United States
Environmental Protection
1=1 m m Agency
EPA/690/R-12/036F
Final
6-12-2012
Provisional Peer-Reviewed Toxicity Values for
Tridymite
(CASRN 15468-32-3)
Superfund Health Risk Technical Support Center
National Center for Environmental Assessment
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, OH 45268

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AUTHORS, CONTRIBUTORS, AND REVIEWERS
CHEMICAL MANAGER
Chris Cubbison, National Center for Environmental Assessment, Cincinnati, OH
DRAFT DOCUMENT PREPARED BY
National Center for Environmental Assessment, Cincinnati, OH
This document was externally peer reviewed under contract to
Eastern Research Group, Inc.
110 Hartwell Avenue
Lexington, MA 02421-3136
Questions regarding the contents of this document may be directed to the U.S. EPA Office of
Research and Development's National Center for Environmental Assessment, Superfund Health
Risk Technical Support Center (513-569-7300).
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TABLE OF CONTENTS
COMMONLY USED ABBREVIATIONS	iii
BACKGROUND	4
DISCLAIMERS	4
QUESTIONS REGARDING PPRTVS	4
INTRODUCTION	5
REVIEW OF POTENTIALLY RELEVANT DATA (CANCER AND NONCANCER)	7
DERIVATION 01 PROVISIONAL VALUES	 14
CANCER WEIGHT-OF-EVIDENCE (WOE) DESCRIPTOR	14
MODE-OF-ACTION (MOA) DISCUSSION	14
REFERENCES	14
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COMMONLY USED ABBREVIATIONS
BMC
benchmark concentration
BMCL
benchmark concentration lower bound 95% confidence interval
BMD
benchmark dose
BMDL
benchmark dose lower bound 95% confidence interval
HEC
human equivalent concentration
HED
human equivalent dose
IUR
inhalation unit risk
LOAEL
lowest-observed-adverse-effect level
LOAELadj
LOAEL adjusted to continuous exposure duration
LOAELhec
LOAEL adjusted for dosimetric differences across species to a human
NOAEL
no-ob served-adverse-effect level
NOAELadj
NOAEL adjusted to continuous exposure duration
NOAELhec
NOAEL adjusted for dosimetric differences across species to a human
NOEL
no-ob served-effect level
OSF
oral slope factor
p-IUR
provisional inhalation unit risk
POD
point of departure
p-OSF
provisional oral slope factor
p-RfC
provisional reference concentration (inhalation)
p-RfD
provisional reference dose (oral)
RfC
reference concentration (inhalation)
RfD
reference dose (oral)
UF
uncertainty factor
UFa
animal-to-human uncertainty factor
UFC
composite uncertainty factor
UFd
incomplete-to-complete database uncertainty factor
UFh
interhuman uncertainty factor
UFl
LOAEL-to-NOAEL uncertainty factor
UFS
subchronic-to-chronic uncertainty factor
WOE
weight of evidence
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PROVISIONAL PEER-REVIEWED TOXICITY VALUES FOR
TRIDYMITE (CASRN 15468-32-3)
BACKGROUND
A Provisional Peer-Reviewed Toxicity Value (PPRTV) is defined as a toxicity value
derived for use in the Superfund Program. PPRTVs are derived after a review of the relevant
scientific literature using established Agency guidance on human health toxicity value
derivations. All PPRTV assessments receive internal review by a standing panel of National
Center for Environment Assessment (NCEA) scientists and an independent external peer review
by three scientific experts.
The purpose of this document is to provide support for the hazard and dose-response
assessment pertaining to chronic and subchronic exposures to substances of concern, to present
the major conclusions reached in the hazard identification and derivation of the PPRTVs, and to
characterize the overall confidence in these conclusions and toxicity values. It is not intended to
be a comprehensive treatise on the chemical or toxicological nature of this substance.
