Benzo[a]pyrene & PAHs Toxicology and Cancer Risk

Chemical-Specific Toxicology Page — Benzo[a]pyrene (representing PAHs)

Benzo[a]pyrene Toxicology

RoseyFields evaluates the toxicology of benzo[a]pyrene (BaP) using the following evidence base. BaP is used throughout this page, and by RoseyFields generally, as the index/benchmark compound for the broader class of carcinogenic polycyclic aromatic hydrocarbons (PAHs), consistent with EPA’s and ATSDR’s risk assessment frameworks for this chemical class. This page summarizes BaP’s physicochemical properties, toxicokinetics, mechanism of action, animal toxicological and carcinogenicity findings, and the relationship between animal and human evidence, drawing on the peer-reviewed primary literature underlying EPA’s 2017 IRIS Toxicological Review of Benzo[a]pyrene.

2. Chemical Identity and Physicochemical Properties Relevant to Toxicity

Benzo[a]pyrene (CAS No. 50-32-8) is a five-ring polycyclic aromatic hydrocarbon and one of more than 100 PAH congeners formed during the incomplete combustion of coal, oil, wood, and other organic matter; it is also present in coal tar, creosote, roofing tar, and tobacco smoke. PAHs as a class are lipophilic, environmentally persistent, and, because they typically co-occur as complex mixtures rather than single compounds, are evaluated toxicologically using potency equivalency factors (PEFs) that express each congener’s carcinogenic potency relative to BaP, the benchmark compound selected because of the depth of toxicological data available for it and the frequency of human exposure to it (Collins et al., 1998). BaP itself has no functional groups that react directly with cellular macromolecules; like benzene, its toxicity depends entirely on metabolic bioactivation, a point addressed further in Sections 3 and 4.

Because PAHs are always encountered as mixtures with varying congener compositions, RoseyFields’ chemical-identity analysis for a PAH-contaminated site begins by identifying which congeners were measured, applying the relevant potency equivalency factors to derive a BaP-equivalent concentration for the mixture, and using that BaP-equivalent value as the basis for the toxicological and dose-response analysis that follows — a mixture-to-index-compound approach conceptually similar to, but methodologically distinct from, EPA’s toxic equivalency factor approach for dioxin-like compounds.

3. Toxicokinetics and Species/Sex Considerations Relevant to Animal Data

BaP is absorbed following oral, inhalation, and dermal exposure and is then rapidly and extensively metabolized — primarily by cytochrome P450 (CYP) enzymes, with only a minor fraction excreted unchanged. Because BaP itself is not directly reactive, essentially all of its toxicological and carcinogenic activity depends on the extent and pathway of this metabolic activation, discussed in detail in Section 4.

BaP and its metabolites are widely distributed in maternal and fetal tissues and readily cross the placenta, and BaP is also detectable in human breast milk, particularly among smokers (Madhavan and Naidu, 1995; Withey et al., 1993). Despite this placental transfer, evaluations of BaP distribution and elimination in lactating animals do not indicate that neonatal lactational exposure is magnified relative to maternal exposure (Lavoie et al., 1987b; West and Horton, 1976).

A key toxicokinetic consideration for interpreting animal carcinogenicity data is species and tissue differences in metabolic capacity. Mice have historically been the most extensively studied species in BaP dermal carcinogenesis studies because of evidence that murine skin may be more sensitive than that of other species to BaP-induced tumorigenesis, though rigorous cross-species comparisons of lifetime dermal sensitivity remain limited. A human-skin-graft xenograft study in immunocompromised mice found no tumors after topical BaP application, in contrast to universal tumor development in non-grafted mouse skin controls, a result the study authors interpreted as evidence that human skin is less susceptible than mouse skin to BaP induced carcinogenesis — potentially reflecting genuine toxicokinetic and toxicodynamic differences (better DNA repair capacity, slower cell turnover, generally slower toxicokinetics in human tissue) rather than an artifact of the model, though the short observation period (limited to 6 months, versus a human squamous cell carcinoma latency in PAH-exposed cohorts typically exceeding 20 years) makes this model’s negative predictive value for human skin cancer risk uncertain (Urano et al., 1995).

Genetic variability in the aryl hydrocarbon receptor (AhR), which governs inducibility of the CYP enzymes responsible for BaP bioactivation, is a second toxicokinetic factor with direct relevance to interpreting animal data across strains. Mouse strains with high-affinity AhR alleles (e.g., C57BL/6) show BaP-induced renal and cardiovascular effects not seen in strains with lower-affinity AhR alleles (e.g., D2N/DBA), and this same polymorphism modulates the severity of BaP-enhanced atherosclerosis in genetically susceptible (ApoE-/-) mice (Nanez et al., 2011; Godschalk et al., 2003). In humans, the analogous CYP1A1 polymorphism that increases enzyme inducibility has been associated with elevated PAH- and BPDE-DNA adduct levels in exposed workers, indicating that this same source of interindividual variability in metabolic bioactivation operates in humans (Alexandrov et al., 2002; Bartsch et al., 2000).

