#1.,	United States
kS^laMJIjk Environmental Protection
^^iniiil mmAgency
EPA/690/R-09/003F
Final
9-29-2009
Provisional Peer-Reviewed Toxicity Values for
Benzene
(CASRN 71-43-2)
Derivation of a Subchronic Oral Provisional-RfD
and a Subchronic Inhalation Provisional-RfC
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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COMMONLY USED ABBREVIATIONS
BMD
Benchmark Dose
IRIS
Integrated Risk Information System
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
p-OSF
provisional oral slope factor
p-RfC
provisional inhalation reference concentration
p-RfD
provisional oral reference dose
RfC
inhalation reference concentration
RfD
oral reference dose
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
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PROVISIONAL PEER-REVIEWED TOXICITY VALUES
FOR BENZENE (CASRN 71-43-2)
DERIVATION OF A SUBCHRONIC ORAL PROVISIONAL-RfD
AND A SUBCHRONIC INHALATION PROVISIONAL-RfC
Background
On December 5, 2003, the U.S. Environmental Protection Agency's (U.S. EPA) Office of
Superfund Remediation and Technology Innovation (OSRTI) revised its hierarchy of human
health toxicity values for Superfund risk assessments, establishing the following three tiers as the
new hierarchy:
1.	U.S. EPA's Integrated Risk Information System (IRIS).
2.	Provisional Peer-Reviewed Toxicity Values (PPRTVs) used in U.S. EPA's Superfund
Program.
3.	Other (peer-reviewed) toxicity values, including:
~	Minimal Risk Levels produced by the Agency for Toxic Substances and Disease
Registry (ATSDR),
~	California Environmental Protection Agency (CalEPA) values, and
~	EPA Health Effects Assessment Summary Table (HEAST) values.
A PPRTV is defined as a toxicity value derived for use in the Superfund Program when
such a value is not available in U.S. EPA's IRIS. PPRTVs are developed according to a Standard
Operating Procedure (SOP) and are derived after a review of the relevant scientific literature
using the same methods, sources of data, and Agency guidance for value derivation generally
used by the U.S. EPA IRIS Program. All provisional toxicity values receive internal review by
two U.S. EPA scientists and external peer review by three independently selected scientific
experts. PPRTVs differ from IRIS values in that PPRTVs do not receive the multiprogram
consensus review provided for IRIS values. This is because IRIS values are generally intended
to be used in all U.S. EPA programs, while PPRTVs are developed specifically for the Superfund
Program.
Because new information becomes available and scientific methods improve over time,
PPRTVs are reviewed on a 5-year basis and updated into the active database. Once an IRIS
value for a specific chemical becomes available for Agency review, the analogous PPRTV for
that same chemical is retired. It should also be noted that some PPRTV documents conclude that
a PPRTV cannot be derived based on inadequate data.
Disclaimers
Users of this document should first check to see if any IRIS values exist for the chemical
of concern before proceeding to use a PPRTV. If no IRIS value is available, staff in the regional
Superfund and Resource Conservation and Recovery Act (RCRA) program offices are advised to
carefully review the information provided in this document to ensure that the PPRTVs used are
appropriate for the types of exposures and circumstances at the Superfund site or RCRA facility
in question. PPRTVs are periodically updated; therefore, users should ensure that the values
contained in the PPRTV are current at the time of use.
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It is important to remember that a provisional value alone tells very little about the
adverse effects of a chemical or the quality of evidence on which the value is based. Therefore,
users are strongly encouraged to read the entire PPRTV document and understand the strengths
and limitations of the derived provisional values. PPRTVs are developed by the U.S. EPA
Office of Research and Development's National Center for Environmental Assessment,
Superfund Health Risk Technical Support Center for OSRTI. Other U.S. EPA programs or
external parties who may choose of their own initiative to use these PPRTVs are advised that
Superfund resources will not generally be used to respond to challenges of PPRTVs used in a
context outside of the Superfund Program.
Questions Regarding PPRTVs
Questions regarding the contents of the PPRTVs and their appropriate use (e.g., on
chemicals not covered, or whether chemicals have pending IRIS toxicity values) 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), or OSRTI.
INTRODUCTION
A streamlined approach was used to derive the provisional subchronic RfD and RfC
values for benzene. Benzene has a chronic RfD, a chronic RfC, and a cancer assessment on
IRIS; only derivation of the subchronic provisional toxicity values is presented. Benzene was
recently reassessed by the IRIS program and a Toxicological Review (U.S. EPA, 2002) is
available. In addition, the Agency for Toxic Substances and Disease Registry (ATSDR)
Toxicological Profile for benzene has been updated recently (ATSDR, 2007). Both the IRIS
Toxicological Review and the ATSDR Toxicological Profile contain comprehensive overviews
of the toxicology and toxicokinetics available for benzene. Given the availability of recent IRIS
and ATSDR reviews, these reports have been used in lieu of literature searches to identify the
critical studies and endpoints for use in deriving the subchronic values.
The derivation of provisional subchronic toxicity values for benzene is discussed below.
Review of the data supporting the chronic toxicity values for benzene on IRIS (U.S. EPA, 2003)
indicated that subchronic data were used to derive the chronic values and, thus, are appropriate to
serve as the basis for the corresponding subchronic toxicity values. A brief rationale is provided
for the selection of the critical study and endpoint; a summary of the critical study is presented,
and the subchronic toxicity value derivations are described. Further information on the
toxicology and toxicokinetics of benzene is provided in Appendix A, Pertinent Sections from
IRIS Summary for Benzene: Chronic Health Hazard Assessments for Noncarcinogenic Effects,
the IRIS Toxicological Review for Benzene (U.S. EPA, 2002), or the ATSDR (2007)
Toxicological Profile for Benzene.
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REVIEW OF PERTINENT DATA AND DERIVATION OF PROVISIONAL
SUBCHRONIC TOXICITY VALUES FOR BENZENE
"3
The chronic RfC (0.03 mg/m ) and the chronic RfD (0.004 mg/kg-day) for benzene on
IRIS (U.S. EPA, 2002) were both based on hematological effects in humans exposed via
inhalation in an occupational setting (Rothman et al., 1996). The mean duration of exposure in
this study was 6.3 years (range 0.7-16 years), and a subchronic-to-chronic UF of 3 was applied
in the derivation of both the RfC and RfD. The ATSDR intermediate-duration inhalation
"3
Minimal Risk Level (MRL) (0.006 ppm or 0.02 mg/m ) was derived in August 2007 and is based
on immunotoxicity in a 28-day study in mice (i.e., Rosenthal and Snyder, 1987). ATSDR (2007)
reviewed two human occupational studies (inhalation exposure) of subchronic duration that were
published after the IRIS (U.S. EPA, 2002) Toxicological Review: Lan et al. (2004) and
Qu et al. (2002, 2003). Mean exposure durations in these studies were 6.1 and 4.5-9.7 years,
respectively. Both studies identified statistically significant (p < 0.05) hematological effects at
concentrations lower than the benchmark concentration lower bound BMCLisd (8.2 mg/m3)
derived from the study by Rothman et al. (1996), which was used as the point of departure
(POD) for deriving the chronic RfC on IRIS (U.S. EPA, 2002). The LOAEL identified by
Lan et al. (2004) was 1.82 mg/m3 and the LOAEL identified by Qu et al. (2002, 2003) was
7.22 mg/m3. ATSDR (2007) applied BMD modeling to the data reported by Lan et al. (2004) to
"3
derive the chronic MRL for benzene (0.003 ppm or 0.01 mg/m ). There is no
intermediate-duration oral MRL for benzene, however, ATSDR (2007) derived a chronic oral
MRL for benzene (0.0005 mg/kg-day) based on route-to-route extrapolation from the POD used
to derive the chronic-duration inhalation MRL.
Given that the chronic RfC and RfD were derived recently and are based on a subchronic
study, the subchronic p-RfC and p-RfD are based on the same critical study (i.e., Rothman et al.,
1996), endpoint (hematological effects) and POD (BMCL of 8.2 mg/m3; BMDL of
1.2 mg/kg-day) as the chronic values, without the UFS factor (subchronic to chronic
extrapolation).
A summary of the critical study is excerpted from the U.S. EPA (2008) IRIS record for
benzene and reproduced below:
Rothman et al. (1996) conducted a cross-sectional study of 44 workers exposed to
benzene and 44 age- and gender-matched unexposed controls. Of the 44 subjects in the
exposed and control groups, 21 were female. Mean (standard deviation) years of
occupational exposure to benzene were 6.3 (4.4), with a range of 0.7-16years. Benzene
exposure was monitored by organic vapor passive dosimetry badges worn by each
worker for a full work shift on 5 days within a 1-2 week period prior to collection of
blood samples. The median 8-hour time-weighted average (TWA) benzene exposure
concentration for all exposed workers was 31 ppm (99 mg/m ). The exposed group was
subdivided into two equal groups of 22 subjects: those exposed to greater than the
median concentration and those exposed to less than the median concentration. The
median 8-hour TWA exposure concentration was 13.6 ppm (43.4 mg/m3) for the
low-exposure group and 91.9 ppm (294 mg/m ) for the high-exposure group.
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There were six hematological measurements that were evaluated: total white blood cell
(WBC) count, absolute lymphocyte count (ALC), hematocrit, red blood cell (RBC) count,
platelet count and mean corpuscular volume (MCV) (Rothman et al., 1996). All six
parameters were significantly different in the high benzene-exposure group (>31 ppm)
when compared to controls. ALC, WBC count, RBC count, hematocrit and platelets were
all significantly decreased and MCV was significantly increased. ALC was the most
3	3
sensitive endpoint; it was reducedfrom 1.9 x 10 uL blood in controls to 1.6 x 10 uL
(p < 0.01) in the <31 ppm group and to 1.3 x I 03 uL (p < 0.001) in the group exposed to
>31 ppm benzene. The ALC was also significantly reduced (1.6 x 10 /p,L; p 0.03) in a
subgroup of 11 workers exposed to a median 8-hour TWA of 7.6ppm (24 mg/m3)
benzene.