The PPRTV review process provides needed toxicity values in a quick turnaround
timeframe while maintaining scientific quality. PPRTV assessments are updated approximately
on a 5-year cycle for new data or methodologies that might impact the toxicity values or
characterization of potential for adverse human health effects and are revised as appropriate. It is
important to utilize the PPRTV database flittp://hhpprtv.ornl.gov) to obtain the current
information available. When a final Integrated Risk Information System (IRIS) assessment is
made publicly available on the Internet (www.epa.gov/iris). the respective PPRTVs are removed
from the database.
DISCLAIMERS
The PPRTV document provides toxicity values and information about the adverse effects
of the chemical and the evidence on which the value is based, including the strengths and
limitations of the data. All users are advised to review the information provided in this
document to ensure that the PPRTV used is appropriate for the types of exposures and
circumstances at the site in question and the risk management decision that would be supported
by the risk assessment.
Other U.S. Environmental Protection Agency (EPA) programs or external parties who
may choose to use PPRTVs are advised that Superfund resources will not generally be used to
respond to challenges, if any, of PPRTVs used in a context outside of the Superfund program.
QUESTIONS REGARDING PPRTVS
Questions regarding the contents and appropriate use of this PPRTV assessment should
be directed to the EPA Office of Research and Development's National Center for
Environmental Assessment, Superfund Health Risk Technical Support Center (513-569-7300).
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INTRODUCTION
Tridymite, CASRN 15468-32-3, is a crystalline form of silica (Si02). Silica occurs as
crystalline, amorphous, microcrystalline, and cryptocrystalline mineral forms that differ in terms
of their structural and physicochemical properties (NIOSH, 1974; IARC, 1997); however, the
most common forms of silica are cristobalite (CASRN 14464-46-1) and quartz
(CASRN 14808-60-7). During industrial and naturally occurring processes, tridymite (usually in
the form of quartz) is heated to create silica bricks or diatomaceous earth, respectively (NIOSH,
1974; IARC, 1997). Tridymite is also used in insulation, filtering media, and as a siliceous
refractory material to line furnaces (NIOSH, 1974). Tridymite is naturally present in volcanic
rocks and soils, which can result in exposure during mineral extraction (NIOSH, 1974; IARC,
1997). Table 1 summarizes the selected physicochemical properties for tridymite.
Table 1. Physicochemical Properties of Tridymite (CASRN 15468-32-3)a
Property (unit)
Value
Boiling point (F)
4046
Melting point (F)
3110
Density (g/cm3)
2.66
Vapor pressure (mmHg at 25C)
0
Solubility in water (g/100 mL at 25C)
ND
Relative vapor density (air =1)
ND
Molecular weight (g/mol)
60.1
aOSHA, 2011a.
ND = no data.
No reference dose (RfD), reference concentration (RfC), or cancer assessment for
tridymite is included in the U.S. Environmental Protection Agency (U.S. EPA) Integrated Risk
Information System (IRIS) (U.S. EPA, 201 la) or on the Drinking Water Standards and Health
Advisories List (U.S. EPA, 201 lb). No RfD or RfC values are reported in the Health Effects
Assessment Summary Tables (HEAST) (U.S. EPA, 2003). The Chemical Assessments and
Related Activities (CARA) list does not include a Health and Environmental Effects Profile
(HEEP) for tridymite (U.S. EPA, 1994). The toxicity of tridymite has not been reviewed by the
Agency for Toxic Substances and Disease Registry (ATSDR, 2012). The World Health
Organization (WHO, 2000) published a Concise International Chemical Assessment Document
(CICAD) on crystalline silica that considered quartz but did not consider experimental studies on
the effects of tridymite. The California Environmental Protection Agency (CalEPA, 2008, 2012)
set a chronic inhalation reference exposure limit (REL) of 3 (J,g/m3 for respirable crystalline silica
based on its effects on the respiratory system. However, CalEPA (2008, 2012) did not specify
the forms of crystalline silica that are regulated or provide any CASRNs. The National Institute
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for Occupational Safety and Health (NIOSH, 2010) set a time-weighted average (TWA) REL of
0.05 mg/m3 for crystalline silica in the form of respirable dust, which it considers a potential
carcinogen. For this REL, NIOSH (2010) listed cristobalite, quartz, and tridymite as synonyms
and trade names and provided the general CASRN for silicon dioxide (CASRN 14808-60-7).