4. Mechanism(s) of Action

BaP’s carcinogenicity operates through a well-characterized mutagenic mode of action, in which metabolic bioactivation generates DNA-reactive intermediates that form mutagenic DNA adducts, leading to characteristic mutations in tumor suppressor genes and oncogenes that drive tumor initiation. Three parallel metabolic activation pathways have been identified.

Diol epoxide pathway

The best-characterized and most potent pathway proceeds through initial CYP1A1-, CYP1B1-, and CYP1A2- catalyzed epoxidation of BaP, followed by epoxide hydrolase-mediated conversion to trans-dihydrodiols, one of which (BaP-7,8-diol) is further oxidized to the potent DNA-binding metabolite benzo[a]pyrene-7,8-diol 9,10-epoxide (BPDE) (Grover, 1986). The stereoisomer (+)-anti-BPDE is the most potent DNA-binding form and reacts preferentially with the exocyclic amino groups of deoxyguanosine and deoxyadenosine, forming the characteristic BPDE-N2-deoxyguanosine (BPdG) adduct (Geacintov et al., 1997). These adducts, if not repaired before replication, give rise predominantly to G→T transversion mutations (Boström et al., 2002).

Radical cation pathway

A second pathway involves one-electron oxidation of BaP by CYP or peroxidase enzymes to form electrophilic radical cation intermediates, which form unstable, depurinating DNA adducts at the N-3/N-7 positions of adenine and the C-8/N-7 positions of guanine (Cavalieri and Rogan, 1995).

o-Quinone/reactive oxygen species pathway

A third pathway, mediated by aldo-keto reductase enzymes, converts BaP-7,8-diol to BaP-7,8-dione and other o-quinone metabolites that are potent cytotoxins and weak mutagens capable of forming both stable and depurinating DNA adducts, and that also generate reactive oxygen species through redox cycling, producing additional oxidative DNA damage (Penning et al., 1999).

Downstream mutational consequences and human relevance

BPDE-DNA adducts preferentially form at, and their repair is comparatively inefficient at, the same codon positions in the p53 tumor suppressor gene that constitute mutational hotspots observed in human lung cancers, directly linking the animal/in vitro mechanistic data to human tumor genetics (Denissenko et al., 1996). G→T transversions — the signature mutation of the diol epoxide pathway — are the predominant p53 and K-ras mutation type observed in lung tumors from smokers and from individuals exposed to PAH-rich smoky coal emissions, with the frequency of this mutation type increasing with smoking intensity (Bennett et al., 1999). BPDE-DNA adducts have been directly detected in human placental tissue, confirming that this activation pathway operates in vivo in humans, not only in animal models (Manchester et al., 1988). Together, this evidence — spanning bacterial mutagenicity assays, mammalian cell systems, animal bioassays with dose concordant adduct formation and tumor development, and human biomonitoring and tumor-genetics data — satisfies EPA’s framework for establishing a mutagenic mode of action and supports EPA’s use of age dependent adjustment factors (ADAFs) in BaP cancer risk assessment, reflecting the generally understood greater carcinogenic susceptibility of early-life mutagenic exposures.

5. Animal Toxicological Evidence by Organ System

BaP has been evaluated in animals for developmental, reproductive, immunological, hepatic, renal, and cardiovascular effects, primarily via the oral route in rats and mice. EPA’s 2017 IRIS assessment treats the developmental and immunological evidence as sufficiently robust to inform hazard identification, while treating the hepatic, renal, and cardiovascular evidence as suggestive but not yet sufficient to support a firm conclusion of human hazard, given the more limited number of studies using environmentally relevant exposure routes and durations.

Developmental and reproductive toxicity

Decreased embryo/fetal survival, reduced litter size, reduced offspring body weight, and impaired offspring fertility are the most consistently reported developmental findings. Gavage exposure of pregnant mice to BaP on gestation days (GDs) 7–16 produced an approximately 40% decrease in litter size at 160 mg/kg-day, with no effect at 10 or 40 mg/kg-day (Mackenzie and Angevine, 1981), while inhalation exposure of pregnant F344 rats on GDs 11–20 produced a dose-related increase in embryo/fetal resorptions beginning at the lowest tested concentration of 25 µg/m3 (Archibong et al., 2002). Offspring fertility is affected at doses as low as 10 mg/kg-day: F1 females from BaP-exposed dams showed a greater than 30% decrease in fertility and a 20% decrease in litter size when mated with untreated males, and gestationally exposed F1 males showed a 35% decrease in fertility when mated with untreated females (Mackenzie and Angevine, 1981), findings corroborated by a second mouse study reporting 63% fewer litters and 30% smaller litters among F1 females (Kristensen et al., 1995). A notable developmental cardiovascular finding is a dose-related increase in offspring systolic (up to 50%) and diastolic (up to 80%) blood pressure following gestational gavage exposure (Jules et al., 2012).

