The use of biomonitoring data in exposure and human health risk assessment: benzene case study.

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by  Arnold, Scott M.

http://informahealthcare.com/txc
ISSN:1040-8444(print),1547-6898(electronic)
CritRevToxicol,2013;43(2):119–153
!2013InformaHealthcareUSA,Inc.DOI:10.3109/10408444.2012.756455
REVIEW ARTICLE
The use of biomonitoring data in exposure and human health risk
assessment: benzene case study
ScottM.Arnold1,JuergenAngerer2,PeterJ.Boogaard3,MichaelF.Hughes4,RaeganB.O’Lone5,StevenH.Robison6,
and A. Robert Schnatter7
1TheDowChemicalCompany,Midland,MI,USA,2BGFA–Forschungsinstitutfu¨rArbeitsmedizinderDeutschenGesetzlichenUnfallversicherung,
Erlangen,Germany,3ShellInternationalB.V.,TheHague,theNetherlands,4UnitedStatesEnvironmentalProtectionAgency,ResearchTrianglePark,
NC,USA,5ILSIHealthandEnvironmentalSciencesInstitute,Washington,DC,USA,6TheProcterandGambleCompany,Cincinnati,OH,USA,and
7ExxonMobilBiomedicalSciences,Inc.,Annandale,NJ,USA,
Abstract Keywords
Aframeworkof‘‘CommonCriteria’’(i.e.aseriesofquestions)hasbeendevelopedtoinformthe Biomarkersofexposure,biomonitoring,ben-
use and evaluation of biomonitoring data in the context of human exposure and risk zene,cancer,riskassessment
assessment. The data-rich chemical benzene was selected for use in a case study to assess
whetherrefinementoftheCommonCriteriaframeworkwasnecessary,andtogainadditional
History
perspective on approaches for integrating biomonitoring data into a risk-based context. The
Received17August2012
availabledataforbenzenesatisfiedmostoftheCommonCriteriaandallowedforarisk-based
Revised30November2012
evaluationofthebenzenebiomonitoringdata.Ingeneral,biomarker(bloodbenzene,urinary
Accepted4December2012
benzene and urinary S-phenylmercapturic acid) central tendency (i.e. mean, median and
geometricmean)concentrationsfornon-smokersareatorbelowthepredictedbloodorurine
concentrationsthatwouldcorrespondtoexposureattheUSEnvironmentalProtectionAgency
referenceconcentration(30mg/m3),butgreaterthanbloodorurineconcentrationsrelatingto
theairconcentrationatthe1(cid:2)10(cid:3)5excesscancerrisk(2.9mg/m3).Smokersclearlyhavehigher
levels of benzene exposure, and biomarker levels of benzene for non-smokers are generally
consistentwithambientairmonitoringresults.Whilesomebiomarkersofbenzenearespecific
indicators of exposure, the interpretation of benzene biomonitoring levels in a health-risk
context are complicated by issues associated with short half-lives and gaps in knowledge
regardingtherelationshipbetweenthebiomarkersandsubsequenttoxiceffects.
TableofContents BenzeneandBradfordHillCriteria....................................... 129
Abstract .................................................................................119 CommonCriteriaandEpidemiology.................................... 129
Introduction ...........................................................................120 Biomarkers/AnalyticalMethodology ..........................................130
Exposure.................................................................................121 Benzene ........................................................................... 130
Primarysourcesandroutesofexposure.............................. 121 Analyticalmethodology...................................................130
Temporalityanddurationofexposure................................. 122 Biomarkerconcentrations................................................130
CommonCriteriaandexposure .......................................... 122 Conclusionforbenzeneasabiomarkerofexposure .........131
Toxicology/Toxicokinetics.........................................................123 S-Phenylmercapturicacid ................................................... 131
Toxicology ........................................................................ 123 Analyticalmethodology...................................................131
Hematotoxicity...............................................................123 Biomarkerconcentrations................................................133
Genotoxicity ..................................................................... 124 Backgroundsources ......................................................133
Carcinogenicity.................................................................. 124 ConclusionforS-phenylmercapturicasabiomarkerof
Carcinogenicity–Humans .............................................124 exposure ..................................................................133
Carcinogenicity–Animals................................................124 Trans,trans-muconicacid................................................... 133
Toxicokinetics .................................................................. 125 Analyticalmethodology...................................................133
Absorption.....................................................................125 Biomarkerconcentrations................................................133
Distribution ..................................................................125 Backgroundsources ......................................................134
Metabolism ..................................................................125 Conclusionfortrans,trans-muconicacidasabiomarkerof
Elimination.....................................................................126 exposure ..................................................................134
ModeofAction.................................................................. 127 Phenol,CatecholandHydroquinone ................................. 134
PharmacokineticModels ................................................... 127 Analyticalmethodology...................................................134
CommonCriteriaandToxicology/Toxicokinetics .................. 128 Biomarkerconcentrations................................................135
Epidemiology...........................................................................129 Backgroundsources ......................................................135
Addressforcorrespondence:ScottArnold,TheDowChemicalCompany,1803Building,Midland,MI48674,USA.Tel:þ19896364843.Fax:þ1989
6382425.E-mail:[email protected]
120 S. M. Arnold et al. CritRevToxicol,2013;43(2):119–153
Conclusionforphenol,catecholandhydroquinoneasa lifestyle factors such as diet, smoking and hobbies), it can
biomarkerofexposure .............................................135 provide valuable perspective to help evaluate aggregate
DNAandProteinAdductsofBenzene................................. 135
exposure to chemicals (Angerer et al., 2006, 2007; Pirkle
DNAadductsofbenzene ................................................136
et al., 1995). Traditionally, biomonitoring data are used to
Proteinadductsofbenzene.............................................136
ConclusionforDNAandproteinadductsasabiomarker assess the efficacy of control measures in occupational
ofexposure...............................................................137 settings. Biomonitoring data are now one commonly used
CommonCriteriaandBiomarkers/AnalyticalMethodology ... 137 tool to determine chemical exposure in the general popula-
RiskAssessment/RiskCharacterization .......................................138
tion. Biomonitoring data can also be used to assess the
Relevanttoxicologydata ................................................... 138
effectiveness of environmental remediation efforts. Thus,
Relationshipbetweenthebiomarkerofexposureand
knownhumanhealtheffect .......................................... 138 when collecting biomonitoring data, one must also consider
PharmacokineticData......................................................... 139 the overall objectives of the evaluation. For example,if there
Benchmarks ..................................................................... 139 is a need tounderstand exposure, then determining the levels
Generalpopulationbenchmarks.......................................139
ofthechemicaland/oritsmetabolitesinanappropriatematrix
Noncancerendpoints......................................................139
(e.g. blood and urine) may be sufficient. Alternatively, if the
Cancerendpoints............................................................139
Occupationalbenchmarks................................................140 biomonitoringdataaretobeusedtounderstandhealtheffects,
RiskCharacterization ......................................................... 140 considerably more information would be needed. For asses-
Generalpopulationexposure ..........................................140 sing risk to a population, in addition to the exposure
Bloodbenzene ...............................................................140
assessment data that biomonitoring may provide, additional
UrinarybenzeneandSPMA.............................................142
information including sources and pathways of exposure and
Urbanworkerexposure...................................................143
Industrialworkerexposure .............................................144 toxicology are needed.