As noted above, subchronic toxicity values for benzene are based on the same critical
study, endpoint, and POD as the corresponding IRIS chronic toxicity values.
Derivation of a Subchronic p-RfD for Benzene
For the chronic RfD, U.S. EPA (2002) utilized route-to-route extrapolation from the POD
used to derive the chronic RfC. A BMCLisd (the lower confidence limit on the benchmark
concentration associated with a benchmark response of 1 standard deviation [SD] from the
control mean response) of 8.2 mg/m3 was estimated from modeling the data on lymphocyte
count in humans exposed via inhalation (Rothman et al., 1996). The BMCLisd was converted to
an equivalent oral dose of 1.2 mg/kg-day, which was then used as the POD for derivation of the
chronic RfD. The text below, excerpted from the IRIS record for benzene, briefly describes the
extrapolation procedure. Further details on the BMD modeling and the route-to-route
extrapolation are available in the IRIS Summary (see Appendix A) and in the Toxicological
Review (U.S. EPA, 2002).
In the support document for the benzene cancer assessment on IRIS (U.S. EPA,
1999), EPA provided a simple methodfor extrapolation of benzene-induced
cancer risk from the inhalation to the oral route. The same method is applied here
for noncancer (hematopoietic) effects. The method is based on the relative
efficiency of benzene absorption across routes of exposure, especially pulmonary
and gastrointestinal barriers. An inhalation absorption rate of 50% and an oral
absorption rate of 100% were used to calculate the absorbed benzene dose. These
values are based on human inhalation absorption studies and the study by
Sabourin et al. (1987) that compared inhalation and oral absorption in rats and
mice. The authors found that during a 6-hour inhalation exposure, the retention of
[14C]benzene decreasedfrom 33 ± 6% to 15 ± 9% for rats andfrom 50 ± 1% to
10 ±2% for mice as exposure concentration increased from 26 to 2,600 mg/m3
(10 to 1,000 ppm). In the same study, gastrointestinal absorption of benzene
administered by gavage was >97% for doses between 0.5 and 150 mg/kg body
weight. At oral doses below 15 mg/kg, >90% of the 14 C excreted was in the urine
as non-ethyl acetate-extractable material. At higher doses, an increasing
percentage of the orally administered benzene was exhaled unmetabolized. Thus,
in the dose range represented by the BMCL from the study by Rothman et al.
(1996), absorption of a comparable oral dose was assumed to be 100%. See also
U.S. EPA (1999) for more details about the route-to-route extrapolation of
benzene inhalation results to oral exposures. To calculate an equivalent oral dose
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rate, the BMCLadj is multiplied by the default inhalation rate, multiplied by 0.5 to
correct for the higher oral absorption, and divided by the standard default human
3	3
body weight of 70 kg: 8.2 mg/m x 20 m /day x 0.5 '¦ 70 kg 1.2 mg/kg/day.
To derive the chronic RfD (0.004 mg/kg-day), the extrapolated POD (1.2 mg/kg-day)
was divided by a composite UF of 300, which includes a 10-fold UF for intraspecies variation, a
3-fold UF for use of an adverse effect level, a 3-fold UF for extrapolation from
subchronic-to-chronic duration, and a 3-fold UF for database deficiencies—especially the lack of
multigeneration reproductive toxicity and developmental toxicity studies. Appendix A contains
additional details on the UF selections.
For the derivation of a subchronic p-RfD, the BMDL-equivalent value of 1.2 mg/kg-day
was divided by a composite UF of 100, including a 10-fold UF for intraspecies variation, a 3-fold
UF for use of an adverse effect level, and a 3-fold UF for database deficiencies. This composite
UF differs from the chronic composite UF by a factor of 3 because there is no extrapolation to
chronic exposure. Derivation of the subchronic p-RfD is show below.
Subchronic p-RfD = POD - UF
= 1.2-100
= 0.01 or 1 x 10"2 mg/kg-day
Confidence in the subchronic p-RfD is medium, which is identical with the IRIS chronic
RfD. As discussed further in the IRIS Summary and Toxicological Review for benzene, the
principal study of Rothman et al. (1996) is well conducted, and a dose-response relationship was
established between ALC and benzene air concentration and benzene urine metabolites. In
addition, the RfD obtained from route-to-route extrapolation of the BMD modeling results from
the Rothman et al. (1996) study is in good agreement with effect levels identified in male rats
based on ALC data from the NTP (1986) chronic rodent gavage study. Further, while
route-to-route extrapolation was used to estimate a POD, this extrapolation introduces less
uncertainty than extrapolating from test animals to humans (U.S. EPA, 1999).
Derivation of a Subchronic p-RfC for Benzene
For the chronic RfC, U.S. EPA (2002) estimated a BMCLisd of 8.2 mg/m3 from BMD
modeling of the data on lymphocyte count in humans exposed via inhalation (Rothman et al.,
1996). The BMCLisd was used as the POD for derivation of the chronic RfC. Further detail on
the BMD modeling is available in the IRIS Summary (see Appendix A) and in the Toxicological
Review (U.S. EPA, 2002). A composite UF of 300 was applied to the BMCLisd to derive the
chronic RfC. The composite UF includes a 10-fold UF for intraspecies variation, a 3-fold UF for
use of an adverse effect level, a 3-fold UF for extrapolation from subchronic-to-chronic duration,
and a 3-fold UF for database deficiencies—especially the lack of multigeneration reproductive
toxicity and developmental toxicity studies. Appendix A contains additional details on the UF
selections.
"3
For the derivation of a subchronic p-RfC, the BMCLisd of 8.2 mg/m was divided by a
composite UF of 100 that includes a 10-fold UF for intraspecies variation, a 3-fold UF for use of
an adverse effect level, and a 3-fold UF for database deficiencies. This composite UF differs
from the chronic composite UF by a factor of 3 because there is no extrapolation to chronic
exposure. Derivation of the subchronic p-RfC is shown below.
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Subchronic p-RfC = POD - UF
= 8.2-100
= 0.08 or 8 x 10"2 mg/m3
Confidence in the subchronic p-RfC is medium, which identical with the IRIS chronic
RfC. As discussed further in the IRIS Summary and Toxicological Review for benzene, the
principal study of Rothman et al. (1996) is well conducted, and a dose-response relationship was
established between ALC and benzene air concentration and benzene urine metabolites. The
availability of good-quality human data for a sensitive endpoint eliminates the uncertainty
associated with basing the RfC on experimental animal data. In addition, the RfC obtained from
the BMD modeling results from the Rothman et al. (1996) study is in good agreement with the
effect levels identified from the Ward et al. (1985) subchronic rodent inhalation study.
REFERENCES
ATSDR (Agency for Toxic Substances and Disease Registry). 2007. Toxicological Profile for
Benzene. Agency for Toxic Substances and Disease Registry, Public Health Service, U.S.
Department of Health and Human Services. Draft for Public Comment. Available at
http://www.atsdr.cdc.gov/toxprofiles/tp3.html.
Lan, Q., L. Zhang, G. Li et al. 2004. Hematotoxicity in workers exposed to low levels of
benzene. Science. 306:1774-1776.
NTP (National Toxicology Program). 1986. Toxicology and Carcinogenesis Studies of Benzene
(CAS No. 71-43-2) in F344/N Rats and B6C3F1 Mice (Gavage Studies). NTP, Research
Triangle Park, NC.
Qu, Q., R. Shore, G. Li et al. 2002. Hematological changes among Chinese workers with a
broad range of benzene exposures. Am. J. Ind. Med. 42(4):275-285.
Qu, Q., R. Shore, G. Li et al. 2003. Appendix A. Analyses of the combined data for year 1 and
year 2. Validation and evaluation of biomarkers in workers exposed to benzene in China: 1-54.
Research number 115.
Rosenthal, G.J. and C.A. Snyder. 1987. Inhaled benzene reduces all aspects of cell-mediated
tumor surveillance in mice. Toxicol. Appl. Pharmacol. 88:35-43.
Rothman, N., G.L. Li, M. Dosemeci et al. 1996. Hematotoxicity among Chinese workers
heavily exposed to benzene. Am. J. Ind. Med. 29:236-246.
Sabourin, P.J., B.T. Chen, G. Lucier, L.S. Birnbaum, E. Fisher, and R.F. Henderson. 1987.
Effect of dose on the absorption and excretion of [C14]benzene administered orally or by
inhalation in rats and mice. Toxicol. Appl. Pharmacol. 87: 325-336.
U.S. EPA. 1999. Extrapolation of the Benzene Inhalation Unit Risk Estimate to the Oral Route
of Exposure. National Center for Environmental Assessment, Office of Research and
Development, Washington, DC. NCEA-W-0517.
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U.S. EPA. 2002. Toxicological Review of Benzene (Noncancer Effects) (CAS No. 71-43-2) in
Support of Summary Information on the Integrated Risk Information System (IRIS). U.S.
Environmental Protection Agency, Washington, DC. EPA/635/R-02/00IF. Available at
http://www.epa.gov/iris/toxreviews/0276-tr.pdf. (Accessed June 2009).
U.S. EPA (Environmental Protection Agency). (2003) Integrated Risk Information System
(IRIS). IRIS Summary of Benzene (CASRN 71-43-2). Office of Research and Development,
National center for Environmental Assessment, Washington, DC. Available online at
http://www.epa.gov/iris/. (Accessed June 2009).
U.S. EPA (Environmental Protection Agency). (2008) Integrated Risk Information System
(IRIS). Office of Research and Development, National Center for Environmental Assessment,
Washington, DC. Online, http://www.epa.gov/iris/.
Ward, C.O., R.A. Kuna, N.K. Snyder, R.D. Alsaker, W.B. Coate, and P.H. Craig. 1985.