NIOSH (2010) also set levels considered to be immediately dangerous to life or health (IDLH) as
25 mg/m3 for tridymite. These limits were based upon effects to the eyes and respiratory system,
including eye irritation, cough, dyspnea (difficulty breathing), wheezing, decreased pulmonary
function, progressive respiratory symptoms (silicosis), and lung cancer (NIOSH, 2010). Based
on the results of epidemiological and animal studies, NIOSH (1974) recommended setting
exposure level standards for tridymite and cristobalite at one-half of those recommended for
quartz. The Occupational Safety and Health Administration (OSHA, 2006) derived two
equations for the calculation of two 8-hour TWA permissible exposure limits (PELs) for
"3
crystalline silica in the form of respirable quartz (10 mg/m ^ [% Si02 + 2]) and quartz in the
form of dust (30 mg/m3 ^ [% Si02 + 2]). The 8-hour TWA PEL for tridymite was defined as
one-half of the value calculated for quartz using the same equations (OSHA, 2006, 201 la). No
occupational exposure limit for tridymite is current or recommended by the American
Conference of Governmental Industrial Hygienists (ACGIH, 2011).
The HEAST (U.S. EPA, 2003) does not include a U.S. EPA (1986) cancer
weight-of-evidence classification or an oral slope factor for tridymite. The International Agency
for Research on Cancer (IARC, 1997) reviewed the carcinogenic potential of tridymite and
determined that there is limited evidence from animal experiments to determine the
carcinogenicity of tridymite. IARC (1997) evaluated all forms of silica for their
carcinogenicities and found that there was sufficient evidence to determine the carcinogenicity of
inhaled quartz or cristobalite that can occur due to exposure to occupational sources of silica.
Likewise, in animals, only quartz and cristobalite have been found to have sufficient evidence of
carcinogenicity (IARC, 1997). The 12th Report on Carcinogens (NTP, 2011) classifies
respirable-size crystalline silica as a known human carcinogen based on sufficient evidence of
carcinogenicity in humans. No CASRNs were identified in this report, so it is unclear whether
this determination is applicable to tridymite (NTP, 2011). Worker exposure to respirable
crystalline silica is associated with elevated rates of lung cancer and silicosis (NTP, 2011). In
rats, intratracheal instillation and inhalation exposure to respirable crystalline silica consistently
causes lung cancer adenocarcinoma and squamous cell carcinoma, and single intrapleural or
intraperitoneal injections of respirable crystalline silica cause lymphoma (NTP, 2011).
CalEPA (2012) has not prepared a quantitative estimate of the carcinogenic potential of
tridymite. In summary, while there are several toxicity values available for various forms of
crystalline silica, toxicity values specific for tridymite are quite limited.
Literature searches were conducted on sources published from January 1, 1900 through
February 12, 2012 for studies relevant to the derivation of provisional toxicity values for
tridymite, CASRN 15468-32-3. Searches were conducted using U.S. EPA's Health and
Environmental Research Online (HERO) database of scientific literature. HERO searches the
following databases: AGRICOLA; American Chemical Society; BioOne; Cochrane Library;
DOE: Energy Information Administration, Information Bridge, and Energy Citations Database;
EBSCO: Academic Search Complete; GeoRef Preview; GPO: Government Printing Office;
Informaworld; IngentaConnect; J-STAGE: Japan Science & Technology; JSTOR: Mathematics
& Statistics and Life Sciences; NSCEP/NEPIS (EPA publications available through the National
Service Center for Environmental Publications [NSCEP] and National Environmental
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Publications Internet Site [NEPIS] database); PubMed: MEDLINE and CANCERLIT databases;
SAGE; Science Direct; Scirus; Scitopia; SpringerLink; TOXNET (Toxicology Data Network):
ANEUPL, CCRIS, ChemlDplus, CIS, CRISP, DART, EMIC, EPIDEM, ETICBACK, FEDRIP,
GENE-TOX, HAPAB, HEEP, HMTC, HSDB, IRIS, ITER, LactMed, Multi-Database Search,
NIOSH, NTIS, PESTAB, PPBIB, RISKLINE, TRI; and TSCATS; Virtual Health Library; Web
of Science (searches Current Content database among others); World Health Organization; and
Worldwide Science. The following databases outside of HERO were searched for toxicity
values: ACGM, AT SDR, CalEPA, U.S. EPA IRIS, U.S. EPA HEAST, U.S. EPA HEEP,
U.S. EPA OW, U.S. EPA TSCATS/TSCATS2, NIOSH, NTP, OSHA, and RTECS.