CommonCriteriaandRiskAssessment/RiskManagement ... 144 The Biomonitoring Technical Committee of the
SummaryandConclusions ......................................................145 International Life Sciences Institute (ILSI) Health and
Acknowledgements..................................................................146
Environmental Sciences Institute (HESI) held an
DeclarationofInterest ............................................................146
International Biomonitoring Workshop in 2004. At this
References ..............................................................................146
workshop, a framework of ‘‘Common Criteria’’ (i.e. a series
of key questions, Table 1) was developed to inform the use
Introduction and evaluation of the use of biomonitoring data in exposure
and human health risk assessment. The criteriawere outlined
Human biomonitoring can be an effective tool for assessing
for the following categories (Albertini et al., 2006):
exposure to a variety of chemicals. As biomonitoring data
(cid:4) exposure,
integratesallroutes(inhalation,dermalandoral)andsources
(cid:4) toxicology/toxicokinetics,
of exposure (i.e. including occupational, environmental and
Table1. Biomonitoringcommoncriteria.
Analyticalmethodology/ Riskassessment/risk
Exposure Toxicology/toxicokinetics Epidemiology biomarkerofexposure management
Source(s)identified? Aretheresufficientdata Arereasonablecause–effect Werestandardreference Aretheresufficientand
includinglongerdura- inferencessupported?* materialsusedinthe relevanttoxicologydata
tionstudies? biologicalmatrixof
interest?
Pathway(s)/route(s) Doroutesusedintoxicol- Hasanadversehealtheffect Havespecificityandsensi- Knownrelationship
understood? ogystudiescompareto beenobservedin tivityofmethodsbeen betweenbiomarkerof
anticipatedhuman humans? described? exposureandhuman
exposure? healtheffect?
Humanexposurerela- Aretoxicokineticdatain Hasthepathogenesisofthe Isbiomarkerofexposure Applicabletoxicokinetic
tionshiptoexisting animalsavailable? healtheffectbeen validforintendeduse?y data?
toxicologydata described?
Exposure–doserelation- Is/arethecriticaleffect(s) Isthereahealtheffectin Doessamplingstrategy Ifapplicable–evidencethat
shipunderstood? known? theexposedpopulation consider potential remediationeffortsare
sourcesoferror? working?
Temporality/duration Isthemode/mechanismof Havetoxicokineticand/or Doessamplingstrategy
understoodz actionunderstood? toxicodynamicgenetic considerstabilityofbio-
polymorphismsbeen markerofexposure?
describedwhichmay
impactrisk?
Didsamplingstrategy
incorporate
toxicokinetics?
*AretheBradford-Hillcriteriafulfilled?
yDoesthebiomarkerofexposureaccuratelyreflecttheintendeduse?
zTemporalityreferstotherelationshipofwhenexposureoccurredrelativetowhenthesamplewascollected.Durationreferstohowlongtheexposure
occurredrelativetowhenthesamplewascollected.
DOI:10.3109/10408444.2012.756455 Benzene biomonitoring data for risk assessment 121
(cid:4) epidemiology, For occupational settings, where the primary exposure
(cid:4) analytical methods/biomarkers ofexposure and routesareinhalationanddermal,exposureassessmentcanbe
(cid:4) risk assessment/risk management. relatively straightforward if the quantity of material used and
Theframeworkemergedthroughtheevaluationofsixcase the work area is relatively well defined. In contrast,
studies of chemicals with varying physical–chemical proper- assessment of benzene exposure for the general population
tiesanddataavailability(Barr&Angerer,2006;Birnbaum& is harder to quantify because individual lifestyles are
CohenHubal,2006;Butenhoffetal.,2006;Calafat&McKee, extremely variable, ambient weather conditions can impact
2006; Hughes, 2006; Robison & Barr, 2006). As a follow-up exposure, and living environments are more diverse. In non-
to the 2004 workshop, the ILSI HESI Biomonitoring occupational settings, inhalation of benzene is the primary
Technical Committee selected benzene, a data-rich com- exposureroutewith minor contributionfromdermaland oral
pound,to assess whether refinement ofthe Common Criteria sources. Outdoor ambient air concentrations of benzene are
framework was necessary, and to gain additional perspective dependent on geographical location (i.e. rural versus urban)
on approaches for integrating biomonitoring data into a risk- (Wallace, 1996). Recent surveys in the San Francisco area
based context. (Harley et al., 2006), Mexico City and Puebla, Mexico
Several biomonitoring datasets are available for benzene, (Tovalin-Ahumada&Whitehead,2007)andinFlorence,Italy
including the Centers for Disease Control and Prevention’s (Fondelli et al., 2008) indicate ambient air concentrations of
(CDC) National Health and Nutrition Examination Survey about 0.2–2mg/m3 (San Francisco), 7mg/m3 (Puebla), 44mg/
(NHANES). There is a wealth of published human epide- m3 (Mexico City) and 2.3–7mg/m3 (Florence). US national
miology, exposure and toxicology data on benzene (ATSDR, trends (1994–2009) indicate a 66% decline in the average
2007; International Agency on Research in Cancer (IARC), ambient air benzene concentration (2.7–0.9mg/m3) for 22
1982,1987;Johnsonetal.,2007).Benzenehasbeenasubject urbanmonitoringsites(USEnvironmentalProtectionAgency
of recent reviews on exposure, health effects and biomarkers (USEPA), 2010). Natural sources of benzene in air include
(Bird et al., 2010; Galbraith et al., 2010; HEI, 2007; Johnson volcanoesandforestfires.Anthropogenicsourcesofbenzene
etal.,2007;Snyder,2012;VCEEP,2006;Weisel,2010).This inairincludecombustiblefuelemissions,exhaustfrommotor
paper applies the Common Criteria to the benzene data to vehicles and evaporation of gasoline and solvents, especially
examine the relationship between benzene exposure and in attached garages, industry or hazardous waste sites, and
human health risk. home products (e.g. paint). The range of urban, rural, indoor
and personal air benzene concentrations vary from 20- to
Exposure about1000-foldintheUSandEurope(Figure1AandB;HEI,
2007andBruinendeBruin,2008).Thiswidevariabilityinair
Human biomonitoring data integrates all sources of possible
benzene concentrations shown in Figure 1(A) (data from
exposuretoachemical,butitdoesnotprovideinformationon
Table 4, HEI, 2007) is due to many factors such as sample
individual routes of exposure. This is because biomonitoring
location (e.g. rural versus urban; outdoor versus indoor),
generally makes an assessment of the exposure by quantitat-
season and time of measurement (e.g. winter; afternoon),
ingabiomarkerofexposureinblood,urineorotherbiological
number of observations, average sampling time and other
media.Thus,itisdifficulttomakeaninformeddecisiononan
factors (e.g. mean versus maximum concentrations). This
individualroute ofexposure. Ifavailable, additional informa-
variability emphasizes the point that public health scientists
tion concerning the primary sources, routes of exposure and
need to be cognizant of the sources of their data when
temporal variability will help inform sampling
assessingthepotentialadversehealtheffectsfromexposureto
strategies, interpret the health implications of the data and
environmental toxicants.
