Benzo[a]pyrene is the most well known polycyclic aromatic hydrocarbon (PAH) in a large group of organic compounds with two or more fused aromatic rings. PAHs are formed mainly as a result of incomplete combustion of organic materials during industrial and other human activities. These activities include processing of coal and crude oil, combustion of natural gas, combustion of refuse, vehicle traffic, cooking and tobacco smoking, as well as natural processes such as forest fires. Motor vehicle exhaust and re-suspension are major contributors of PAHs, including benzo[a]pyrene, to urban air. PAHs in urban air have also been associated with residential wood burning emissions.
Benzo[a]pyrene will tend to be incorporated onto particulates during cooling and condensation in the atmosphere. Consequently, benzo[a]pyrene will generally exist in the particle phase at normal ambient temperatures in the atmosphere. Particle sizes will be mostly ≤2.5 µm in aerodynamic diameter. Processes governing the fate of benzo[a]pyrene in the atmosphere are the same processes that govern transport and removal of these small particles from the atmosphere.
Primary endpoints of toxicity associated with chronic benzo[a]pyrene exposures at doses >1 mg/m in animals are carcinogenicity, reproductive toxicity, and developmental toxicity. Other on-carcinogenic effects reported at doses >100 mg/m to animals include growth inhibition, immunosuppression, liver damage, and irritation/hypersensitivities. Severity of the effects depends on dose, administration of dose (route and vehicle of administration), and animal species, age and genotype. Epidemiological studies in coke-oven, coal-gas, and aluminum production workers have provided evidence of the role of inhaled benzo[a]pyrene at doses >0.1 mg/m in the causation of lung cancers.
PAHs are common contaminants of terrestrial and aquatic ecosystems. Plants acquire PAHs, including benzo[a]pyrene, by absorption through their roots. Once entry has occurred, it is thought that PAHs can then be translocated to all plant tissues. PAHs can also enter the plant by foliar deposition. Toxicological studies for defining potential hazards of PAHs have been conducted in the absence of UV radiation. Current evidence indicates that coexposure of UV light and near UV light and specific PAHs, including benzo[a]pyrene, can result in increased toxicity to plants.
Inhalation exposure to single PAH compounds, for example benzo[a]pyrene alone, does not occur without other PAHs being present. Several PAHs with four of more rings are treated as having the potential to cause cancer in addition to benzo[a]pyrene. As a result, benzo[a]pyrene is proposed as an indicator for the carcinogenic fraction of these PAHs which are all present as mixtures in ambient air. Further, a method using factors of 10 to represent the potency of individual PAHs relative to benzo[a]pyrene is recommended to address mixtures of PAHs in ambient air.
The ability of benzo[a]pyrene and other PAHs to contribute to health effects in the general human population as a result of low-level environmental exposure is much less certain than what has been reported in animal studies and in the workplace. As a result regulatory agencies adopt air quality guidelines for benzo[a]pyrene and other PAHs in order to protect humans. Benzo[a]pyrene concentrations in indoor air, in air outside of homes, and in urban (city) air reported in scientific literature tend to be within the range of concentrations representing air quality guidelines developed by regulatory agencies for protection of human receptors.