REVIEW OF POTENTIALLY RELEVANT DATA
(CANCER AND NONCANCER)
Table 2 provides an overview of the relevant databases for information on tridymite and
includes potentially relevant repeated short-term-, subchronic-, and chronic-duration studies.
The literature search revealed no human or animal studies (i.e., acute-, short-term-, or
chronic-duration) sufficient for development of toxicity values for tridymite. Table 3 provides a
summary of available toxicity values for tridymite and other forms of crystalline silica. Table 4
provides a summary of other available studies on tridymite. None of the studies summarized
herein provide information that is sufficient to derive provisional toxicity values for oral or
inhalation exposure to tridymite.
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Table 2. Summary of Potentially Relevant Data for Tridymite (CASRN 15468-32-3)
Category
Number of
Male/Female, Strain,
Species, Study Type,
Study Duration
Dosimetry"
Critical Effects
NOAEL
BMDL/
BMCL
LOAEL
Reference
Comments
Human
1. Oral
Acuteb
ND
Short-term0
ND
Long-termd
ND
Chronic6
ND
2. Inhalation
Acuteb
ND
Short-term0
ND
ND
6 reported cases of
silicosis
ND
NC
ND
Beskow,
1978
Workers exposed to
kieselguhr (contains quartz,
cristobalite, and tridymite);
unknown exposure duration;
information only available
from the abstract.
Long-termd
ND
ND
Silicosis and dyspnea
ND
NC
ND
Hansen, 1983
Workers exposed to dust
containing silicic acid in
unacceptable concentrations
that was mixed with quartz,
cristobalite, and tridymite;
information only available
from the abstract.
Chronic0
ND
ND
Stomach and lung
cancer
ND
NC
ND
Greenberg,
1986
Worker exposed to mixed
dusts presumed to contain
quartz, cristobalite, and
tridymite; information only
available from abstract.
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Table 2. Summary of Potentially Relevant Data for Tridymite (CASRN 15468-32-3)
Category
Number of
Male/Female, Strain,
Species, Study Type,
Study Duration
Dosimetry"
Critical Effects
NOAEL
BMDL/
BMCL
LOAEL
Reference
Comments
Animal
1. Oral
Subchronic
ND
Chronic
ND
Developmental
ND
Reproductive
ND
Carcinogenic
ND
2. Inhalation
Acute
30/0 Worcester white
rat strain; single
intrapulmonary
injection; observation
until natural death (up
to 365 d)
200 mg/kg
High mortality (20/30)
in first 120 d. Severe
(Grade 5) lung fibrosis
in the survivors.



King et al.,
1953
Tridymite demonstrated
greater mortality and fibrotic
potency than other forms of
silica.
70 (sex not specified),
Wistar, rat,
intratracheal
instillation of
tridymite, observed
24 wk
50 mg
Cellular reaction to
formation of
argyrophil fibers.
ND
NC
ND
Kosztolanyi
et al., 1972
Raw data not provided;
histological methods unclear;
general poor reporting.