provide the basic information for advice to limit exposure, if
An overarching consideration for both occupational and
necessary.
generalpopulationsourcesofexposuretobenzeneexposureis
tobaccosmoking.Benzeneconcentrationscanbe10–20times
Primary sources and routes of exposure
higher in exhaled breath of cigarette smokers than in non-
Several recent reviews summarize the sources of benzene smokers (Gordon et al., 2002). For cigarette smokers,
exposure(ATSDR,2007;Johnsonetal.,2007;VCEEP,2006; smoking accounts for about 90% of this group’s exposure to
Weisel, 2010). The two primary sources of industrial benzene (Wallace, 1996). For non-smokers, environmental
exposure to benzene are activities associated with the tobaccosmoke,dependinguponlifestyleandlocalrestrictions
production and synthesis of benzene and the use of benzene on smoking, can be a significant source of benzene exposure
tosynthesizeotherchemicals.Anumberofotheroccupations (Wallace, 1996). Thus, any biomonitoring study of benzene
such as aviation workers, service station workers (Carrieri must take into account smoking history and whether
et al., 2006), bus drivers, police (Capleton & Levy, 2005), individuals are exposed to second-hand smoke.
cargotankerworkers(Kirkeleitetal.,2006a,b),urbanworkers Considerablylower exposures tobenzene (usually51% of
(Fustinonietal., 2005a,b;Maninietal., 2006)andfishermen the total body burden) can occur from consumption of food,
(Kirraneetal.,2007)maybeexposedtobenzenethroughthe water and beverages (Wallace, 1996). Benzene is detected in
use of petroleum products. Exposure to benzene in solvents raw,processedandcookedfoodswithconcentrationsranging
hasalsobeendemonstratedforshoeproductionworkers(Kim from1to190partsperbillion(ppb,mg/kg)(Fleming-Jones&
etal.,2006a,b;Wangetal., 2006).Theairconcentrations for Smith, 2003). It has been suggested that the presence of
these various occupational settings ranged from 1mg/m3 to benzene in food is by its uptake from air (Grob et al., 1990).
over 1000mg/m3. Currently available data indicate that the mean concentration
122 S. M. Arnold et al. CritRevToxicol,2013;43(2):119–153
(A) (B) 1000
100
e
n
e 10
z
n
e
B
3
m 1
g/
μ
0.1
0.01
Personal Exposure Indoor Offices Indoor Ho m e Outdoor W ork
Figure1. (A)BenzeneambientairconcentrationsintheUSA,HEI(2007).ReprintedwithpermissionfromtheHealthEffectsInstitute,Boston,MA.
(B) Benzene ambient air concentrations in European metropolitan areas. Adapted from Bruinen de Bruin et al. (2008) with kind permission from
SpringerScienceþBusinessMedia.
of benzene in drinking water is 0.27mg/L (95th percentile fueling automobiles or yard equipment (e.g. lawn mowers,
¼0.5mg/L and maximum¼355mg/L) (ATSDR, 2007). Page snow blowers or other gasoline powered equipment) can
et al. (1993) detected benzene in one of 182 bottled drinking result in transient exposures to air concentrations of benzene
water samples at a concentration of 2mg/L. Dermal uptake that approximate occupational air concentrations (Egeghy
canalsocontributetosystemicexposurefollowingcontactof et al., 2000).Inaddition,exposuretosolventsused incertain
the skin with solvents or fuels containing benzene. The site hobbies may result in higher cyclic exposure to benzene.
and surface area of the skin contacted is of particular Lifestyle choices such as cigarette smoking or frequent
importance when evaluating occupational skin exposure to exposure to second-hand smoke must also be factored in to
benzene and fuels or solvents containing benzene. any consideration of background exposure to benzene.
The assessment of benzene exposure must take into
Temporality and duration of exposure considerationthatbenzeneanditsmetaboliteshaverelatively
shorthalf-lives((cid:5)16h)(Quetal.,2000;Rickertetal.,1979).
There is some understanding of temporality and duration of
Additionally, biomonitoring data have documented daily and
occupational exposures since workers typically have defined
weekly human intra- and inter-individual variability for
schedules with known durations of contact with benzene,
benzene and other volatile organic chemicals (Sexton et al.,
benzene in the air, or solvents containing benzene. It is more
2005). The sampling strategy and analysis of the data should
difficult to establish patterns of exposure for the general
also take this into consideration.
populationsinceavarietyoffactorsimpactexposureandlead
to greater variability in exposure (Johnson et al., 2007).
Common Criteria and exposure
Backgroundambientairconcentrationsofbenzeneareknown
orcanbedeterminedforthegeneralpopulationandarehighly Overall, large amounts of data are available to adequately
dependent on the living environment of the population being address the Common Criteria questions related to exposure
evaluated. For example, mean ambient air levels of benzene (Table1).Theprimarysourcesofoccupational(chemicaland
arereportedtorangefrom0.6–0.7mg/m3inruralsettingsand petroleum industries, use of organic solvents in the manu-
0.3–3.9mg/m3 in urban settings (HEI, 2007). Thus, indivi- facturingindustry,proximitytocombustionoffuelsandother
dualslivingincitieswilllikelyhavehigherbackgroundlevels flammable material) and non-occupational (tobacco smoke,
of benzene when compared with individuals living in a rural refueling of combustion engines, emissions from combustion
environment. Knowledge of the ambient benzene air con- engines) exposures are well characterized. The predominant
centrations may provide information about a somewhat pathway of benzene occupational and non-occupational
constant low-level background exposure with little variation exposure is inhalation. Dermal exposure to benzene is more
intemporalityandduration.Incontrast,highershort-duration of an occupational than non-occupational concern. With
exposuresareoftenmoredifficulttoassesssincetheytendto regard to relating human exposure to animal toxicology
occur at irregular intervals. Ambient air concentrations are studies (Table 1), there are a number of inhalation toxicity
most often reported as an average concentration measured studies of varying duration in rats and mice (ATSDR, 2007;
overan8h,24horlongerduration.Therefore, thesetypesof National Toxicology Program (NTP), 1986; VCEEP, 2006),
measures should be considered average levels with no and this topic is covered in ‘‘toxicology/toxicokinetics’’
information on temporal variation within the sampling time section. In addition, there is a relatively good understanding
frame or over more extended time periods. Activities such as of the exposure–dose relationship for benzene toxicity in
DOI:10.3109/10408444.2012.756455 Benzene biomonitoring data for risk assessment 123
animals and humans following inhalation exposure as there one of the most sensitive parameters to benzene exposure.