Subchronic inhalation toxicity of benzene in rats and mice. Am. J. Ind. Med. 7: 457-473.
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APPENDIX A. PERTINENT SECTIONS FROM IRIS SUMMARY FOR BENZENE:
CHRONIC HEALTH HAZARD ASSESSMENTS FOR
N ON CARCINOGENIC EFFECTS
Benzene; CASRN 71-43-2; 04/17/2003
Health assessment information on a chemical substance is included in IRIS only after a
comprehensive review of chronic toxicity data by U.S. EPA health scientists from several
Program Offices and the Office of Research and Development. The summaries presented in
Sections I and II represent a consensus reached in the review process. Background information
and explanations of the methods used to derive the values given in IRIS are provided in the
Background Documents.
STATUS OF DATA FOR Benzene
File First On-Line 03/01/1988
Category (section)
Oral RfD Assessment (I. A.)	online
Inhalation RfC Assessment (I.B.)	online
Carcinogenicity Assessment (II.)	online
I. Chronic Health Hazard Assessments for Noncarcinogenic Effects
I.A. Reference Dose for Chronic Oral Exposure (RfD)
Substance Name—Benzene
CASRN—71-43-2
Last Revised—04/17/2003
The oral Reference Dose (RfD) is based on the assumption that thresholds exist for certain toxic
effects such as cellular necrosis. It is expressed in units of mg/kg/day. In general, the RfD is 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. Please refer to the IRIS Background Document for an
elaboration of these concepts. The U.S. EPA has evaluated this substance for potential human
carcinogenicity. A summary of that evaluation is found in Section II of this file.
04/17/2003
04/17/2003
01/19/2000
I.A.1. Oral RfD Summary
Critical Effect
Decreased lymphocyte	BMDL = 1.2 mg/kg/day	300 1 4.0 xlO"3
count (Human occupational	mg/kg/day
inhalation study;
Rothman et al., 1996)
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*Conversion factors: MW = 78.11. Assuming 25°C and 760 mm Hg, BMCL (mg/m3) = 7.2 ppm
x MW/24.45 = 23 mg/m3. BMCLadj = 23 mg/m3 x 10 m3/20 m3 x 5 days/7days = 8.2 mg/m .
The BMDL was derived by route-to-route extrapolation with the assumptions that inhalation
absorption was 50% and oral absorption was 100% in the dose range near the BMC. BMDLadj =
8.2 mg/m3 x 20 rnVday x 0.5 70 kg = 1.2 mg/kg/day. (The original BMC was based on a
benchmark response of one standard deviation change from the control mean.)
	I.A.2. Principal and Supporting Studies (Oral RfD)
The RfD is based on route-to-route extrapolation of the results of benchmark dose (BMD)
modeling of the absolute lymphocyte count (ALC) data from the occupational epidemiologic
study by Rothman et al. (1996), in which workers were exposed to benzene by inhalation. A
comparison analysis based on BMD modeling of data from the National Toxicology Program's
(NTP's) experimental animal gavage study (NTP, 1986) was also conducted. In addition,
comparison analyses using the lowest-observed-adverse-effect levels (LOAELs) from the
Rothman et al. (1996) and NTP (1986) studies were performed.
Rothman et al. (1996) conducted a cross-sectional study of 44 workers exposed to benzene and
44 age- and gender-matched unexposed controls. Twenty-one of the 44 subjects in the exposed
and control groups were female. Mean (standard deviation) years of occupational exposure to
benzene were 6.3 (4.4), with a range of 0.7-16 years. Benzene exposure was monitored by
organic vapor passive dosimetry badges worn by each worker for a full workshift on 5 days
within a 1-2 week period prior to collection of blood samples. The median 8-hour time-weighted
average (TWA) benzene exposure concentration for all exposed workers was 31 ppm (99
mg/m ). The exposed group was subdivided into two equal groups of 22 subjects: those exposed
to greater than the median concentration and those exposed to less than the median
concentration. The median 8-hour TWA exposure concentration was 13.6 ppm (43.4 mg/m3) for
"3
the low-exposure group and 91.9 ppm (294 mg/m ) for the high-exposure group.
Six hematological measurements were evaluated: total white blood cell (WBC) count, ALC,
hematocrit, red blood cell (RBC) count, platelet count, and mean corpuscular volume (MCV).
All six parameters were significantly different in the high-benzene exposure group (>31 ppm)
when compared to controls. ALC, WBC count, RBC count, hematocrit, and platelets were all
significantly decreased, and MCV was significantly increased. ALC was the most sensitive
3	3
endpoint; it was reduced from 1.9 x 10 /|iL blood in controls to 1.6 x 10 /|iL (p <0.01) in the <31
ppm group and to 1.3 x 103/|iL (p<0.001) in the group exposed to >31 ppm benzene. The ALC
"3
was also significantly reduced (1.6 x 10 /|iL; p=0.03) in a subgroup of 11 workers exposed to a
median 8-hour TWA of 7.6 ppm (24 mg/m3) benzene. For additional details about this study see
Section I.B.2.
BMD modeling of the ALC data of Rothman et al. (1996) yielded a benchmark concentration
(BMC) of 13.7 ppm (8-hr TWA) and a BMCL (the 95% lower bound on the BMC) of 7.2 ppm
(8-hr TWA) for the default benchmark response of one standard deviation change from the
control mean (see Section I.B.2 for details of the analysis). Converting the units and adjusting for
"3
continuous exposure results in a BMCLadj of 8.2 mg/m . [According to the Ideal Gas Law,
concentration in mg/m3 = concentration in ppm x MW/24.45 at 25°C and 760 mm Hg. Thus,
BMCL (mg/m3) = 7.2 x 78.11/24.45 = 23.0 mg/m3. BMCLadj = 23.0 mg/m3 x 10 m3/20 m3 x 5
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3	3
days/7 days = 8.2 mg/m , where 10 m is the default human occupational volume of air inhaled
in an 8-hour workshift, and 20 m3 is the default human ambient volume of air inhaled in a 24-
hour day (U.S. EPA, 1994).]
In the support document for the benzene cancer assessment on IRIS (U.S. EPA, 1999), EPA
provided a simple method for extrapolation of benzene-induced cancer risk from the inhalation
to the oral route. The same method is applied here for noncancer (hematopoietic) effects. The
method is based on the relative efficiency of benzene absorption across routes of exposure,
especially pulmonary and gastrointestinal barriers. An inhalation absorption rate of 50% and an
oral absorption rate of 100% were used to calculate the absorbed benzene dose. These values are
based on human inhalation absorption studies and the study by Sabourin et al. (1987) that
compared inhalation and oral absorption in rats and mice. The authors found that during a 6-hour
inhalation exposure, the retention of [14C]benzene decreased from 33 ± 6% to 15 ± 9% for rats
and from 50 ± 1% to 10 ± 2% for mice as exposure concentration increased from 26 to 2,600
"3
mg/m (10 to 1,000 ppm). In the same study, gastrointestinal absorption of benzene administered
by gavage was >97% for doses between 0.5 and 150 mg/kg body weight. At oral doses below 15
mg/kg, >90% of the 14C excreted was in the urine as non-ethyl acetate-extractable material. At
higher doses, an increasing percentage of the orally administered benzene was exhaled
unmetabolized. Thus, in the dose range represented by the BMCL from the study by Rothman et
al. (1996), absorption of a comparable oral dose was assumed to be 100%. See also U.S. EPA
(1999) for more details about the route-to-route extrapolation of benzene inhalation results to
oral exposures.
To calculate an equivalent oral dose rate, the BMCLadj is multiplied by the default inhalation
rate, multiplied by 0.5 to correct for the higher oral absorption, and divided by the standard
default human body weight of 70 kg: 8.2 mg/m3 x 20 m3/day x 0,5 ^ 70 kg = 1,2 mg/kg/day. The
RfD is then derived by dividing the equivalent oral dose by the overall uncertainty factor (UF) of
300: RfD = equivalent oral dose/UF = 1.2 mg/kg/day ^ 300 = 4 x 10"3 mg/kg/day. The overall
UF of 300 comprises a UF of 3 for effect-level extrapolation, 10 for intraspecies differences
(human variability), 3 for subchronic-to-chronic extrapolation, and 3 for database deficiencies
(see Section I.A.3).
For comparison, an RfD was also calculated based on the LOAEL of 7.6 ppm (8 hr TWA) from
the Rothman et al. (1996) study (see Section I.B.2). Converting the units and adjusting for
continuous exposure results in a LOAELadj of 8.7 mg/m3. Then the equivalent oral exposure is
3	3
calculated as above: 8.7 mg/m x 20 m /day x 0.5 70 kg = 1.2 mg/kg/day. The equivalent oral
exposure is then divided by an overall UF of 1000 to obtain the RfD: 1.2 mg/kg/day ^ 1000 = 1 x
"3
10" mg/kg/day. The combined UF of 1000 represents UFs of 10 to account for the use of a
LOAEL because of the lack of an appropriate no-observed-adverse-effect level (NOAEL), 10 for
intraspecies differences in response (human variability), 3 for subchronic-to-chronic
extrapolation, and 3 for database deficiencies. The value of 1 x 10"3 mg/kg/day is in good
"3
agreement with the value of 4 x 10" mg/kg/day calculated from the BMDL (the 95% lower
bound on the BMD).
A comparison RfD derivation was also performed using the results of the NTP (1986)
experimental animal gavage study. In that study, F344 rats and B6C3F1 mice of both sexes were
administered benzene by gavage, 5 days/week for 103 weeks. Male rats (50/group) were
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administered doses of 0, 50, 100, or 200 mg/kg, and females (50/group) were administered doses
of 0, 25, 50, or 100 mg/kg. B6C3F1 mice (50/sex/group) were administered doses of 0, 25, 50, or
100 mg/kg. Blood was drawn from 10 randomly preselected animals per species/sex/dose group
at 12, 15, 18, and 21 months, as well as from all animals at the terminal kill at 24 months.
