PAHs (see Section 10.8), are among the urban aerosol particle constituents of most concern, particularly because metabolites of some of these compounds (see the 7,8-diol-9,10-epoxide of benzo(a)pyrene in Figure 22.18) are carcinogenic. PAHs include unsubstituted compounds as well as those with alkyl, oxygen, or nitrogen substituents, or O or N hetero atoms: