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Since then, long-term sampling data show that annual averages of BaP concentrations in major urban areas have dropped dramatically over a period of several decades. For example, at a roadside sampling site (1.5 m above ground level) located on Fleet Street, London, BaP fell from 39 ng m"3 in 1962-1963 to 10 ng m~3 in f972-f973 (Commings and Hampton, 1976), similar to 2.0 ng m~3 in 1987 measured at a site 5 m above ground in Central London (Baek et al., 1992). (Note that concentrations in urban areas tend to decrease with sampling height, i.e., with distance from mobile sources.) Key reasons for the major decline in BaP concentrations in London and other urban airsheds throughout the world have been the enactment and enforcement of "clean air" legislation and the trend to cleaner fuels (see, for example, NRC, f972, 1983; Hoffman and Wynder, f977; Bj0rseth, 1983; Grimmer, f983a, f983b; Holmberg and Ahlborg, 1983; Osborne and Crosby, 1987; Baek et al., f 992; California Air Resources Board, f994, f998; and references therein).

There is, however, a word of caution. Holmberg and Ahlborg point out in their 1983 Consensus Report: Mutagenicity and Carcinogenicity of Car Exhaust and Coal Combustion Emissions, "ft should be stressed, however, that a reduction in the BaP level does not necessarily mean a reduction in the potential health hazards, since the spectrum of pollutants has also changed with time." For example, in their long-term study of BaP concentrations and the bacterial mutagenicity of ambient particles collected in Sapporo, Japan, from 1974 to 1992, Matsumoto and co-workers (1998) reported that BaP levels dropped 75-80%, but the level of overall bacterial mutagenicity remained "relatively unchanged." Similar concerns with focusing solely on BaP have been expressed by other researchers (e.g., see Lane and Katz, 1977; Pitts, 1983;

Rosenkranz and Mermelstein, 1985a; Tokiwa and Ohnishi, 1986; Lewtas, 1993a; Atkinson and Arey, 1994; Rosenkranz, 1996; Nielsen et al., 1996; Finlayson-Pitts and Pitts, 1997; and Collins et al., 1998).

However, BaP has often been used as a "marker" for POM to set air quality and emission standards (e.g., see Nielsen et al., 1995). For example, the Netherlands Environmental Programme 1988-1991 draft document gives an "acceptable level" for the annual average concentration of BaP in ambient air of 0.5 ng m 3 and a "tolerable level" of 5 ng m"3 (Montizaan et al., 1989). Based on "technical and economic feasibility" as well as concentrations in other western European cities, Germany has an "orienting value" of an annual average of BaP of 10 ng m~3 (Montizaan et al., 1989); a basis for this approach, as stated by The Umwelt Bundesamt (Federal Environmental Agency), is that "dose-effect relationships for man do not exist" (Montizaan et al., 1989; Nielsen et al., 1995, 1999a). Such relationships have, however, been established for PAHs in occupational exposures (e.g., see Mastrangelo et al., 1996).

Because certain PAHs are carcinogens, and thus there is no "safe level," the WHO (1987) does not recommend one. However, it has developed risk assessments based on BaP "as an indicator" (Nielsen et al., 1995).

2. Carcinogenicity of PAHs, Cancer Potencies, and Potency Equivalence Factors

There are a variety of sources of data on the carcinogenicity of environmental PAHs and PACs. A key source is a series of monographs published by the International Agency for Research on Cancer (IARC), The Evaluation of Carcinogenic Risks to Humans. This series includes Polynuclear Aromatic Compounds, Vol. 32, Part 1, Chemical, Environmental, and Experimental Data (1983); Overall Evaluations of Carcinogenicity: An Updating of IARC Monographs, Volumes 1-42 (1987); and Diesel and Gasoline Engine Exhausts and Some Nitroarenes, Vol. 46 (1989). Other useful evaluations of the carcinogenicity of specific PAHs and PACs include, for example, those of the U.S. Environmental Protection Agency (1986), the California Air Resources Board (f 994), and the U.S. Department of Health and Human Services, Public Health Service, National Toxicology Program (U.S. DHHS, 1998).

Evaluations of the carcinogenicity of selected PAHs and PACs are summarized in Tables 10.13 and 10.14. Designations are defined in Box 10.7.

Unless otherwise noted, we use IARC definitions and symbols for the carcinogenicity of a given agent; for example, BaP and benzo[6]fluoranthene (BbF) are

Class 2A and 2B animal carcinogens and "probable" and "possible" human carcinogens, respectively. A note of caution is appropriate, however, regarding differences in the classification schemes. For example, the EPA classifications for BaP and BbF are both B2, "sufficient evidence from animal studies."

Because it is not only BaP but also a variety of other PAHs and PACs that are of concern in terms of the possible inhalation cancer risk to humans of complex mixtures of combustion-generated POM, a number of approaches have been developed for evaluating the potencies of various compounds (e.g., Holmberg and Ahlborg, 1983; IARC, 1989; Nisbet and La Goy, 1992; Lewtas, 1985b, 1993a, 1993b; 1994; Heinrich et al., 1994; CARB, 1994; Mastrangelo et al., 1996; Nielsen et al., 1996; OEHHA, 1998; CARB, 1998; Collins et al., 1998; Tokiwa et al., 1998). One approach is to calculate the inhalation "unit risks" for excess lung cancer for BaP and each of its copollutant carcinogenic PAHs and PACs in polluted ambient air. The latter values are divided by the unit risk for BaP to obtain their individual potency equivalence factors, PEFs, based on BaP = f.00, e.g., Nielsen et al., 1996, and Collins et al., 1998. These PEFs are listed in Table 10.13 for PAHs and Table 10.14 for PACs. The PEFs range from 0.01 for chrysene and 2-nitrofluorene to 10 for each of the three 6-ring dibenzopyrenes (C24HI4, MW 302) and for the N-PACs 6-nitrochrysene and 1,6-dinitropyrene. Interestingly, Cavalieri et al. (1989, 1994) reported that the dibenzo[a,/]pyrene isomer is actually 100-200 times more tumorigenic than BaP, and they termed it "the most potent carcinogenic aromatic hydrocarbon."

To assess both the relative and absolute contributions of various PAHs and PACs to health impacts, the potencies must be combined with concentrations of the individual PAHs and PACs in air. The levels of compounds other than BaP can be quite substantial and hence contribute significantly to the overall carcinogenicity and mutagenicity. For example, Allen and co-workers (1998) reported that, while the individual concentrations of the biologically active 6-ring PAHs identified and quantified in urban Boston air are relatively small (e.g., the concentration of dibenzo[«,e]pyrene was 0.133 ng m~3), their total concentration of ~1.5 ng m~3 is comparable to the BaP present as a copollutant in the sample.

Collins et al. (1998) applied the PEF data for PAHs and PACs (Tables 10.13 and 10.14) to daytime concentrations measured in ambient air in Riverside, California (~90 km east and downwind of Los Angeles; see Fig. 10.23), during the months of July and August 1994 (Atkinson and Arey, 1997; Krieger et al., 1997). As seen in Table 10.15, the "PEF adjusted concentration," 269 pg m"3, is ~40% of the total mass of PAHs and PACs

TABLE 10.13 Carcinogenicities of Selected PAHs in Ambient Air As Evaluated by IARC, U.S. EPA, and U.S. DHHS, Cancer Potency Equivalence Factors Relative to BaP = 1.00 from Nielsen et al. (1996), CARB (1994), and Collins et al. (1998), and Human Cell Mutagenicities Relative to BaP = 1.00

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