Immunotoxicity

Subchronic oral exposure produces consistent thymic and splenic effects: decreased thymus weight (up to 62%) and increased incidence of thymic atrophy have been observed in rats given 10–90 mg/kg-day for 35– 90 days (Kroese et al., 2001; De Jong et al., 1999), together with reduced splenic cellularity driven primarily by decreased B-cell numbers (up to 61% reduction) (De Jong et al., 1999). Functional immunosuppression has also been demonstrated: subcutaneous BaP exposure produced dose-related decreases in antigen-specific serum IgM following sheep red blood cell challenge in both rats and mice (Temple et al., 1993), and reductions in serum IgA and IgG have been reported following short-term parenteral exposure in mice.

Hepatic, renal, and cardiovascular effects

Increased liver weight (15–29%) and dose-dependent induction of hepatic CYP1A1 activity (measured as ethoxyresorufin-O-deethylase, or EROD, activity) are the most consistently reported hepatic findings, observed across multiple subchronic gavage studies in rats, though these changes were generally not accompanied by clear histopathological correlates except at the highest doses tested, where oval cell hyperplasia was observed (Kroese et al., 2001; De Jong et al., 1999). Renal findings are less consistent: decreased kidney weight was observed in one 35-day rat study but not replicated in a similarly designed study, while increased incidence of abnormal renal tubular casts was reported in F344 rats following 90-day dietary exposure (De Jong et al., 1999; Knuckles et al., 2001); a gestational exposure study found that renal effects (decreased podocyte number, increased urine albumin) were strain- and AhR-genotype-dependent, occurring only in a high-AhR-affinity mouse strain (Nanez et al., 2011). Cardiovascular findings in adult animals are limited to studies combining BaP with an atherogenic diet or in genetically atherosclerosis-prone (ApoE-/-) mice, which show modest enhancement of atherosclerotic lesion development and aortic DNA adduct formation (Uno et al., 2014; Godschalk et al., 2003), together with an increase in serum LDL-cholesterol following short-term gavage exposure in otherwise healthy rats (Chung and Jung, 2003).

6. Animal Carcinogenicity Bioassay Findings

BaP is carcinogenic in every species and by every route in which it has been adequately tested — oral, inhalation, and dermal — producing tumors at both the site of first contact and, in some studies, at distant sites. The most extensive oral database comes from a 2-year dietary bioassay in female B6C3F1 mice, which found dose-related, statistically significant increases in forestomach papillomas/squamous cell carcinomas (46/47 at the highest dose vs. 1/48 in controls), together with esophageal and tongue papillomas/carcinomas (Beland and Culp, 1998; Culp et al., 1998); a parallel 2-year rat bioassay using both gavage and dietary administration similarly found dose-related forestomach tumors, some malignant (Brune et al., 1981). The only lifetime inhalation bioassay, conducted in Syrian golden hamsters, found concentration-dependent tumors of the larynx and trachea beginning at 9.5 mg/m3, together with tumors of the nasal cavity, pharynx, and esophagus that the study authors attributed to mucociliary clearance of inhaled particles into the upper digestive tract (Thyssen et al., 1981). Dermal bioassays, conducted predominantly in mice because of evidence of greater sensitivity in that species, consistently demonstrate dose-dependent skin tumor induction with decreasing latency at higher doses, across multiple mouse strains and dosing regimens (Poel, 1959, and subsequent replications by other laboratories).

7. Comparison of Animal-Derived Findings to Human Epidemiological Evidence

Human and animal evidence for BaP converge clearly on the respiratory tract, urinary bladder, and skin as target sites, mirroring the tissue-level tropism seen in the animal bioassays (upper respiratory/digestive tract in the hamster inhalation study; skin in the mouse dermal studies). The strongest human evidence comes from three geographically and industrially independent occupational cohort studies — spanning aluminum reduction plants and other PAH-exposed industries — each of which found increasing lung cancer risk with increasing cumulative BaP exposure (measured in µg/m3-years) after accounting for smoking, with roughly a two-fold increased risk at the highest exposures (Armstrong and Gibbs, 2009; Spinelli et al., 2006; Xu et al., 1996). For bladder cancer, occupational cohorts of aluminum smelter workers found an exposure-response relationship at cumulative exposures above roughly 80 µg/m3-years (Gibbs et al., 2007), while a study of asphalt pavers with a substantially narrower and lower exposure range (up to approximately 2 µg/m3-years) did not find an association (Burstyn et al., 2007) — a pattern the EPA Working Group considered consistent with a genuine threshold-like exposure-response rather than conflicting evidence, since the asphalt workers’ entire exposure range corresponded only to the lowest exposure categories examined in the smelter-worker studies. For skin cancer, studies of creosote-exposed timber workers and brick makers found standardized incidence ratios of 1.5–2.4 for non-melanoma skin cancer (Karlehagen et al., 1992; Törnqvist et al., 1986), and a cohort of asphalt roofers employed 20 or more years showed a standardized mortality ratio of 4.0 for the same outcome (Hammond et al., 1976) — findings that likely underestimate true risk because non-melanoma skin cancers are rarely fatal and are frequently excised before entering mortality or even some incidence registries.

At the mechanistic level, the human and animal evidence is unusually tightly linked for a carcinogen: the same BPDE-DNA adduct chemistry and the same G→T transversion mutational signature at the same p53 codons are documented in both BaP-treated laboratory animals and in lung tumors from PAH- and tobacco-smoke exposed humans (Denissenko et al., 1996), a degree of mechanistic concordance between species that is relatively uncommon and that substantially strengthens confidence that the animal bioassay findings are directly relevant to human carcinogenic risk, notwithstanding species differences in absolute potency.

Where the animal and human evidence diverge is less in direction than in tissue specificity and magnitude. The animal oral bioassays are dominated by forestomach tumors — a tissue with no direct human anatomical equivalent (humans lack a discrete forestomach), and one recognized to be susceptible to local, high concentration irritant/gavage-vehicle effects in rodent bioassays generally — while human epidemiological evidence for BaP-associated cancer is concentrated in the lung, bladder, and skin. This divergence is attributable primarily to differences in the site of first contact and local metabolic activation capacity between rodent forestomach and human upper GI/respiratory tissue, rather than to any difference in fundamental carcinogenic mechanism, and RoseyFields treats the forestomach tumor data as supportive evidence of BaP’s general carcinogenic potential rather than as directly predictive of a specific human tumor site. Species differences in dermal sensitivity (Section 3) likewise mean that the substantial database of mouse skin painting bioassays, while clearly establishing BaP as a complete dermal carcinogen, may somewhat overstate the potency of dermal BaP exposure in humans relative to what the epidemiological skin cancer data would independently suggest.

8. Data Gaps and Areas of Ongoing Research

Several data gaps are relevant to RoseyFields’ risk assessment and litigation support work involving BaP and PAH mixtures.

• Liver, kidney, and cardiovascular effects remain insufficiently characterized to support a firm conclusion regarding human hazard. Available studies are limited in number, use predominantly high, non-environmentally-relevant doses, and in the case of cardiovascular effects, rely heavily on genetically susceptible (ApoE-/-) mouse models or short-duration protocols rather than chronic, environmentally relevant exposure regimens in wild-type animals.
• Human cardiovascular epidemiology is confounded by concurrent exposure to complex PAH mixtures and other combustion byproducts; the two most informative occupational studies (Friesen et al., 2010; Burstyn et al., 2005) relied on statistical exposure-reconstruction models rather than direct BaP measurement and could not fully account for co-exposures in the occupational settings studied.
• Comparative cross-species sensitivity to dermal carcinogenesis has not been rigorously quantified; the human skin-graft xenograft evidence (Urano et al., 1995) is suggestive that human skin may be less susceptible than mouse skin, but methodological limitations (short observation period, absence of an adequate surgical control) prevent a confident quantitative cross-species comparison.
• The relative contribution of BaP specifically, versus co-occurring PAH congeners, to the cancer risk observed in human cohorts cannot be fully isolated in any available epidemiological study, since all identified epidemiological studies involve mixed PAH exposure; this is the central rationale for, and also a persistent limitation of, the potency-equivalency-factor approach used to translate mixture exposures into a BaP-equivalent dose.
• Non-melanoma skin cancer risk in PAH-exposed cohorts is likely underestimated in the existing epidemiological literature because of low case-fatality and common preventive excision of

precancerous lesions, meaning the true magnitude of dermal PAH carcinogenic risk in humans may exceed what current registry-based studies suggest.

• PAH congeners without established potency equivalency factors — including several classified by IARC as possibly (Group 2B) or probably (Group 2A) carcinogenic, such as cyclopenta[c,d]pyrene, benzo[c]phenanthrene, and dibenzo[a,l]pyrene derivatives — are excluded from BaP-equivalent risk calculations entirely, representing an acknowledged gap in current PAH mixture risk assessment methodology pending further toxicological characterization.

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