Additional groups of 10 animals of each sex and species were administered benzene for 51
weeks at the same doses of the 103-week (2-year) study, and blood was drawn at 0, 3, 6, 9, and
12 months. This study identified a LOAEL of 25 mg/kg for leukopenia and lymphocytopenia in
female F344 rats and male and female B6C3F1 mice and 50 mg/kg in male F344 rats. These
were the lowest doses tested, and thus no NOAEL was identified.
Reductions in lymphocyte count was the critical effect, and attempts were made to model the
dose-response relationships using a BMD modeling approach. Modeling was performed for each
dataset in two data groupings within which the datasets are comparable (6- and 9-month; and 12-
,15-, 18-, and 21-month), and ranges of results are presented. Each of these datasets had at most
10 animals/dose, so the dose-response results are not very robust. The males of each species
exhibited more dramatic and consistent reductions in lymphocyte count, but it was not clear a
priori which species was more sensitive; therefore, dose-response analyses were performed for
both the male mouse and the male rat.
The continuous linear, polynomial, and power models in EPA's Benchmark Dose Modeling
Software (version 1.20) were used for the modeling. The software estimates the parameters using
the method of maximum likelihood. Most of the data were supralinear (i.e., the magnitude of the
reductions in lymphocyte count decreased with increasing unit dose), and it was necessary to
transform the dose data according to the formula d' = ln(d+l) in order to fit the available models.
The results are summarized in Table 1. For each dataset, the selected model was chosen based on
the lowest Akaike's Information Criterion (AIC) value, with consideration of the graphical
display, as suggested in EPA's draft Benchmark Dose Technical Guidance Document (U.S. EPA,
2000). For selecting between models within a family of models, for example, between a linear
and a two-degree polynomial model, consideration was given to the log-likelihood values to
evaluate the statistical significance of adding an extra parameter. There was substantial
variability in these data, but it appeared to be random and not amenable to modeling. Therefore,
constant variance was assumed for all the models, although in some cases the variances failed the
test for homogeneity.
In the absence of a clear definition for an adverse effect for this endpoint, a default benchmark
response of one standard deviation change from the control mean response was selected, as
suggested in the draft technical guidance document. This definition of the benchmark response is
highly sensitive to the substantial variability in data such as these, and thus the benchmark
response itself is not very robust. The usefulness of this default definition would be strengthened
by the use of a larger dataset of historical control data, but such data were not located. The
software uses the estimated "constant" standard deviation as the standard deviation for all the
group means. The 95% lower confidence limits (BMDLs) on the BMDs are calculated using the
likelihood profile method.
The results shown in Table 1 suggest that the male rat is more sensitive than the male mouse to
lymphocyte count reductions from exposure to benzene in this NTP gavage bioassay because the
ranges of BMDs/BMDLs are substantially lower for the male rat, especially for year 2. The
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ranges for the male rat are fairly tight, and the models selected provide good fits to all the male
rat datasets. However, all but one of the calculated BMDs for the male rat are over an order of
magnitude below the lowest exposure dose of 50 mg/kg. Ideally, BMDs should be closer to the
low end of the range of observation, that is, the range of the actual exposure doses, to reduce the
impacts of model selection and the uncertainties inherent in extrapolating to lower doses.
Nevertheless, data from two drinking water studies provide support for selecting a BMD in this
range. These two studies were of shorter duration and used fewer experimental animals than the
NTP (1986) study; however, they do provide dose-response data for BMD modeling, and they
also have the advantage of being drinking water studies; thus the benzene exposure scenario is
more relevant to human oral benzene exposures. In one study, Hsieh et al. (1988) exposed male
CD-I mice (five/group) to 0, 8, 40, or 180 mg/kg/day benzene in drinking water for 28 days.
Hematological effects were observed at all exposure levels. BMD modeling of the ALC yielded
a BMD of 2.2 mg/kg/day and a BMDL of 1.4 mg/kg/day, based on a linear model with
transformed doses and a benchmark response of one standard deviation change from the control
mean, as above. In the second study, White et al. (1984) exposed female B6C3F1 mice to 0, 12,
195, or 350 mg/kg/day benzene in drinking water for 30 days. BMD modeling of the ALC (five
to six mice/group) resulted in a BMD of 11.6 mg/kg/day and a BMDL of 5.3 mg/kg/day (also
based on a linear model with transformed doses and a benchmark response of one standard
deviation change from the control mean, as above).
The results in Table 1 from BMD modeling of the male rat ALC data from the NTP (1986) study
show the lowest BMDL of about 1 mg/kg at three time points in the second year;
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Table 1. BMD modeling results for NTP (1986) male mouse and male rat lymphocyte
counts, with transformed dose data
Dataset
Model
Variance
Homogeneity
Fit
BMD"
(mg/kg)
BMDLa (mg/kg)
Male Mouse
6-month
two-degree
polynomial
ok
borderline
p=0.047
19.68
6.57
9-month
linear
no
yes, /?=0.35
9.07
4.05
year 1 range
9.07-19.68
4.05-6.57
12-month
linear
ok
yes, /?=0.30
3.74
2.32
15-month
power
no
yes, p=0.31
47.46
18.55
18-month
power
no
borderline
p=0.09
28.93
13.99
21-month
power
no
yes, /?=0.15
23.34
5.80
year 2 range
3.74-47.46
2.32-18.55
Male Rat |
6-month
power
ok
yes, /?=0.30
9.92
4.52
9-month
linear
no
yes, p=0.11
3.71
2.30
year 1 range
3.71-9.92
2.30-4.52
12-month
linear
no
yes, p=0.22
1.34
0.95
15-month
linear
ok
yes, p=0.93
1.34
0.95
18-month
linear
no
yes, p=0.22
2.73
1.74
21-month
linear
ok
yes, p=0.54
1.69
1.10
year 2 range
1.34-2.73
0.95-1.74
aUnadjusted animal dose in mg/kg, after transforming the results back according to the formula
dose = exp(transformed dose) - 1. (The BMD was based on a benchmark response of one
standard deviation change from the control mean.)
thus this was selected as the point of departure for an RfD calculation. Adjusting for exposure 7
days/week yields a BMDLadj of 0.7 mg/kg/day. This value is divided by an overall UF of 1000
to obtain the RfD: RfD = 0.7 mg/kg/day 1000 = 7 x 10"4 mg/kg/day. The overall UF of 1000
comprises UFs of 3 for effect-level extrapolation, 10 for interspecies extrapolation for oral
studies, 10 for intraspecies variability, and 3 for database deficiencies. This RfD value is in
reasonably good agreement (within an order of magnitude) with the RfD of 4 x 10" mg/kg/day
derived from the Rothman et al. (1996) human inhalation study.
For comparison purposes, an RfD can also be derived from the LOAEL of 25 mg/kg identified
for hematological effects in the NTP (1986) study (there was no NOAEL). Adjusting from 5-day
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to 7-day exposure yields a LOAELadj of 18 mg/kg/day, which can be used to calculate an RfD
for benzene as follows: RfD = LOAELadj UF =18 mg/kg/day ^ 3000 = 6 x 10"3 mg/kg/day,
where the combined UF of 3000 is made up of component factors of 10 for LOAEL-to-NOAEL
extrapolation, 10 for interspecies extrapolation, 10 for intraspecies variability, and 3 for database
"3
deficiencies. This value is in good agreement with the RfD of 4 x 10" mg/kg/day calculated from
the BMD analysis of the Rothman et al. (1996) human data.
	I.A.3. Uncertainty and Modifying Factors (Oral RfD)
UF = 300 for the BMCL-oral-equivalent from the Rothman et al. (1996) study.
First, because the BMC is considered to be an adverse effect level, an effect level extrapolation
factor analogous to the LOAEL-to-NOAEL UF is used. EPA is planning to develop guidance for
applying an effect level extrapolation factor to a BMD. A factor of 3 will be used in this analysis,
based on the professional judgement that, although the BMD corresponds to an adverse effect
level at the low end of the observable range, the endpoint is not very serious in and of itself.
Decreased ALC is a very sensitive sentinel effect that can be measured in the blood, but it is not
a frank effect, and there is no evidence that it is related to any functional impairment at levels of
decrement near the benchmark response. For a more serious effect, a larger factor, such as 10,
might be selected. Second, a factor of 10 was used for intraspecies differences in response
(human variability) as a means of protecting potentially sensitive human subpopulations. Third, a
subchronic-to-chronic extrapolation factor was applied because the mean exposure duration for
the subjects in the principal study was 6.3 years, which is less than the exposure duration of 7
years (one-tenth of the assumed human life span of 70 years) that has been used by the
Superfund program as a cut-off for deriving a subchronic human reference dose (U.S. EPA,
1989). Furthermore, the exposure duration varied from 0.7 years to 16 years. However, because
the mean exposure duration was near the borderline of what would be considered chronic (i.e.,
6.3 years vs. 7 years), a value of 3 (vs. 10) was felt to be appropriate for the UF. Finally, a UF of
3 was chosen to account for database deficiencies because no two-generation reproductive and
developmental toxicity studies for benzene are available. Therefore, an overall UF of 3 x 10 x 3
x 3 = 300 is used to calculate the chronic oral RfD.
For the comparison analysis based on the Rothman et al. (1996) LOAELADj-equivalent oral dose
rate value of 1.2 mg/kg/day, the following UFs were selected: a factor of 10 for use of a LOAEL
due to lack of an appropriate NOAEL, a factor of 10 for intraspecies variability, a factor of 3 for
subchronic-to-chronic extrapolation, and a factor of 3 for database deficiencies, as above. Hence,
an overall UF of 10x10x3x3 = 1000 was used in the comparison analysis.
For the comparison analysis based on the BMDLadj calculated from BMD modeling of the male
rat data from the NTP (1986) gavage study, the following UFs were used: a UF of 3 for effect-
level extrapolation, which is analogous to the LOAEL-to-NOAEL extrapolation factor, because
the BMC is considered an adverse effect level; a UF of 10 for interspecies extrapolation for oral
studies; a UF of 10 for intraspecies variability; and a UF of 3 for database deficiencies. Thus, an
overall UF of 3x 10x 10x3 = 1000 was used in this comparison analysis.