are a number of physiologically-based pharmacokinetic Rothman et al. (1996) reported a decrease of the absolute
(PBPK) models for benzene following inhalation exposure, lymphocyte count in a group of workers exposed to 8ppm
whichareevaluatedinthe‘‘Pharmacokineticmodel’’section. benzene (median 8h TWA). The USEPA (2003) used data
Temporality and duration of exposure to benzene are not from this study to derive the benzene inhalation reference
well studied, but are recognized to be important in the concentration (RfC) of9.4ppb(30mg/m3).
exposure assessment of benzene. It is recognized that More recent studies have examined exposures to lower
exposures to benzene vary with regard to temporality and levelsofbenzenethanthosereportedbyRothmanetal.(1996).
duration across occupations, geographic regions and popula- Lan et al. (2004) examined shoe manufacturers exposed to
tions.Patternswithinoccupationalexposurescanbeassessed benzene over a 16-month period. There was also a group of
morecloselythannon-occupationalexposures.Biomonitoring age- and gender-matched controls not exposed to benzene
results for short-lived compounds such as benzene indicate (5limit of detection (LOD) of 0.04ppm). The workers were
large intra-individual daily variability in exposure and grouped according to their exposure (control,51, 1–510 and
represent only recent exposure. Care must be taken to (cid:6)10ppm,meanlevelsover1monthmonitoringperiod).Blood
extrapolate the results to long-term exposure. Therefore, was drawn from these individuals and hematological evalua-
benzeneexposuresstudiesneedtobecarefullydesignedifthe tionswereconducted.Inthelowestexposuregroup,leukocyte
goal is to allow interpretation of benzene biomonitoring data andplateletcountsweresignificantlydecreasedrelativetothe
in a health-risk context. This is discussed further in ‘‘risk controlvalues (8–15% lower). In the highest exposure group,
assessment/risk characterization’’section. these cells were decreased 15–36% compared to controls.
However,Lamm&Grunwald(2006)reanalyzedtheLanetal.
data and concluded that while hematotoxicity was demon-
Toxicology/toxicokinetics
strated at benzene concentrations greater than 10ppm, it was
Toxicology inconsistentatlowerconcentrations.IntwostudiesbyCollins
etal.(1991,1997),nosignificantcorrelationwasobservedin
The presence of a chemical in the body does not itself mean
workers between benzene exposure (range, 0.01–1.4 ppm;
that the chemical will cause harm (CDC, 2009). The
mean, 0.55ppm, 8h TWA) and prevalence of abnormal
concentration of a chemical detected in the body needs to
becomparedtoconcentrationsknowntocausehealtheffects. hematological values. More recently, Swaen et al. (2010)
There is awealth of literature on the human health effects of assessed low-level occupational exposure to benzene and its
effect on hematological parameters. Approximately 20000
benzene; indeed, it is one of the most studied compounds in
blood samples of non-exposed and benzene-exposed workers
commerce due to its ubiquity in the environment, the ability
wereanalyzedandtherewereasimilarnumberofbenzeneair
toidentifyitatlowlevelsandconcernaboutitshealtheffects
measurements. Depending on the job and operational status,
(ATSDR, 2007; Ahmad Khan, 2007; Krewski et al., 2000;
themean8hTWAbenzeneairconcentrationrangedfrom0.14
Snyder, 2002, 2007; Snyder et al., 1993; World Health
to0.92ppm.Therewasnodifferencebetweenthelymphocyte
Organization (WHO), 1993). The health effects of benzene
are primarily documented from inhalation exposure. For counts of exposed and non-exposed groups. There were no
short-term acute toxicities, outcomes from benzene exposure differences in hematological parameters (e.g. hemoglobin,
hematocrit, white blood cells) among exposed subgroups
can range from dizziness to death. For chronic toxicities,
(50.5ppm,0.5–1ppmand41ppm)andwiththenon-exposed
major outcomes include hematotoxicity and cancer. For the
group. Schnatter et al. (2010) examined a largepopulation of
purposes of this paper, only the key endpoints used to derive
shoe and rubber workers from Shanghai, China. Exposure to
chronicbenzenetoxicologicalbenchmarksforriskassessment
benzene ranged from 0.02 to 273ppm. These authors found
are summarized.
decrementsinmostbloodcelltypes,whilefittingchangepoint
regressionmodelstoeachcelltype.Theseanalysessuggested
Hematotoxicity
decreased blood cell counts down to approximately 8ppm,
The hematopoietic system is the most critical target tissue benzene; but, clear signals below this concentrationwere not
followinginhalationexposuretobenzeneineitherhumansor evident. This concentration is similar to the concentration
animals. A reduction in the number of the three major blood reportedasano-effectlevelinRothmanetal.(1996),although
components, erythrocytes (anemia), leukocytes (leukopenia) the Schnatter et al. (2010) study is much larger. Thus, the
and platelets (thrombocytopenia), can develop following concentration of benzene in air where hematological effects
exposure to benzene. This effect has been noted in humans begintooccurarestillbeingdebated,althoughmorethanone
froma2doccupationalexposuretobenzeneataconcentration studysuggestshematologicaleffectsbegintoappearbetween5
greaterthan60ppm(partspermillion;Midzenskietal.,1992). and10ppm,andnosucheffectatlowerlevels.
Pancytopenia occurswhen there isa reduction in the number In cases of high exposure to benzene, aplastic anemia can
of more than one type of blood cell. These effects in blood develop. Aplastic anemia occurs when the bone marrow no
cellsarereversibleiftheexposuretobenzeneisremovedand longerfunctionsadequatelyandthestemcellsfromwhichthe
the individual provided medical assistance. Rothman et al. blood cells are derived are unable to mature. Aksoy et al.
(1996) observed decreases in counts of total leukocytes, (1971,1972)reportedontheeffectsofoccupationalexposure
platelets, lymphocytes and red blood cells and hematocrit in tobenzenecontainedinadhesives.Theworkerswereexposed
workersexposed tobenzene (median level:31ppm,8h time- to benzene for 5 months to 17 years. Recordings of working
weightedaverage(TWA)).Lymphocytecountisthoughttobe environment benzene exposures reached 210ppm, and in
124 S. M. Arnold et al. CritRevToxicol,2013;43(2):119–153
some cases, 640ppm. In one study (Askoyet al., 1971), with reported a RR of 9, while workers exposed to over
maximum exposures up to 210ppm, 25% of the study 1000ppm-years showed an unprecedented RR of 83 AMML
participants displayed hematological effects including leuko- (Crump, 1994). One of the several large occupational studies
penia,thrombocytopenia,eosinophiliaandpancytopenia.Yin in China reported significant increased RR of benzene-
et al. (1987a) documented 24 cases ofaplastic anemia out of exposed workers for all hematologic neoplasms (RR¼2.6),
over 500000 individuals exposed occupationally to benzene all leukemias (RR¼2.5) and acute non-lymphocytic leuke-
mixed with other solvents or benzene alone. In the latter mia (RR¼3.0) (Hayes et al., 1997). For lower benzene
exposures, the workplace benzene air concentration ranged exposures, risks are less clear, as expected. For example,
from 0.02 to 264ppm. Schnatter et al. (1996a,b), Rushton & Romaniuk (1997) and
Decreased blood cell counts are observed in laboratory Glass et al. (2003) have all studied petroleum workers where
animals following repeated acute, intermediate and chronic benzeneexposurerangeduptoapproximately220ppm-years,
inhalation exposure to benzene. Anemia and lymphocytope- with averageexposures mostly below5ppm. Resultsof these
niawere observed in mice chronicallyexposed (26 weeks) to studies have been inconsistent. Limitations of many of the
302ppm benzene (Green et al., 1981). Additional studies in published occupational studies are that exposures to other
animals show there are effects on bone marrow cellularity solvents occur with benzene exposure, exposure monitoring
(hypo- and hypercelluarity) and colony forming stem cells, was inadequate in some cases and the overall low number of
which are indicative of the development of aplastic anemia study subjects in several of the studies decreased the
(ATSDR, 2007). Snyder et al. (1978a, 1980) reported a statistical power of the association (ATSDR, 2007).
