabolism and toxicity of inhaled 144Ce in rats. Radiat Res 49:589-
11 610.
12 U.S. EPA (Environmental Protection Agency). (1986a) Guidelines for the health risk assessment of chemical
13 mixtures. Federal Register 51(185):34014-34025. Available online at http://www.epa.gov/ncea/raf/rafguid.htm.
14 U.S. EPA (Environmental Protection Agency). (1986b) Guidelines for mutagenicity risk assessment. Federal
15 Register 51(185):34006-34012. Available online at http://www.epa.gov/ncea/raf/rafguid.htm.
16 U.S. EPA (Environmental Protection Agency). (1987) Recommendations for and documentation of biological values
17 for use in risk assessment. Environmental Criteria and Assessment Office, Office of Health and Environmental
18 Assessment, Cincinnati, OH; EPA/600/6-87/008. Available from the National Technical Information Service,
19 Springfield, VA; PB88-179874/AS, and online at http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=34855.
20 U.S. EPA (Environmental Protection Agency). (1991) Guidelines for developmental toxicity risk assessment.
21 Federal Register 56(234):63798-63826. Available online at http://www.epa.gov/ncea/raf/rafguid.htm.
22 U.S. EPA (Environmental Protection Agency). (1994a) Interim policy for particle size and limit concentration issues
23 in inhalation toxicity: notice of availability. Federal Register 59(206):53799. Available online at
24 http://www.epa.gov/EPA-PEST/1994/October/Day-26/pr-l 1.html.
25 U.S. EPA (Environmental Protection Agency). (1994b) Methods for derivation of inhalation reference
26 concentrations and application of inhalation dosimetry. Environmental Criteria and Assessment Office, Office of
27 Health and Environmental Assessment, Cincinnati, OH; EPA/600/8-90/066F. Available from the National Technical
28 Information Service, Springfield, VA, PB2000-500023, and online at
29 http://cfpub.epa.gov/ncea/raf/recordisplay.cfm?deid=71993.
30 U.S. EPA (Environmental Protection Agency). (1995) Use of the benchmark dose approach in health risk
31 assessment. Risk Assessment Forum, Washington, DC; EPA/630/R-94/007. Available from the National Technical
32 Information Service, Springfield, VA, PB95-213765, and online at
33 http://cfpub.epa.gov/ncea/raf/raf_pubtitles.cfm?detype=document&excCol=archive.
34 U.S. EPA (Environmental Protection Agency). (1996) Guidelines for reproductive toxicity risk assessment. Federal
35 Register 61(212):56274-56322. Available online at http://www.epa.gov/ncea/raf/rafguid.htm.
36 U.S. EPA (Environmental Protection Agency). (1996) Proposed guidelines for carcinogen risk assessment. Federal
37 Register 61:17960-18011. Available online at http://www.epa.gov/cancerguidelines.
38 U.S. EPA (Environmental Protection Agency). (1998) Guidelines for neurotoxicity risk assessment. Federal
39 Register 63(93):26926-26954. Available online at http://www.epa.gov/ncea/raf/rafguid.htm.
40 U.S. EPA (Environmental Protection Agency). (2000a) Science policy council handbook: risk characterization.
41 Office of Science Policy, Office of Research and Development, Washington, DC. EPA/100-B-00-002. Available
42 online at http://www.epa.gov/OSA/spc/pdfs/prhandbk.pdf.
May, 2007
75
DRAFT - DO NOT CITE OR QUOTE
-------
1 U.S. EPA (Environmental Protection Agency). (2000b) Benchmark dose technical guidance document [external
2 review draft]. Risk Assessment Forum, Washington, DC; EPA/630/R-00/001. Available online at
3 http://cfpub.epa.gov/ncea/cfm/ nceapublication.cfm?ActType=PublicationTopics&detype=DOCUMENT&subject
4 =BENCHMARK+DOSE&subjtype=TITLE&excCol=Archive.