Finally, for the comparison analysis based on the LOAEL from the NTP (1986) gavage study,
the following UFs were used: 10 for LOAEL-to-NOAEL extrapolation, 10 for interspecies
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extrapolation, 10 for intraspecies variability, and 3 for database deficiencies. Therefore, an
overall UF of 3000 was used in this comparison analysis.
	I.A.4. Additional Studies/Comments (Oral RfD)
Benzene is toxic by all routes of administration. Hematotoxicity and immunotoxicity have been
consistently reported to be the most sensitive indicators of noncancer toxicity in both humans
and experimental animals, and these effects have been the subject of several reviews (Aksoy,
1989; Goldstein, 1988, Snyder et al., 1993; Ross, 1996; U.S. EPA, 2002). The bone marrow is
the target organ for the expression of benzene hematotoxicity and immunotoxicity.
Leukocytopenia has been consistently shown to be a more sensitive indicator of benzene toxicity
in experimental animal systems than anemia, and lymphocytopenia has been shown to be an
even more sensitive indicator of benzene toxicity than overall leukocytopenia. Neither
gastrointestinal effects from oral exposure nor pulmonary effects due to inhalation exposure have
been reported, (see Section I.B.4 for a more detailed summary of benzene toxicity).
For more detail on Susceptible Populations, exit to the toxicohwcal review. Section 4.4
(PDF).
	I.A.5. Confidence in the Oral RfD
Study — Medium
Database — Medium
RfD — Medium
The overall confidence in this RfD assessment is medium. The principal study of Rothman et al.
(1996) was well conducted, and the availability of good-quality human data for a sensitive
endpoint eliminates the uncertainty associated with basing the RfD on experimental animal data.
A dose-response relationship was established between ALC and benzene air concentration and
benzene urine metabolites. Six blood parameters measured (ALC, WBC count, RBC count,
hematocrit, platelets, and MCV) were significantly different in the high- benzene-exposure group
when compared with controls. However, only the ALC was reduced in a subgroup of 11 subjects
exposed to a median 8-hour TWA of 7.6 ppm benzene, suggesting that this exposure level may
be at the low end of the range of benzene exposures eliciting hematotoxic effects in humans.
"3
In addition, the RfD of 4 x 10" mg/kg/day obtained from route-to-route extrapolation of the
BMD modeling results from the Rothman et al. (1996) study is in good agreement with the value
3	•	•	•
of 1 x 10" mg/kg/day based on the oral equivalent LOAEL. The RfD is also in good agreement
with the value of 7 x 10"4 mg/kg/day, based on BMD modeling of the male rat ALC data from
"3
the NTP (1986) chronic rodent gavage study and the value of 6 x 10" mg/kg/day based on the
LOAEL from the NTP (1986) study.
With continuous endpoints such as hematological parameters, there is uncertainty about when a
change in a parameter that has inherent variability becomes an adverse effect. Other uncertainties
explicitly recognized in the quantitative derivation of the chronic oral RfD include intraspecies
variability (to accommodate sensitive human subgroups), the applicability of the subchronic
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inhalation data to chronic oral exposures, and database deficiencies due to the lack of a two-
generation reproductive/developmental toxicity study for benzene.
Route-to-route extrapolation was used to estimate oral equivalent doses from inhalation
exposures resulting from analysis of the Rothman et al. (1996) occupational data. In experiments
conducted to compare the metabolite doses to the target organ following oral or inhalation
exposure, Sabourin et al. (1987, 1989) found that there was no simple relationship between the
two routes of exposure. All published experimental animal models of the in vivo metabolism and
disposition of benzene have used the physiologically based approach to pharmacokinetics, and
they conclude that formation of metabolites follow Michaelis-Menten kinetics. Although these
models predict the urinary metabolites formed from benzene exposures, they offer no
information regarding the dosimetry of oxidative metabolites in the bone marrow, a site of
action. However, the target specificity of benzene toxicity for the bone marrow progenitor cells
irrespective of route of administration is well documented in both humans and experimental
animal models. Thus, route-to-route extrapolation is justified and introduces a lower degree of
uncertainty than extrapolating from test animals to humans (U.S. EPA, 1999). Use of a
modifying factor of 3 was considered to recognize uncertainties in the route-to-route
extrapolation; however, it was deemed unnecessary. The RfD is based on human data for a
sensitive endpoint; thus, it was felt that the composite UF of 300 provides sufficient protection.
For more detail on Characterization of Hazard and Dose Response, exit to the toxicohwcal
review. Section 6 (PDF).
	I.A.6. EPA Documentation and Review of the Oral RfD
Source Document — U.S. EPA, 2002
This assessment was peer reviewed by external scientists as well as in response to public
comments. Their comments have been evaluated carefully and incorporated in the finalization of
this IRIS summary. The peer review document (12 pages, 135 Kbytes) is available in Adobe
PDF format.
Other EPA Documentation — U.S. EPA, 1985, 1999
Date of Agency Consensus — January 23, 2002
	I.A.7. EPA Contacts (Oral RfD)
Please contact the IRIS Hotline for all questions concerning this assessment or IRIS, in general,
at (202)566-1676 (phone), (202)566-1749 (FAX) or hotline.iris@epa .gov (internet address).
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I.B. Reference Concentration for Chronic Inhalation Exposure (RfC)
Substance Name — Benzene
CASRN —71-43-2
Last Revised — 04/17/2003
The inhalation Reference Concentration (RfC) is analogous to the oral RfD and is likewise based
on the assumption that thresholds exist for certain toxic effects such as cellular necrosis. The
inhalation RfC considers toxic effects for both the respiratory system (portal-of-entry) and for
effects peripheral to the respiratory system (extrarespiratory effects). It is generally expressed in
units of mg/cu.m. In general, the RfC is an estimate (with uncertainty spanning perhaps an order
of magnitude) of a daily inhalation exposure of the human population (including sensitive
subgroups) that is likely to be without an appreciable risk of deleterious effects during a lifetime.
Inhalation RfCs were derived according to Methods for Derivation of Inhalation Reference
Concentrations and Application of Inhalation Dosimetry (U.S. EPA, 1994). RfCs can also be
derived for the noncarcinogenic health effects of substances that are carcinogens. Therefore, it is
essential to refer to other sources of information concerning the carcinogenicity of this substance.
If the U.S. EPA has evaluated this substance for potential human carcinogenicity, a summary of
that evaluation will be contained in section II of this file.
	I.B.I. Inhalation RfC Summary	
Critical Effect	Exposures*	UF MF RfC
Decreased lymphocyte	BMCL = 8.2 mg/m3	300 1 3 x 10"2
count (Human occupational	mg/m
inhalation study of
Rothman et al., 1996)
*Conversion factors: MW = 78.11. BMCL = 7.2 ppm, 8-hour TWA. Assuming 25°C and 760
mm Hg, BMCL (mg/m3) = 7.2 ppm x MW/24.45 = 23.0 mg/m3. BMCLadj = 23.0 mg/m3 x 10
3	3	3
m /20 m x 5 days/7days = 8.2 mg/m . (The BMC was based on a benchmark response of one
standard deviation change from the control mean.)
	I.B.2. Principal and Supporting Studies (Inhalation RfC)
The RfC is based on BMD modeling of the ALC data from the occupational epidemiologic study
of Rothman et al. (1996), in which workers were exposed to benzene by inhalation. A
comparison analysis based on BMD modeling of hematological data from the Ward et al. (1985)
subchronic experimental animal inhalation study was also conducted. In addition, comparison
analyses using the LOAEL from the Rothman et al. (1996) study and the NOAEL from the Ward
et al. (1985) study were performed.
Rothman et al. (1996) conducted a cross-sectional study of 44 workers exposed to a range of
benzene concentrations and 44 age- and gender-matched unexposed controls, all from Shanghai,
China. Twenty-one of the 44 subjects in the exposed and control groups were female. The
exposed workers were from three workplaces where benzene was used-a factory that
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manufactured rubber padding for printing presses, a factory that manufactured adhesive tape, and
a factory that used benzene-based paint. The unexposed workers were from two workplaces: a
factory that manufactured sewing machines and an administrative facility. Workers who had a
prior history of cancer, therapeutic radiation, chemotherapy, or current pregnancy were excluded.
Requirements for inclusion in the study were current employment for at least 6 months in a
factory that used benzene, minimal exposure to other aromatic solvents, and no exposure to other
chemicals known to be toxic to bone marrow or to ionizing radiation. Controls who had no
history of occupational exposure to benzene or other bone marrow-toxic agents were frequency-
matched to the exposed subjects on age (5-year intervals) and gender.
Benzene exposure was monitored by organic vapor passive dosimetry badges worn by each
worker for a full workshift on 5 days within a 1-2 week period prior to collection of blood
samples. Benzene exposure of controls in the sewing machine factory was monitored for 1 day,
but no exposure monitoring was performed in the administrative facility. Benzene exposure was
also evaluated by analyzing for benzene metabolites in urine samples collected at the end of the
benzene exposure period for the exposed subjects. Historical benzene exposure of the subjects
was evaluated by examining employment history. Data on age, gender, current and lifelong
tobacco use, alcohol consumption, medical history, and occupational history were collected by
interview. Six hematological measurements were evaluated: total WBC count, ALC, hematocrit,
RBC count, platelet count, and MCV. Total WBC counts and ALC were performed using a
Coulter T540 blood counter. Abnormal counts were confirmed. Benzene metabolites in urine
were measured by an isotope dilution gas chromatography/mass spectometry assay. Correlation
analyses were performed with Spearman rank order correlation. The Wilcoxon rank sum test was
used to test for hematological differences.