20% and 81% incidence of bone marrow hypoplasia in Occupationalexposuretobenzenehasalsobeenrelatedto
mice exposed to benzene for life at 100 or 300ppm, the development of myelodysplastic syndrome (MDS; Irons
respectively. et al., 2005, 2010; Schnatter et al., 2012). MDS is a diverse
arrayofneoplasticdisorderscharacterizedbyvaryingdegrees
Genotoxicity ofpancytopeniaanddysplasiaofmyeloidcells.Itspathogen-
esisisnotwellunderstood,althoughMDSandAMLareoften
Benzene is considered not mutagenic in bacterial systems or
observed as secondary cancers subsequent to treatment with
in vitro mammalian test systems. However, benzene is
chemotherapeutic agents (Smith et al., 2003). MDS has also
reported to be genotoxic when evaluated in mammalian
been previously termed pre-leukemia since it can progress to
systems in vivo. Furthermore, there is evidence that benzene
AML. However, recent estimates report that only about 20–
induces DNA strand breaks in lymphocytes from humans
30%ofcasesprogresstoAML,thustheterm‘‘pre-leukemia’’
exposed to benzene (Andreoli et al., 1997; Nilsson et al.,
is outdated (Albitar et al. 2002). Since bone marrow smears
1996; Sul et al., 2002). There has been one evaluation of
are required for the definitive diagnosis of AML, manyearly
mutations in bone marrow cells from humans exposed to
studiesmayhavemisclassifiedMDSasAML,aplasticanemia
benzene (Rothman et al., 1995) using the glycophorin A
or other blood disorders (Layton & Mufti, 1986). There are
mutationassay.Theresultssuggestthatmutationsaccumulate
several different subtypes of MDS (e.g. refractory anemia;
in long-lived bone marrow stem cells. However, it was
refractory anemia with ringed sideroblasts) and the number
observed that gene duplication occurred as opposed to gene
has been changing as diagnostic tools have improved
inactivation (Rothman et al., 1995).
(Swerdlow et al., 2008). Only recent studies have examined
benzeneexposurewithrespecttoMDSsubtypes(Ironsetal.,
Carcinogenicity 2005, 2010). Using the WHO criteria for the diagnosis of
MDS (Swerdlow et al., 2008), Irons et al. (2010) found that
Carcinogenicity – humans
MDS-unclassifiable (MDS-U) case subtypes had high ben-
Occupational exposure to benzene via inhalation has been zeneexposurewhencomparedtocasesofMDSwithouthigh
associated through epidemiological studies with the develop- benzene exposure (odds ratio¼11.1). More recently,
ment of cancer (ATSDR, 2007; IARC, 1982, 1987; USEPA, Schnatter et al. (2012) suggested that lower benzene
1998). The cancer type is predominantly acute myeloid exposures may be related to MDS, although specific MDS
leukemia (AML), although there is suggestive evidence that subtypes were not examined.
other leukemia cell types, non-Hodgkin lymphoma and
multiple myeloma, may also develop (Hayes et al., 1997;
Carcinogenicity – animals
Rinskyetal.,1987;Schnatteretal.,2005).Thepliofilmstudy
(Rinsky et al., 1987) of occupational benzene exposure Benzene is carcinogenic in rats and mice following exposure
stronglysuggestsarelationshipwithleukemia,asthestandard via inhalation. Maltoni et al. (1989) observed carcinomas of
mortalityratiofordeathfromleukemiaoftheexposedcohort the Zymbal gland and oral cavity in male and female
was3.4andstatisticallysignificant.Crump(1994)performed Sprague–Dawley rats exposed to benzene at concentrations
additional analyses on these data and reported that for the of 200–300ppm. The animals were exposed for 4–7h/d, 5d/
development of acute myelogenous and monocytic leukemia week, up to 104 weeks. There were also small increases in
combined(termed‘‘AMML’’,ofwhichthemajorityisAML), hepatomasandcarcinomasofthenasalcavitiesandmammary
therelativerisk(RR)forcancerdeathbycumulativebenzene glands of these exposed animals. Mice develop thymic and
exposure was 5–6 and statistically significant. For higher lymphocytic lymphomas, Zymbal gland, lung and ovarian
exposures, the RRs were much higher. For example, workers tumors,andmyelogenousleukemiasfromchronicexposureto
exposed to benzene between 400 and 1000ppm-years benzene (Cronkite et al., 1984, 1989; Farris et al., 1993).
DOI:10.3109/10408444.2012.756455 Benzene biomonitoring data for risk assessment 125
Theassociationbetweenoccupationalexposuretobenzene Benzene can be absorbed through the skin. Modjtahedi &
and development of cancer as well a number of animal Maibach(2008)reportedthatinhumanvolunteers,0.07%and
bioassayswhere carcinogenic effectswere observedprovided 0.13% of the applied dose of benzene (0.1mL, dose area of
sufficient evidence whereby the IARC (1982, 1987), the 25.8cm2)wasabsorbedafterdirectapplicationtotheforearm
USEPA (1998) and the NTP (2005) of the Department of andpalm,respectively.Invitrodermalabsorptionofbenzene
Human and Health Services have classified benzene as a in monkey and mini-pig was 0.19% and 0.23% of the dose
known human carcinogen. (5mL/cm2), respectively (Franz, 1984). Benzene vapor has
alsobeenreportedtopenetrateanimalskin(McDougaletal.,
Toxicokinetics 1990; Tsuruta, 1989), which should be considered in
assessments of occupational exposure to benzene.