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Table 2. Summary of Potentially Relevant Data for Tridymite (CASRN 15468-32-3)
Category
Number of
Male/Female, Strain,
Species, Study Type,
Study Duration
Dosimetry3
Critical Effects
NOAEL
BMDL/
BMCL
LOAEL
Reference
Comments

10-15/0, S-D, rat,
intratracheal
instillation of
hydrofluoric
acid-etched tridymite,
observed 3, 6, and
12 mo
50 mg
3 mo: small interstitial
macrophagic,
nonconfluent
granulomas; dust
particles in alveoli
6 and 12 mo: increased
volume of ganulomas;
moderate central
fibrosis; dust in
alveolar spaces.
ND
NC
ND
Chiappino
and Vigliani,
1982
Animals kept in specific
pathogen free conditions
were compared with those
exposed to bacteria.
Bacterial exposure increased
silicosis development.
Subchronic
ND
Chronic
ND
Developmental
ND
Reproductive
ND
Carcinogenic
16/16 WISTAR-
derived rats Alderly
Park strain; single
intrapulmonary
injection; observation
until natural death
20 mg
tridymite/rat;
body weights
not given
16 malignant
lymphomas of the
histiocytic type
(MLHT) counted in 11
of 32 treated rats vs
0 tumors in
16/16 saline control
rats. Distribution of
tumors among males
and females was not
reported.



Wagner et al.,
1980
Tridymite form of silica was
more potent than other forms
tested.
20/20 AGUS AgB1 rat
strain; single intra
injection; observation
until natural death
20 mg
tridymite/rat;
body weights
not given
2 MLHT/40 rats vs
0 MLHT in 8/4 saline
controls.



Wagner et al.,
1980

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Table 2. Summary of Potentially Relevant Data for Tridymite (CASRN 15468-32-3)
Category
Number of
Male/Female, Strain,
Species, Study Type,
Study Duration
Dosimetry3
Critical Effects
NOAEL
BMDL/
BMCL
LOAEL
Reference
Comments

12/12 PVG rat strain:
single intrapulmonary
injection; observation
until natural death
20 mg
tridymite/rat;
body weights
not given
2 MLHT/24 rats vs
0 MLHT in 8/4 saline
controls.



Wagner et al.,
1980

aThese values have not been converted to adjusted daily dose (ADD in mg/kg-d), human equivalent dose (HED in mg/kg-d), or human equivalent concentration (HEC in
mg/m3) units.
bAcute = exposure for <24 hr (U.S. EPA, 2002).
Short-term = repeated exposure for >24 hr but <30 d (U.S. EPA, 2002).
dLong-term = repeated exposure for >30 d but <10% of the total lifespan (based on 70-yr typical lifespan) (U.S. EPA, 2002).
"Chronic = repeated exposure for >10% lifespan (U.S. EPA, 2002).
NC = not calculated; ND = no data; S-D = Sprague-Dawley.
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Table 3. Available Toxicity Values for Tridymite (CASRN 15468-32-3) and
Crystalline Silica (various CASRNs)
Site
RfV (date published)
Notes
Date
Accessed
Tridymite
Crystalline Silica,
Various Forms
ACGIH; 8-hr
TWA, cancer
classification
Withdrawn due
to insufficient
data (2008)
0.025 mg/m3 (2008); suspected
human carcinogen (2008)
Threshold limit values for
a-quartz (14808-60-7,
1317-95-9) and cristobalite
(14464-46-1).
9/22/11
ATSDR
NV
NV
None
9/22/11
CalEPA; chronic
inhalation REL,
Proposition 65
NV
3.0 |ig/m3 (2008); evidence of
carcinogenicity (2012)
No CASRN provided; REL
based on effects to the
human respiratory system.
9/22/11
IARC
Limited
evidence of
toxicity in
animals (1997)
Sufficient evidence of
carcinogenicity in humans and
animals (1997)
"Crystalline silica inhaled in
the form of quartz or
cristobalite from
occupational sources is
carcinogenic to humans."