Mean (standard deviation) years of occupational exposure to benzene were 6.3 (4.4) with a range
of 0.7-16 years. The median 8-hour TWA benzene exposure concentration for all exposed
3	3
workers was 31 ppm (99 mg/m ). Exposure to toluene and xylene was <= 0.2 ppm (0.6 mg/m )
in all groups. The exposed group was subdivided into two equal groups of 22-one group
comprising workers who were exposed to greater than the median concentration and the other
containing those exposed to less than the median concentration. The median (range) 8-hour
TWA exposure concentration was 13.6 (1.6-30.6) ppm (43.4 [5.1-97.8] mg/m3] for the low-
exposure group and 91.9 (31.5-328.5) ppm (294 [101-1049] mg/m3) for the high-exposure group.
A subgroup of the low-exposure group composed of 11 individuals who were not exposed to >31
ppm (100 mg/m3) at any time during the monitoring period was also examined in some
comparisons. The median (range) 8-hour TWA exposure of these individuals was 7.6 (1-20) ppm
(24 [3.2-64] mg/m3). The urinary concentrations of the metabolites phenol, muconic acid,
hydroquinone, and catechol were all significantly correlated with measured benzene exposure.
All six blood parameters measured were significantly different in the high-benzene exposure
group as compared to controls. ALC, WBC count, RBC count, hematocrit, and platelets were all
significantly decreased, and MCV was significantly increased. The ALC was reduced from 1.9 x
103/|iL blood in controls to 1.6 x 103/|u.L (p<0.01) in the <31 ppm (99 mg/m3) group and to 1.3 x
"3
10 /|iL (/K0.001) in the group exposed to >31 ppm benzene. In the subgroup of 11 workers
exposed to a median 8-hour TWA of 7.6 ppm (24 mg/m3) benzene, the ALC (1.6 x 103/(J.L) was
also significantly reduced (p=0.03). The RBC and platelet counts were also significantly reduced
in the <31 ppm exposure group, but only ALC was significantly different in the low-exposure
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subgroup. The fact that no other measured blood cell parameters were significantly different in
this subgroup suggests that ALC was the most sensitive measure of benzene hematotoxicity and
that this exposure level (median 8-hour TWA of 7.6 ppm) may be at the low end of the range of
benzene exposures eliciting hematotoxic effects in humans.
ALC is also thought to have a potential role as a "sentinel" effect for a cascade of early
hematological and related biological changes that might be expected to result in the more
profound examples of benzene poisoning observed in other cohorts of the National Cancer
Institute/Chinese Academy of Preventive Medicine study, as described by Dosemeci et al.
(1996). That ALC depletion is accompanied by gene-duplicating mutations in somatic cells
under the same range of exposure conditions suggests that benzene can cause repeated damage to
longer-lived stem cells in human bone marrow, further implicating the compound as etiologically
important in the onset of benzene-associated leukemia. This finding underlines the importance of
basing public health concern for benzene on a toxicological effect that is representative of the
earliest biological changes induced by the compound.
BMD modeling of the ALC exposure-response data from Rothman et al. (1996) was done using
U.S. EPA's Benchmark Dose Modeling Software (version 1.20). The data are rather supralinear,
that is, the change in ALC per unit change in exposure decreases with increasing exposure;
therefore, in order to fit the data with one of the available continuous models, the exposure levels
were first transformed according to the equation d' = ln(d+l). Then the exposure-response data
were fitted using the continuous linear model, which provided a good fit (p=0.54). A two-degree
polynomial and a power model also fit the data, but the linear model was selected because it is
the most parsimonious. The parameters were estimated using the method of maximum
likelihood. A constant variance model was used.
In the absence of a clear definition for an adverse effect for this continuous endpoint, a default
benchmark response of one standard deviation change from the control mean was selected, as
suggested in EPA's draft Benchmark Dose Technical Guidance Document (U.S. EPA, 2000).
This default definition of a benchmark response for continuous endpoints corresponds to an
excess risk of approximately 10% for the proportion of individuals below the 2nd percentile (or
above the 98th percentile) of the control distribution for normally distributed effects (see U.S.
EPA, 2000). A 95% lower confidence limit (BMCL) on the resulting BMC was calculated using
the likelihood profile method. Transforming the results back to the original exposure scale yields
a BMC of 13.7 ppm (8-hr TWA) and a BMCL of 7.2 ppm (8-hr TWA).
As suggested in the draft technical guidance document (U.S. EPA, 2000), the BMCL is chosen
as the point of departure for the RfC derivation. An adjusted BMCL is calculated by converting
"3
ppm to mg/m and adjusting the 8-hour TWA occupational exposure to an equivalent continuous
environmental exposure. The BMCL is first converted to mg/m3 using the molecular weight of
78.11 for benzene and assuming 25°C and 760 mm Hg: 7.2 ppm x 78.11/24.45 = 23.0 mg/m3.
The converted value is then adjusted from the 8-hour occupational TWA to a continuous
exposure concentration using the default respiration rates (U.S. EPA, 1994): BMCLadj = 23.0
mg/m3 x (10 m3/20 m3) x 5 days/7 days = 8.2 mg/m3.
The RfC is then derived by dividing the adjusted BMCL by the overall UF of 300: RfC =
BMCLadj/UF = 8.2 mg/m3 300 = 3 x 10"2 mg/m3. The overall UF of 300 comprises a UF of 3
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for effect-level extrapolation, 10 for intraspecies differences (human variability), 3 for
subchronic-to-chronic extrapolation, and 3 for database deficiencies (see Section I.B.3).
For comparison, an RfC was also calculated based on the LOAEL of 7.6 ppm (8-hr TWA) from
the Rothman et al. (1996) study. Converting the units and adjusting for continuous exposure as
above results in a LOAELadj of 8.7 mg/m3. The LOAELadj is then divided by an overall UF of
1000 to obtain the RfC: 8.7 mg/m3 1000 = 9 x 10"3 mg/m3. The combined UF of 1000
represents UFs of 10 to account for the use of a LOAEL because of the lack of an appropriate
NOAEL, 10 for intraspecies differences in response (human variability), 3 for subchronic-to-
chronic extrapolation, and 3 for database deficiencies. The value of 9 x 10"3 mg/m3 is in good
2	3
agreement with the RfC of 3 x 10" mg/m calculated from the BMC.
A comparison RfC derivation based on BMD modeling of hematological data from the Ward et
al. (1985) subchronic experimental animal inhalation study was also conducted. The Ward study
was selected because it used a relatively long inhalation exposure duration and an adequate
number of animals, and it provided dose-response data. Ward et al. exposed male and female
CD-I mice and Sprague-Dawley rats to 0, 1, 10, 30 or 300 ppm (0, 3.2, 32, 96 or 960 mg/m3)
benzene, 6 hours/day, 5 days/week for 91 days and measured various hematological endpoints.
The study identified both a LOAEL of 300 ppm and a NOAEL of 30 ppm. The male mouse
appeared to be the most sensitive sex/species in this study. The exposure-response relationships
for the different hematological endpoints for the male mouse were modeled using a BMD
modeling approach and decreased hematocrit (i.e., volume percentage of erythrocytes in whole
blood) was chosen as the critical effect.
U.S. EPA's Benchmark Dose Modeling Software (version 1.20) was used for the modeling. An
assumption of constant variance was used, although the test for homogeneity of the variances
failed. The continuous linear, polynomial, and power models all resulted in the same BMC and
BMCL estimates; however, the linear model had better results for the fit statistics. The linear
model had a p-value of 0.09, which is of borderline adequacy (the draft technical guidance
document [U.S. EPA, 2000] recommends a p-value of >= 0.1), and the other models had p-
values of 0.04. Thus the continuous linear model was selected. The parameters were estimated
using the method of maximum likelihood.
In the absence of a clear definition for an adverse effect for this continuous endpoint, a default
benchmark response of one standard deviation from the control mean was selected, as suggested
in the draft technical guidance document (U.S. EPA, 2000). The software uses the estimated
standard deviation. A 95% lower confidence limit (BMCL) on the resulting BMC was calculated
using the likelihood profile method. A BMC of 100.7 ppm and a BMCL of 85.0 ppm were
obtained.
It should be noted that the dose spacing in this study was less than ideal. Responses in the three
lower exposure groups for all the hematological endpoints tended to clump near control group
levels, and significant deviations in response were generally seen only in the 300 ppm group,
with a large exposure range in between, including where the BMC is located, for which there are
no response data. Therefore, there is some uncertainty about the actual shape of the exposure-
response curve in the region of the benchmark response and, thus, some corresponding
uncertainty about the values of the BMC and BMCL estimates.
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ALCs were not reported in Ward et al. (1985), so this endpoint could not be compared to the
human ALC results. Total WBC counts were reported and exhibited the largest percent change in
response between the control and the 300 ppm group; however, the data for this endpoint also
had substantial variance, and because the benchmark response used for this analysis is a function
of the standard deviation, WBC count did not yield the lowest BMC estimate. The actual lowest
BMC estimates were obtained for increased mean cell hemoglobin (MCH) (78 ppm; BMCL = 67
ppm) and increased mean cell volume (79 ppm; BMCL = 68 ppm); however, these endpoints are
probably not adverse per se. On the other hand, they are likely to be compensatory effects and,
thus, markers of toxicity, and one could probably justify using them as the critical effects. In any
event, the BMC estimates are not much different from the BMC of 100 ppm obtained for
decreased hematocrit. The results are also similar for total blood hemoglobin (BMC = 104 ppm,
BMCL = 88 ppm). RBC count results were in between those for MCV and MCH and those for
hematocrit and total hemoglobin; however, the model fits were not adequate for the RBC data
and, thus, the RBC results have more uncertainty.