Toxicokinetics is important for helping understand the
relationship between exposure and the measured concentra-
tion of benzene and its metabolites in the body, and helps Distribution
relate the toxicological observations at a given dose level in
Following absorption, benzene distributes throughout the
animal studies to what might happen at similar exposure
body to a number of tissues. Benzene is detected in blood,
levels in humans.
brain,kidney,fat,liver,placenta,cordbloodandothertissues
There are a number of toxicokinetic studies available for
of individuals exposed by inhalation (Dowty et al., 1976;
benzene. Generally, there are more data from laboratory
Winek&Collom,1971).Animalstudiesshowthatbenzeneis
animalthanhumanstudies,withthelattereitherfromcasesof
distributed widely after inhalation, oral, or dermal exposure,
poisonings or a small number of human volunteers.
being detected in brain, fat, liver, kidney and in pregnant
Examination of the benzene data from the perspective of
animals, the placenta and fetus (Ghantous & Danielsson,
the absorption, distribution, metabolism and elimination, the
1986; Low et al., 1989; Rickert et al., 1979).
metabolic aspect is the best characterized. Since benzene has
a relatively short biological half-life (Yu & Weisel, 1996a),
understanding when exposure occurs relative to when blood, Metabolism
urine or expired air samples are collected is an important
The metabolism of benzene has been extensively studied in
component of the exposure and risk assessment.
humansandlaboratoryanimals(ATSDR,2007;Lovernetal.,
2001; Monks et al., 2010; Snyder, 2004). These include
Absorption
in vivo studies to identify metabolites of benzene as well as
The majority of absorption data for benzene is focused on in vitro studies to better understand the mechanism of its
inhalation as the route of exposure. Benzene is detected in metabolism. The mouse, rat and non-human primates share
blood of smokers, firefighters, mechanics and individuals in the same Phases I and II pathways of benzene metabolism
occupations that produce or use benzene, demonstrating with humans (Henderson, 1996). However, there are species
inhalation of benzene is an important route of exposure. A differences in the capacity of these pathways to metabolize
studywithhumanvolunteers(Srbovaetal.,1950)showedthat benzene which results in differences in the fractional
absorption of benzene is rapid at first, with approximately distribution of metabolites formed. These metabolites have
70%ormoreofthedoseabsorbedduringthefirstfewminutes potentially different toxicological potency. A scheme of the
of exposure. By 1h, absorption of benzene then declined, metabolic pathways of benzene and its metabolites is shown
whichisduetotheincreasingbenzeneconcentrationinblood, in Figure 2.
thus reducing the concentration gradient between benzene in Thefirststepinthemetabolismofbenzeneisoxidationto
airandblood.Overall,thegeneralthoughtisthattheextentof theintermediatebenzeneoxide.Thisoxidationiscatalyzedby
the absorption of benzene in humans via inhalation is about cytochromeP450(CYP)2E1.CYP2B4hassomeactivity,but
50% (can vary between 20% and 60%; USEPA, 2002). is less efficient than CYP2E1. Benzene oxide is in
Laboratory animals absorb benzene following inhalation equilibrium with the intermediate benzene oxepin. Benzene
exposure to a similar extent. At 10ppm, during a 6h oxide (or the oxepin) can undergo non-enzymatic rearrange-
exposure, rats and mice absorb and retain 33% and 50% of ment to phenol, hydrolysis to a dihydrodiol, ring opening to
the dose, respectively. The percentage of the dose absorbed trans,trans-muconic acid (ttMA) (which is believed to occur
decreases with increasing concentration of benzene in both through the intermediate trans,trans-muconaldehyde), or
species (Sabourin et al., 1987). Mice appear to absorb a react with glutathione to form a pre-mercapturic acid
greater cumulative inhaled dose of benzene than rats conjugate. The metabolites can undergo further metabolism
(Eutermoser et al., 1986; Sabourin et al., 1987). by oxidation, dehydrogenation or conjugation with sulfate or
Cases of accidental or intentional poisoning by ingestion glucuronic acid. For example, a primary metabolite of
indicatethat benzene isabsorbedsystemically because ofthe benzene oxidation, phenol, can undergo an additional
toxicity(centralnervoussystemeffects,death)thatdeveloped oxidationcatalyzedbyCYP2E1tohydroquinoneandcatechol
in the exposed individuals (Thienes, 1972). Benzene admi- or can be conjugated with sulfate to form phenyl sulfate.
nistered by gavage in corn or olive oil to animals is rapidly Hydroquinone and catechol can undergo further oxidation
(peakconcentrationat1h)andreadilyabsorbed((cid:6)90%ofthe catalyzed by peroxidases to their respective quinones. These
dose) (Low et al., 1989; Parke & Williams, 1953; Sabourin quinones, which are reactive species, can be reduced back to
et al., 1987). It is generally held that the extent of the hydroquinone and catechol by NAD(P)H:quinone oxidore-
absorption of benzene via the oral route is 100%. ductase (NQO).
126 S. M. Arnold et al. CritRevToxicol,2013;43(2):119–153
O OH O gly
OH benzene O
S S O
NH NH OH
[O] OH
O γγ-glu O
S-phenyl mercapturic acid GST + GSH CYP2E1 [O] t,t-muconaldehyde O
(S-PMA) t,t-muconic acid (ttMA)
OH
OH OH
EH EH
O O
+ H2O + H2O [O]
OH OH
benzene oxide benzene oxepin O
OH
epoxybenzenediol
DHDH DHDH
OH
OH OH
CYP2E1 CYP2E1
OH
catechol
phenol
OH
hydroquinone [O] [O]
NQO1 MPO
NQO1 MPO
O OH O
OH O
O OH o-benzoquinone
p-benzoquinone 1,2,4-trihydroxybenzene
Figure2. Aschematicoflivermetabolismofbenzene.FromBoogaard(2009);reproducedwithpermissionofJohnWiley&SonsLtd.EH,epoxide
hydrolase;GSH,glutathione;GST,glutathione-S-transferase;DHDH,dihydrodioldehydrogenase;MPO,myeloperoxidase;NQO1,NADPHquinone
oxidoreductase1.
Theimportance ofCYP2E1 inthe metabolismofbenzene Both enzymes were important in the metabolism of benzene,
was shown by Valentine et al. (1996). This group exposed although it appears that CYP2F1 has a higher affinity but
benzene to CYP2E1 knockout and wild-type mice via lower activity than CYP2E1 toward this hydrocarbon. This
inhalation. The urinary excretion of benzene metabolites difference in affinity/activity may explain the observation by
was significantly decreased in the knockout mice and there Valentine et al. (1996) that CYP2E1 knockout mice exposed
was a change in the metabolite distribution. Phenyl sulfate to benzene by inhalation do not display cytotoxicity or
levels in urine of the knockout mice increased significantly genotoxicity as opposed to similarly exposed wild-type (and
compared to wild-type mice, indicating that there are other B6C3F1) mice.
