Hydrocarbons in Ambient Air and Cancer

Polycyclic aromatic hydrocarbons (PAHs), which are generated from the incomplete combustion of organic (carbonaceous) material, are ubiquitous contaminants in ambient air (IARC, 1983, 1984a, 1984b, 1985, 2010; WHO, 1998). Their occurrence in the air we breathe has been substantial during the past centuries due to emissions from industrial processes and energy production, motor vehicular traffic, incineration of refuse, and residential heating. PAHs consist of two or more fused aromatic rings made up of carbon and hydrogen atoms. The ring systems can be present in multiple configurations and may be unsubstituted or substituted. PAHs range from semivolatile molecules to molecules with high boiling points. Thus, they may be found both in the gas and the particulate phase of ambient air or in mixtures of both phases. About 500 different PAHs have been detected in air, but often the measurements focus on benzo[a]pyrene (B[a]P) as a representative of the whole PAH family (WHO, 1998; Boström et al., 2002). Many of the PAHs in ambient air are carcinogenic (IARC, 1983, 1984a, 1984b, 1985, 2010) (Figure 7.1), and a recent reassessment of their carcinogenic potential led to B[a]P being upgraded to a Group 1 known human carcinogen (IARC, 2010). Thus there is considerable concern about the relationship between PAH exposure in the ambient air and the potential to contribute to human cancer incidence. The United States Environmental Protection Agency (EPA) monitors 16 priority PAHs in air due to health concerns: naphthalene, acenaphthylene, acenaphthene, fluorene, anthracene, phenanthrene, fluoranthene, pyrene, chrysene, benz[a] anthracene, benzo[b]fluoranthene, benzo[k] fluoranthene, B[a]P, indeno[1,2,3-cd]pyrene, benzo[g,h,i]-perylene, and dibenz[a,h]anthracene (in order of number of aromatic rings per structure) (Figure 7.1). Of particular note is that several PAHs (naphthalene, chrysene, benzo[b] fluoranthene, benzo[k]fluoranthene, B[a]P, dibenz[a,h]anthracene, dibenzo[a,e]pyrene and dibenzo[a,l]pyrene, and anthanthrene) have been found to be carcinogenic in experimental animals after inhalation or intratracheal ingestion, increasing concern about the levels of these carcinogens in ambient air (Figure 7.1). CHAPTER 7 . POLYCYCLIC AROMATIC HYDROCARBONS IN AMBIENT AIR AND CANCER