To derive the RfC, the BMCL is used as the point of departure, as suggested in the draft
Benchmark Dose Technical Guidance Document (U.S. EPA, 2000). For conversion of the
inhalation exposures across species, ppm equivalence was assumed; this is identical to using
EPA's inhalation dosimetry methodology with Regional Gas Dose Ratio for the respiratory tract
region (RGDRr) = 1 (U.S. EPA, 1994). The BMCL is first converted to mg/m3 using the
molecular weight of 78.11 for benzene and assuming 25°C and 760 mm Hg: BMCL (mg/m3) =
"3
85.0 ppm x 78.11/24.45 = 272 mg/m . The converted value is then adjusted to an equivalent
continuous exposure: BMCLadj = 272 mg/m3 x (6 hrs/24 hrs) x 5 days/7 days = 48.5 mg/m3.
The RfC is then obtained by dividing the adjusted BMCL by the overall UF of 1000: RfC = 48.5
mg/m3 1000 = 5 x 10~2 mg/m3. The overall UF of 1000 comprises a UF of 3 for effect-level
extrapolation, 3 for interspecies extrapolation (inhalation), 10 for intraspecies differences, 3 for
subchronic-to-chronic extrapolation, and 3 for database deficiencies (see Section I.B.3). This
value is in good agreement with the RfC of 3 x 10~2 mg/m3 calculated from the BMC from the
Rothman et al. (1996) human study.
For further comparison, an RfC was also calculated, based on the NOAEL of 30 ppm from the
Ward et al. (1985) study. Converting the units and adjusting for continuous exposure as above
results in a NOAELadj of 17.1 mg/m3. The NOAELadj is then divided by an overall UF of 300
to obtain the RfC: 17.1 mg/m3 300 = 6 x 10"2 mg/m3. The combined UF of 300 represents a UF
of 3 for interspecies extrapolation (inhalation), 10 for intraspecies differences, 3 for subchronic-
to-chronic extrapolation, and 3 for database deficiencies. The value of 6 x 10"2 mg/m3 is also in
2	3
good agreement with the RfC of 3 x 10" mg/m calculated from the BMC from the Rothman et
al. (1996) human study.
It should be noted, however, that other experimental animal studies have reported significant
hematological effects at benzene exposures of 10-25 ppm, which are lower than the NOAEL of
30 ppm from the Ward et al. (1985) study. These studies have insufficient data for dose-response
modeling, and they used shorter exposure durations and/or fewer experimental animals than did
the Ward et al. (1985) study; nonetheless, they observed statistically significant hematological
effects at 10-25 ppm. Baarson et al. (1984), for example, exposed male C57BL/6J mice
(five/group) to 10 ppm benzene, 6 hours/day, 5 days/week, for 178 days and observed
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statistically significant reductions in blood lymphocytes at each of the three monitoring time
points (32, 66, and 178 days) when compared to controls. The magnitude of the reduction in
lymphocytes ranged from about 53% at 32 days to about 68% at 178 days. Cronkite et al. (1985)
exposed male and female C57BL/6 BNL mice to various concentrations of benzene 6 hours/day,
5 days/week for 2 weeks and observed no decrease in blood lymphocytes at 10 ppm, but they did
observe a statistically significant reduction of about 21% at 25 ppm as compared to controls (5-
10 mice/group). Thus, lower RfCs than those calculated above for the Ward et al. (1985) study
are possible, based on other experimental animal results. In the most extreme case, using a
LOAEL of 10 ppm and an overall UF of 3000 yields a LOAELadj of 5.7 mg/m3 and an RfC of 2
x 10"3 mg/m3.
	I.B.3. Uncertainty and Modifying Factors (Inhalation RfC)
UF = 300 for the BMCL from the Rothman et al. (1996) study.
First, because the BMC is considered to be an adverse effect level, an effect level extrapolation
factor analogous to the LOAEL-to-NOAEL UF is used. U.S. EPA is planning to develop
guidance for applying an effect level extrapolation factor to a BMD. In the interim, a factor of 3
will be used in this analysis (see Section I.A.3). For a more serious effect, a larger factor, such as
10, might be selected. Second, a factor of 10 was used for intraspecies differences in response
(human variability) as a means of protecting potentially sensitive human subpopulations. Third, a
UF of 3 for subchronic-to-chronic extrapolation was applied (see Section I.A.3). Finally, a UF of
3 was chosen to account for database deficiencies, because no two-generation reproductive and
developmental toxicity studies for benzene are available. Therefore, an overall UF of 3 x 10 x 3
x 3 = 300 is used to calculate the RfC.
For the comparison analysis based on the Rothman et al. (1996) LOAEL, the following UFs
were selected: a factor of 10 for use of a LOAEL due to lack of an appropriate NOAEL, a factor
of 10 for intraspecies variability, a factor of 3 for subchronic-to-chronic extrapolation, and a
factor of 3 for database deficiencies. Hence, an overall UF of 10x10x3x3 = 1000 was used in
the comparison analysis.
For the comparison analysis based on the BMCL calculated from BMD modeling of the male
mouse data from the Ward et al. (1985) subchronic inhalation study, the following UFs were
used: a UF of 3 for effect-level extrapolation, which is analogous to the LOAEL-to-NOAEL
extrapolation factor, because the BMC is considered an adverse effect level; a UF of 3 for
interspecies extrapolation for inhalation studies; a UF of 10 for intraspecies variability; and a UF
of 3 for database deficiencies. In addition, a partial UF of 3 was used to extrapolate from
subchronic to chronic exposure. This partial value was selected based on the observation that
hematological fluctuations such as reductions in RBCs and WBCs in the high-dose mice were
noted at interim sacrifice (14 days) as well as at termination (91 days), suggesting that the
responses occurred early in the exposure cycle and then remained comparatively unchanged.
Thus, an overall UF of 3x3x10x3x3 = 1000 was used in this comparison analysis.
Finally, for the comparison analysis based on the NOAEL from the Ward et al. (1985)
subchronic inhalation study, the following UFs were used: 3 for interspecies extrapolation for
inhalation studies, 10 for intraspecies variability, 3 for database deficiencies, and 3 for
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subchronic-to-chronic extrapolation, as above. Therefore, an overall UF of 300 was used in this
comparison analysis.
MF = None. No modifying factor was considered necessary.
	I.B.4. Additional Studies/Comments (Inhalation RfC)
Benzene is toxic by all routes of administration. Hematotoxicity and immunotoxicity have been
consistently reported to be the most sensitive indicators of noncancer toxicity in both humans
and experimental animals, and these effects have been the subject of several reviews (Aksoy,
1989; Goldstein, 1988, Snyder et al., 1993; Ross, 1996; U.S. EPA, 2002). The bone marrow is
the target organ for the expression of benzene hematotoxicity and immunotoxicity. Neither
gastrointestinal effects from oral exposure nor pulmonary effects due to inhalation exposure have
been reported.
Chronic exposure to benzene results in progressive deterioration in hematopoietic function.
Anemia, leukopenia, lymphocytopenia, thrombocytopenia, pancytopenia, and aplastic anemia
have been reported after chronic benzene exposure (Aksoy, 1989; Goldstein, 1988). In an earlier
follow-up study of benzene-exposed workers, Aksoy et al. (1972) reported that 8 of 32 workers
who had been diagnosed with pancytopenia died, mainly from infection and bleeding. In contrast
to these blood cellularity depression effects, benzene is also known to induce bone marrow
hyperplasia. Acute myelogenous leukemia has been frequently observed in studies of human
cohorts exposed to benzene, and there is evidence linking benzene exposure to several other
forms of leukemia. Whether the hematotoxic/immunotoxic effects of benzene exposure and its
carcinogenic effects are due to a common mechanism is not yet known. This is in part due to the
fact that although the bone marrow depressive effects of exposure to benzene in humans can be
readily duplicated in several experimental animal model systems, a suitable experimental animal
system for the induction of leukemia has not been found. The hematotoxicity/immunotoxicity
effects of benzene exposure lead to significant health effects apart from potential induction of
leukemia, as several deaths due to aplastic anemia have been reported (ATSDR, 1997).
Leukocytopenia has been consistently shown to be a more sensitive indicator of benzene toxicity
in experimental animal systems than anemia, and lymphocytopenia has been shown to be an
even more sensitive indicator of benzene toxicity than overall leukocytopenia (Snyder et al.,
1980, Ward et al., 1985; Baarson et al., 1984). Rothman et al. (1996) also found that a decrease
in ALC was the most sensitive indicator of benzene exposure in a group of workers. Ward et al.
(1996) observed a strong relationship between benzene exposure and decreased WBC counts in a
rubber worker cohort, but no significant relationship with RBC counts was found.
Bogardi-Sare et al. (2000) found that exposure to benzene concentrations of less than 15 ppm can
induce depression of circulating B-lymphocytes. Dosemeci et al. (1996) were able to
demonstrate the presence of benzene poisoning (WBC <4000 cells/mm3 and platelet count
"3
<80,000/mm ) at levels of exposure in the 5-19 ppm range.
As is the case with many other organic solvents, benzene has been shown to produce neurotoxic
effects in test animals and humans after short-term exposures to relatively high concentrations
(U.S. EPA, 2002). The neurotoxicity of benzene, however, has not been extensively studied, and
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no systematic studies of the neurotoxic effects of long-term exposure have been conducted.
Additionally, there is some evidence from human epidemiologic studies of reproductive and
developmental toxicity of benzene, but the data did not provide conclusive evidence of a link
between exposure and effects (U.S. EPA, 2002). Some test animal studies provide limited
evidence that exposure to benzene affects reproductive organs; however, these effects were
limited to high exposure concentrations that exceeded the maximum tolerated dose (U.S. EPA,
2002). Results of inhalation studies conducted in test animals are fairly consistent across species
and have demonstrated that at concentrations of greater than 150 mg/m3 (47 ppm) benzene is
fetotoxic and causes decreased fetal weight and/or minor skeletal variants (U.S. EPA, 2002).
Exposure of mice to benzene in utero has also been shown to cause changes in the hematogenic
progenitor cells in fetuses, 2-day neonates, and 6 week-old adults (Keller and Snyder, 1986,
1988).