5 U.S. EPA (Environmental Protection Agency). (2000c) Supplementary guidance for conducting health risk
6 assessment of chemical mixtures. Risk Assessment Forum, Washington, DC; EPA/630/R-00/002. Available online
7 at http://cfpub.epa.gov/ncea/raf/chem_mix.cfm.
8 U.S. EPA (Environmental Protection Agency). (2002) A review of the reference dose concentration and reference
9 concentration processess. Risk Assessment Forum, Washington, DC; EPA/630/P-02/002F. Available online at
10 http://cfpub.epa.gov/ncea/raf/raf_pubtitles.cfm?detype=document&excCol=archive.
11 U.S. EPA (Environmental Protection Agency). (2005a) Guidelines for carcinogen risk assessment. Federal Register
12 70(66):17765-18717. Available online at http://www.epa.gov/cancerguidelines.
13 U.S. EPA (Environmental Protection Agency). (2005b) Supplemental guidance for assessing susceptibility from
14 early-life exposure to carcinogens. Risk Assessment Forum, Washington, DC; EPA/630/R-03/003F. Available
15 online at http://www.epa.gov/cancerguidelines.
16 U.S. EPA (Environmental Protection Agency). (2006a) Science policy council handbook: peer review. 3rd edition.
17 Office of Science Policy, Office of Research and Development, Washington, DC; EPA/100/B-06/002. Available
18 online at http://www.epa.gov/OSA/spc/2peerrev.htm.
19 U.S. EPA. (2006b) A framework for assessing health risk of environmental exposures to children. National Center
20 for Environmental Assessment, Washington, DC; EPA/600/R-05/093F. Available online at
21 http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=158363.
22 Valiathan, MS; Kartha, CC; Eapen, JT; et al. (1989) A geochemical basis for endomyocardial fibrosis. ICMR Centre
23 for Research in Cardiomyopathy, Trivandrum, India. Cardiovasc Res 23(7):647-648.
24 Vocaturo, G; Colombo, F; Zanoni, M; et al. (1983) Human exposure to heavy metals: rare earth pneumoconiosis in
25 occupational workers. Chest 83(5):780-783.
26 Vogt, P; Spycher, MA; Ruttner, JR. (1986) [Pneumoconiosis caused by "rare earths" (cer-pneumoconiosis)].
27 Schweiz Med Wochenscher 116:1303-1308. (German with English summary).
28 Waring, PM; Watling, RJ. (1990) Rare earth deposits in a deceased movie projectionist: a new case of rare earth
29 pneumoconiosis? Med J Aust 153(11/12):726—730.
30 Wells, WH, Jr; Wells, VL. (2001) The lanthanides, rare earth metals. In: Bingham, E; Cohrssen, B; Powell, C, II;
31 eds. Patty's toxicology. 5th edition. New York, NY: John Wiley and Sons, Inc.; pp 423-458.
32 Wiener-Schmuck, M; Lind, I; Polzer, G; et al. (1990) In vivo and in vitro effects of rare earth compounds. J Aerosol
33 Sci 21(1):S505-S508.
34 Witschi, HR; Last, JA. (2001) Toxic responses of the respiratory system. In: Klaassen, CD; ed. Casarett and Doull's
35 toxicology: the basic science of poisons. 6th edition. New York, NY: McGraw-Hill; pp. 515-534.
36 Wulfsberg, G. (2000) Inorganic chemistry. Sausalito, CA: University Science Books; p. 59.
37 Yoon, HK; Moon, HS; Park, SH; et al. (2005) Dendriform pulmonary ossification in patient with rare earth
38 pneumoconiosis. Thorax60(8):701-703.
39 Yu, L; Chen, Z; Wang, Y. (2001) Effect of rare earth element cerium (Ce) on abnormality rate of sperm and
40 testosterone secretion in sera of male mice. Nanjing Nongye Daxue Xuebao 24(l):77-80. (Chinese with English
41 abstract).
May, 2007
76
DRAFT - DO NOT CITE OR QUOTE
-------
1
May, 2007 77 DRAFT - DO NOT CITE OR QUOTE
------- |