CYPsthatoxidizebenzeneandthatCYP2E1isalsoimportant CYP2E1 is detected in bone marrow of laboratory
in the metabolism of phenol. animals and humans (Bernauer et al., 1999, 2000; Powley
Metabolismofbenzeneoccursinorgansinadditiontothe & Carlson, 2000). However, the extent of benzene
liver. The liver would be the primary site for metabolism of metabolism in bone marrow, at least based on in vitro
benzene following oral absorption. For inhalation exposure, results, does not appear to be great (Irons et al., 1980;
the lung would be a major site of benzene metabolism. Lindstrom et al., 1999), which suggests that metabolites of
Chaney & Carlson (1995) compared the metabolism of benzene formed in other organs are the causative agents for
benzene by rat hepatic and pulmonary microsomes. The myelotoxicity.
hepatic microsomes metabolized benzene five times faster
than pulmonary microsomes. There was also a difference in
Elimination
the amount of metabolites formed. For example, hydroqui-
none comprised 2% and 39% of the metabolites formed in Benzene absorbed by inhalation but not metabolized is
hepatic and pulmonary microsomes, respectively. This primarily expired in the breath with much lower amounts
differencecouldbeduetotheCYPisozymesthatmetabolize (51%) excreted in the urine (Nomiyama & Nomiyama,
benzene.Invitrostudieswithlivermicrosomespreparedfrom 1974a,b; Srbova et al., 1950). Nomiyama & Nomiyama
CYP2E1 knockout and wild-type mice show that CYP2E1 is (1974a,b) reported that in human volunteers exposed to
the most important CYP isozyme in this preparation that benzenevapors(ca50–60ppmfor4h),approximately17%of
metabolizes benzene. In lung microsomes prepared from the theabsorbeddosewasexpiredasparentcompound.Similarly,
same mice, it appears that CYP2E1 and isozymes from the animals exhale unmetabolized benzene following inhalation
CYP2F subfamily metabolize benzene at equal rates (Powley andoralexposure(Rickertetal.,1979;Sabourinetal.,1987).
& Carlson, 2000). The roles of CYP2E1 and CYP2F1 in Severalbenzenemetabolitesareexcretedinurineandinclude
benzene metabolism were evaluated in bronchiolar- and conjugated phenol, hydroquinone, catechol and trihydroxy-
alveolar-derived human cell lines by Sheets et al. (2004). benzene. However, as the concentration of benzene in air or
DOI:10.3109/10408444.2012.756455 Benzene biomonitoring data for risk assessment 127
the administered oral dose increases, the metabolic pathways The critical data gaps described by Meek & Klaunig
become saturated, and greater levels of benzene are expired. (2010) include the need for additional perspective on
In mice orally administered 10 or 200mg/kg of radiolabeled oxidative damage in DNA and other critical cellular
benzene, urinary excretion was the predominant pathway of macromolecules or cells in humans, as well as how the
elimination at the low dose (McMahon & Birnbaum, 1991). benzene metabolites interact with cells to induce transforma-
The major urinary metabolites were hydroquinone glucur- tion, and the mechanism for mutation induction. An analysis
onide (40% of the dose), phenyl sulfate (28%) and ttMA ofconcordancebetweenhumanandanimaldatasuggeststhat
(15%). At the higher dose, the relative amount of urinary thereisgoodconcordanceforthehypothesizedcriticalevents
excretion decreased considerably and exhalation of volatile includingthemetabolismofbenzenetobenzeneoxideandthe
radioactivity increased. Fecal elimination was low for both clonalproliferatonofmutatedcells(Meek&Klaunig,2010).
doses.Followingdermalexposureofradiolabeledbenzenein There is a general consensus that the metabolism of benzene
humans and laboratory animals, benzene-derived radioactiv- has a role in its toxicity (Atkinson, 2009; Smith, 1996;
ity was detected in urine (Franz, 1984; Modjtahedi & Snyder,2004,2007).Butwhichofthemetabolite/metabolites
Maibach,2008;Skowronskietal.,1988).Insimilarlyexposed (benzene oxide, phenol, hydroquinone, ttMA, etc., Figure 2)
rats,benzene-derivedradioactivitywasdetectedinexpiredair is/are the causative agent(s) is not known. Effects that may
(Skowronski et al., 1988). occur from these metabolites include covalent binding to
critical macromolecules (i.e. proteins and DNA), generation
ofoxidantspeciesresultinginoxidativestress,impairmentof
Mode of action
tubulin,histone proteins,topoisomerase II andDNAitselfby
Acute and chronic adverse health effects can arise from protein-DNA cross-linking or DNA strand breakage; inter-
exposure to benzene. Most evidence is from inhalation ference with spindle formation and tubulin function that
exposure (ATSDR, 2007; HEI, 2007; Wallace, 1996). The segregate chromosomes during mitosis; chromosomal
critical acute effects are thoseoften associated with exposure abnormalities, particularly chromosome 5 and 7 (Regev
to many organic solvents and include narcosis, non-specific et al. 2012; Zhang et al., 1998a,b) which are affected in
central nervous system toxicity, respiratory depression and AML in general (Irons & Stillman, 1996) and others (Khan,
death (Khan, 2007; Snyder et al., 1993). The mode of action 2007; Smith, 1996; Snyder, 2007). Some of the metabolites
of the acute effects of benzene is not completely understood. may also interact with one another so that the toxic effect of
The critical non-cancer effect from chronic exposure to one is increased. Eastmond et al. (1987) reported that the
benzene is toxicity to the hematopoietic and immune system coadministration of phenol and hydroquinone to mice
(Lan et al., 2004; Rothman et al., 1996; Snyder, 2002). mimicked the myelotoxicity detected with exposure to
Benzene can cause a decrease of the three major circulating benzene.
cell types: platelets (thrombocytopenia), red blood cells There is some degree of understanding of the genotoxic
(anemia) and white blood cells (leukopenia) and an increase potentialofkeybenzene metabolites (Gaskellet al.,2005a,b;
inmeancorpuscularvolume(Quetal.,2002;Rothmanetal., Snyder,2007),butalinkagebetweenanyspecificmetabolites
1996). These effects tend to arise at benzene exposures and the carcinogenic effect of benzene in rodents or humans
exceeding 8ppm, but the effect is also influenced by the has not been elucidated. These include formation of DNA-
duration of exposure (Rothman et al., 1996; Schnatter et al., reactive metabolites, induction of oxidative DNA damage,
2010).If hematopoietic effects are detected soonenough, the inhibition of DNA topoisomerase II and interference or
effects are likely to be reversible upon cessation of benzene damage to mitotic apparatus. Hydroquinone can be oxidized
exposure. However, sustained exposure may result in to benzoquinone, which in turn can react with cellular
continued marrow depression involving multiple lineages. macromolecules, such as tubulin and histones or can lead to
This multi-lineage depression of blood counts is also known the formation of DNA addicts. In addition, catechol and
as pancytopenia (Snyder, 2000). Continued exposure may benzenetriol could contribute to the formation of reactive
eventually lead to damage to the bone marrow concomitant oxygen species. There are also data indicating that the
with pancytopenia, or aplastic anemia. Alternatively, MDS, metabolites hydroquinone and benzoquinone are human
which is characterized by abnormal maturation and develop- DNA topoisomerase II inhibitors (Hutt & Kalf, 1996;
ment of hematopoietic precursor cells in the bone marrow, Lindsey et al., 2005).