For more detail on Susceptible Populations, exit to the toxicohwcal review. Section 4.4
(PDF).
	I.B.5. Confidence in the Inhalation RfC
Study — Medium
Database — Medium
RfC — Medium
The overall confidence in this RfC assessment is medium. The principal study of Rothman et al.
(1996) was well conducted, and the availability of good-quality human data for a sensitive
endpoint eliminates the uncertainty associated with basing the RfC on experimental animal data.
In addition, the RfC of 3 x 10~2 mg/m3 obtained from the BMD modeling results from the
3	3
Rothman et al. (1996) study is in good agreement with the value of 9 x 10" mg/m based on the
LOAEL. The RfC is also in good agreement with the values of 5 x 10~2 mg/m3 and 6 x 10"2
"3
mg/m based on the BMC and the NOAEL, respectively, from the Ward et al. (1985) subchronic
rodent inhalation study. This consistency in results provides increased confidence in the RfC.
With continuous endpoints such as hematological parameters, there is uncertainty about when a
change in a parameter that has inherent variability becomes an adverse effect. Other uncertainties
explicitly recognized in the quantitative derivation include intraspecies variability (to
accommodate sensitive human subgroups), subchronic-to-chronic extrapolation, and database
deficiencies due to the lack of two-generation reproductive and well-conducted developmental
toxicity studies for benzene.
For more detail on Characterization of Hazard and Dose Response, exit to the toxicohwcal
review. Section 6 (PDF).
	I.B.6. EPA Documentation and Review of the Inhalation RfC
Source Document — U.S. EPA, 2002.
This assessment was peer reviewed by external scientists as well as in response to public
comments. Their comments have been evaluated carefully and incorporated in the finalization of
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this IRIS summary. The peer review document (12 pages, 135 Kbytes) is available in Adobe
PDF format.
Other EPA Documentation — None
Date of Agency Consensus — January 23, 2002
	I. II. 7. EPA Contacts (Inhalation RfC)
Please contact the IRIS Hotline for all questions concerning this assessment or IRIS, in general,
at (202)566-1676 (phone), (202)566-1749 (FAX) or hotline.iris@epa.gov (internet address).
Top of page
VI. Bibliography
Benzene
CASRN —71-43-2
Last Revised — 04/17/2003
VI.A. Oral RfD References
Aksoy, M. 1989. Hematotoxicity and carcinogenicity of benzene. Environ. Health Perspect. 82:
193-197.
Goldstein, B.D. 1988. Benzene toxicity. Occupational medicine. State of the Art Reviews. 3:
541-554.
Hsieh, G.C., R.P. Sharma, andR.D.R. Parker. 1988. Subclinical effects of groundwater
contaminants. I. Alteration of humoral and cellular immunity by benzene in CD-I mice. Arch.
Environ. Contam. Toxicol. 17: 151-158.
NTP (National Toxicology Program). 1986. Toxicology and Carcinogenesis Studies of Benzene
(CAS No. 71-43-2) in F344/N Rats and B6C3F1 Mice (Gavage Studies). NTP, Research
Triangle Park, NC.
Ross, D. 1996. Metabolic basis of benzene toxicity. Eur. J. Haematol. 57: 111-118.
Rothman, N., G.L. Li, M. Dosemeci, W.E. Bechtold, G.E. Marti, Y.Z. Wang, M. Linet, L.Q. Xi,
W. Lu, M.T. Smith, N. Titenko-Holland, L.P. Zhang, W. Blot, S.N. Yin, and R.B. Hayes. 1996.
Hematotoxicity among Chinese workers heavily exposed to benzene. Am. J. Ind. Med. 29: 236-
246.
Sabourin, P.J., B.T. Chen, G. Lucier, L.S. Birnbaum, E. Fisher, and R.F. Henderson. 1987. Effect
of dose on the absorption and excretion of [C14]benzene administered orally or by inhalation in
rats and mice. Toxicol. Appl. Pharmacol. 87: 325-336.
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Sabourin, P.J., W.E. Bechtold, W. Griffith, L.S. Birnbaum, G. Lucier and R.F. Henderson. 1989.
Effect of exposure concentration, exposure rate, and route of administration on metabolism of
benzene by F344 rats and B6C3Fi mice. Toxicol. Appl. Pharmacol. 99: 421-444.
Snyder, R., G. Witz, and B.D. Goldstein. 1993. The toxicology of benzene. Environ. Health
Perspect. 100: 293-306.
U.S. EPA (U.S. Environmental Protection Agency). 1985. Final Draft for Drinking Water
Criteria Document on Benzene. Office of Drinking Water, Washington, DC. PB86-118122.
U.S. EPA. 1989. Workgroup for Risk Assessment Guidance for Superfund. Volume 1. Human
Health Evaluation Manual. Part A. Office of Solid Waste and Emergency Response,
Washington, DC. EPA/540/1-89/002.
U.S. EPA. 1994. Methods for derivation of inhalation reference concentrations and application of
inhalation dosimetry, EPA/600/8-90/066F, dated October, 1994.
U.S. EPA. 1999. Extrapolation of the Benzene Inhalation Unit Risk Estimate to the Oral Route
of Exposure. National Center for Environmental Assessment, Office of Research and
Development, Washington, DC. NCEA-W-0517.
U.S. EPA. 2000. Benchmark Dose Technical Guidance Document (External Review Draft).
EPA/63 0/R-00/001.
U.S. EPA. 2002. Toxicological Review of Benzene (Noncancer Effects). Available online at:
www.epa.gov/iris.
White, K.L. Jr., H.H. Lysy, J. A. Munson, et al. 1984. Immunosuppression of B6C3F1 female
mice following subchronic exposure to benzene from drinking water. TSCA 8E Submission.
OTS Fiche # OTS0536214.
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VLB. Inhalation RfC References
Aksoy, M. 1989. Hematotoxicity and carcinogenicity of benzene. Environ. Health Perspect. 82:
193-197.
Aksoy, M., K. Dincol, K. Erdem, T. Akgun, and G. Dincol. 1972. Details of blood changes in 32
patients with pancytopenia associated with long-term exposure to benzene. Br. J. Ind. Med. 29:
56-64.
ATSDR (Agency for Toxic Substances and Disease Registry) 1997. Toxicological profile for
benzene (Update). Public Health Service, U.S. Department of Health and Human Services,
Atlanta, GA.
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Baarson, K.A., C.A. Snyder, and R.E. Albert. 1984. Repeated exposure of C57B1 mice to
inhaled benzene at 10 ppm markedly depressed erythropoietic colony formation. Toxicol. Lett.
20: 337-342.
Bogardi-Sare, A., M. Zavalic, I. Trosic et al. 2000. Study of some immunological parameters in
workers occupationally exposed to benzene. Int. Arch. Occup. Environ. Health. 73: 397-400.
Cronkite, E.P., R.T. Drew, T. Inoue and J.E. Bullis. 1985. Benzene hematotoxicity and
leukemogenesis. Am. J. Ind. Med. 7: 447-456.
Dosemeci, M., S-N. Yin, M. Linet et al. 1996. Indirect validation of benzene exposure
assessment by association with benzene poisoning. Environ. Health Perspect. 104(Suppl. 6):
1343-1347.
Goldstein, B.D. 1988. Benzene toxicity. Occupational medicine. State of the Art Reviews. 3:
541-554.
Keller, K.A. and C.A. Snyder. 1986. Mice exposed in utero to low concentrations of benzene
exhibit enduring changes in their colony forming hematopoietic cells. Toxicology. 42: 171-181.
Keller, K.A. and C.A. Snyder. 1988. Mice exposed in utero to 20 ppm benzene exhibit altered
numbers of recognizable hematopoietic cells up to seven weeks after exposure. Fund. Appl.
Toxicol. 10: 224-232.
Ross, D. 1996. Metabolic basis of benzene toxicity. Eur. J. Haematol. 57: 111-118.
Rothman, N., G.L. Li, M. Dosemeci, W.E. Bechtold, G.E. Marti, Y.Z. Wang, M. Linet, L.Q. Xi,
W. Lu, M.T. Smith, N. Titenko-Holland, L.P. Zhang, W. Blot, S.N. Yin, and R.B. Hayes. 1996.
Hematotoxicity among Chinese workers heavily exposed to benzene. Am. J. Ind. Med. 29: 236-
246.
Snyder, C.A., B.D. Goldstein, A.R. Sellakumar, I. Bromberg, S. Laskin, and R.E. Albert. 1980.
The inhalation toxicity of benzene: Incidence of hematopoietic neoplasms and hematoxicity in
AKR/J and C57BL/6J mice. Toxicol. Appl. Pharmacol. 54: 323-331.
Snyder, R., G. Witz, and B.D. Goldstein. 1993. The toxicology of benzene. Environ. Health
Perspect. 100: 293-306.
U.S. EPA (U.S. Environmental Protection Agency). 1994. Methods for Derivation of Inhalation
Reference Concentrations and Application of Inhalation Dosimetry. Prepared by the Office of
Health and Environmental Assessment, Research Triangle Park, NC. EPA/600/8-90/066F.
U.S. EPA. 2000. Benchmark Dose Technical Guidance Document (External Review Draft).
EPA/63 0/R-00/001.
U.S. EPA. 2002. Toxicological Review of Benzene (Noncancer Effects) (CAS No. 71-43-2).
Available online at: www.epa.gov/iris.
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Ward, E., R. Hornung, J. Morris, R. Risnsky, D. Wild, W. Halperin, and W. Guthrie. 1996. Risk
of low red or white blood cell count related to estimated benzene exposure in a rubberworker
cohort (1940-1975). Am. J. Ind. Med. 29: 247-257.
Ward, C.O., R.A. Kuna, N.K. Snyder, R.D. Alsaker, W.B. Coate, and P.H. Craig. 1985.
Subchronic inhalation toxicity of benzene in rats and mice. Am. J. Ind. Med. 7: 457-473.
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