9/22/11
NIOSH; 10-hr
TWA, IDLH
NV; 25 mg/m3
(2010)
0.05 mg/m3 (2010); considered to
be a potential occupational
carcinogen (2010)
CASRN 14808-60-7 lists
cristobalite, quartz, and
tridymite as synonyms and
trade names.
9/22/11
NTP
NV
Known to be a human carcinogen
based on sufficient evidence of
carcinogenicity in humans (2011)
No CASRN assigned.
9/22/11
OSHA; 8-hr TWA
Use half the
value calculated
from the
formula for
quartz (2006,
2011a)
Respirable quartz is either
250 mppcf divided by the value
"%Si02 + 5" or 10 mg/m3 divided
by the value "%Si02 + 2"; total
quartz is 30 mg/m3 divided by the
value "%Si02 + 2" (2011b)
None
9/22/11
U.S. EPA; HEEP
NV
Not available
None
9/22/11
U.S. EPA; CARA
NV
NV
None
9/22/11
U.S. EPA; HEAST
NV
NV
None
9/22/11
U.S. EPA;
carcinogen
NV
NV
None
9/22/11
U.S. EPA;
drinking water
NV
NV
None
9/22/11
U.S. EPA; IRIS
NV
NV
U.S. EPA conducted a
noncancer health assessment
of crystalline and
amorphous silica
(U.S. EPA, 1996).
9/22/11
WHO
NV
Concise International Chemical
Assessment (CICAD) of crystalline
silica and quartz (2000)
CICAD did not review
studies on the other forms of
silica.
9/22/11
NV = not available.
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Table 4. Other Studies
Category
Number of Male/Female, Strain,
Species, Study Type, Study
Duration
Dosimetry
Effects
Comments
References
Sister Chromatid
Exchange (SCE)
Human lymphocytes cultured in
vitro with and without monocytes
0, 0.5, 5.0, 50 ng
tridymite/cm2 of
culture dish
Slightly but statistically significantly
elevated SCEs at 50 |ig tridymite/cm2
only in the presence of monocyte
coculture
No effect of tridymite on pure
lymphocyte cultures.
Pairon et al.
(1990)
DNA strand
breakage
In vitro incubation of tridymite and
other forms of silica with X Hindlll
digest DNA or PM2 supercoiled
DNA
Silica samples
standardized by
surface area of
particles at
0.03 m2/mL
Human DNA strand breaks; Thymine
glycol production; Hydroxyl radical
generation
Most results were not quantified.
Relative potency of tridymite is
midrange for most endpoints
compared with other forms of silica.
Daniel et al.
(1995)
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DERIVATION OF PROVISIONAL VALUES
Limitations in the available data preclude development of either cancer and noncancer
toxicity values for tridymite.
CANCER WEIGHT-OF-EVIDENCE (WOE) DESCRIPTOR
Limitations in the available data preclude development of a WOE descriptor for
tridymite.
MODE-OF-ACTION (MOA) DISCUSSION
Limitations in the available data preclude determination of a MOA discussion.
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values for chemical substances and physical agents and biological exposure indices. ACGIH,
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ACGIH (American Conference of Governmental Industrial Hygienists). (2011) 2011 TLVs and
BEIs: Based on the documentation of the threshold limit values for chemical substances and
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ATSDR (Agency for Toxic Substances and Disease Registry). (2012) Toxicological profile
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Beskow, R. (1978) Silicosis in diatomaceous earth factory workers in Sweden. Scand JRespir
Dis 59(4):216221.
CalEPA (California Environmental Protection Agency). (2008) All OEHHA acute, 8-hour and
chronic reference exposure levels (chRELs) as of December 18, 2008. Office of Environmental
Health Hazard Assessment, Sacramento, CA. Search all RELs. Available online at
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factors in the development of silicosis. Br JIndMed 39(3):253-258. 040528.
Daniel, LM; Mao, Y; Tao-chin, L; et al. (1995) DNA strand breakage, thymine glycol
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NTP (National Toxicology Program). (2011) Report on carcinogens, twelfth edition.
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