" For definitions of the classifications by the various organizations, see Table 10.13. h From IARC (Supplement 7, 1987; 1989). ' From U.S. EPA (1986). NC, not classified.

'' From u.S. Department ol Health and Human Services (1998); RAHC, reasonably anticipated to be a human carcinogen. ' From California Air Resources Board (1994) and Office of Environmental Health Hazard Assessment, California Environmental Protection Agency (Cal EPA) (1998). ' From Collins et al. (1998). * From Durant et al. (1996).

(651 pg/m3). That is, the "total inhalation risk" associated with these PAHs and PACs is -40% of that predicted if one assumes each compound has the same carcinogenic potency as BaP. One important caveat was pointed out by Collins et al. (1998): since PAHs are "multipathway" carcinogens, their calculated inhalation risk may account for only 5-f0% of the estimated total multipath exposure risk.

Nielsen and co-workers (1996) employed a similar "potency equivalence factor" methodology in their risk assessment evaluation of the health impacts of PAHs and PACs for typical ambient levels encountered at several sites in Copenhagen, Denmark. For other ex amples of comparative risk evaluations, see, for example, the calculation by Collins et al. (1998) of the equivalent inhalation risk of ambient aerosols in London, using the 1991 data of Halsall et al. (1994), and Lewtas (f993a, 1993b, 1994, and references therein).

Finally, while several volatile and semivolatile PAHs, e.g., naphthalene, the methylnaphthalenes, phenan-threne, pyrene, and fluoranthene, are not significant mutagens or carcinogens (hence not included in Table 10. f 3), they are precursors to powerful direct bacterial mutagens formed in gas-phase atmospheric reactions with hydroxyl during the day and nitrate radicals at night (see Section F). Furthermore, 2-nitrofluoranthene,

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