may result, and often is a precursor to leukemia, especially
AML (Layton & Mufti, 1986; Rossi et al., 2000; Snyder,
Pharmacokinetic models
2002). AML is characterized by uncontrolled proliferation of
immature myeloid cells. PBPK models provide the necessary link to translate the key
The mode of action was recently the subject of a 2009 toxicological endpoints (e.g. an external dose-based no
conference in Munich, Germany, which was subsequently observed adverse effect level) to an internal-based biomarker
published as series of papers in Chemico–Biological concentration (e.g. blood or urine). This information, there-
Interactions (Vol. 184, Issues 1–2, 2010). The key mode of fore, allows derivation of toxicologically-based biomarker
action events have been described as: (1) metabolism of concentrations that can be used to evaluate biomonitoring
benzene to benzene oxide, (2) interaction of this metabolite datainahumanhealthriskcontext.PBPKmodelsincorporate
with critical bone marrow cells, (3) initiated bone marrow physiological (e.g. blood flow) and biochemical processes
cells, (4) clonal proliferation of initiated cells and (5) (e.g. metabolic rates) to quantitatively describe the absorp-
development of leukemia (Meek & Klaunig, 2010). tion, metabolism, distribution and elimination of a chemical.
128 S. M. Arnold et al. CritRevToxicol,2013;43(2):119–153
Table2. HumanPBPKmodelparametersforbenzene(reproducedfrom toxicology data are derived frominhalation exposurestudies.
Brownetal.(1998)withpermissionfromJohnWiley&Sons).
The primary toxic effects of benzene in humans are: (1) the
suppressive effects on formation of the three main types of
Parameter Male Female
blood cells (erythrocytes, leukocytes and thrombocytes)
Bodyweight(kg) 70 60 resulting in hematotoxicity and on the immune system; and
Alveolarventilation(L/h) 450 363
(2)thedevelopmentofAML,whicharethekeyendpointson
Cardiacoutput(L/h) 336 288
which regulatory agencies such as the USEPA base their
Bloodflowfractions(%)
Liver 25 25 toxicological benchmarks. MDS is also a toxic effect of
Fat 8 8 concern from chronic benzene exposure. However, currently
Slowperfused(muscleandskin) 28.5 28.5 itisnotusedasakeyendpointbyregulatoryagencies,sinceit
Richperfused(brain,kidneyandheart) 38.5 38.5
has only recently been confirmed as a relevant endpoint, and
Tissuevolumefractions(%)
there are few exposure/response studies available.
Liver 2.6 2.3
Fat 20 30 Although benzene is one of the most extensively studied
Slowlyperfused 64 55 chemicals in the world from a toxicological standpoint, the
Richlyperfused 6 5 modeofactionisnotcompletelyunderstood.Itiswell-known
Partitioncoefficients that the metabolism of benzene is required prior to the
Blood/air 7.8 8.2
development of hematotoxicity and cancer, but the actual
Liver/blood 2.95 2.8
Fat/blood 54.5 51.8 metabolite(s) that is/are responsible and how the blood cells
Slowperfused/blood 2.05 2 areaffectedhavenotbeencompletelyelucidated.Theanimal
Richperfused/blood 1.92 1.8 data on the toxicity and disposition of benzene have some
Metabolicconstants relevance for human exposure to benzene. The effects of
Km–Michaelis–Menten(mg/L) 0.35 0.35
benzene on blood cells and the immune system observed in
V –maximumvelocity(mg/h) 13.89 19.47
max
animals are pertinent to similar observations in humans.
Animals are good models to use to study these two
toxicological effects that are observed in humans from
exposure to benzene. However, only humans have been
AnumberofPBPKmodelshavebeendevelopedforbenzene
clearly shown to develop AML from benzene exposure
inanimalsandhumans(Boisetal.,1996;Brownetal.,1998;
whereas animals develop different types of leukemia and
Sinclair et al., 1999; Travis et al., 1990; Yokley et al., 2006;
tumors in several organs. Because of these differences in the
andreviewedinATSDR,2007andVCEEP,2006).Ingeneral,
types of cancer developed between humans and animals
thesemodelsprovidegoodsimulationsofbenzenedisposition
exposed to benzene, the animal data may not lend itself to
inseveralspeciesfollowinganacuteinhaledoringestedacute
determinethemechanismofactionforthecarcinogeniceffect
dose and lactational transfer of benzene in humans. A major
of benzene in humans. While humans and animals have
limitationisthatmostofthesemodelsdonotsimulatethefate
common metabolic pathways for benzene, there are differ-
of metabolites and they have not been evaluated for repeated
ences between the preferred pathways, such that different
exposure to benzene. Recently, Hays et al. (2012) evaluated
amounts of metabolites, some potentially more toxic than
the existing human PBPK models for benzene (Bois et al.,
others are formed. Essentially, there does notappear tobe an
1996; Brown et al., 1998; Sinclair et al., 1999; Travis et al.,
animal model that mimics exactly human metabolism of
1990; Yokley et al., 2006) in derivation of a blood and
benzene (Henderson, 1996). However, animals are useful to
urinary-based benzene biomonitoring guidance value termed
gain a better understanding on how benzene and its
thebiomonitoringequivalent.Theauthorsselectedthemodel
metabolites distribute to target organs. Also, animals
of Brown et al. (1998) to estimate blood benzene concentra-
specificallybredtoimitatehumanpolymorphismsinbenzene
tions because of its simplicity, consistency with human
metabolizingenzymeswouldprovidevaluableinformationon
kinetic data and being gender specific accounting for
the susceptibility to the toxic effects of this compound. In
differences in body fat (Hays et al., 2012). The parameters
addition, humanized mouse models implanted with tissue-
usedforthemodelarelistedinTable2.Theapproachusedby
engineered human liver (Chen et al., 2011) could potentially
Haysetal.(2012)isusedinthe‘‘riskassessment’’sectionfor
develop leukemia following chronic exposure to benzene.
theinterpretationofbloodbenzenebiomonitoringdata.There
Although presently not known, the humanized mice
are currently no PBPK models available to predict urinary
could theoretically metabolize benzene differently than
benzene; therefore, linear regression equations that relate air
wild-type mice.
benzene concentrations to urinary biomarker concentrations
For human biomonitoring, ideally the metabolite(s)
are used to derive biomonitoring guidance values for urinary
responsible for benzene’s toxic effects would be monitored
benzene and a similar approach is used for S-phenylmercap-
inbloodorurineandrelatedtotheadversehealtheffect.This
turic acid (SPMA).
is not possible for benzene, but there are a number of human
PBPKmodelsthatprovidethelinkagebetweeninternal-based
Common Criteria and toxicology/toxicokinetics
benzene biomarker concentrations such as benzene in blood
The toxicology of benzene has been extensively studied in and the toxicological endpoints. This information allows
laboratory animals and in humans and most of the related derivation of toxicologically-based biomarker concentrations
Common Criteria questions were adequately addressed that can be used to evaluate biomonitoring data in a risk
(Table 1). Because of benzene’s volatility, the majority of assessment context. However, care must be taken when