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22 alkylated polycyclic aromatic hydrocarbons (alk-PAHs) were characterized in ambient air individually for the first time in urban and semi-urban locations in Toronto, Canada. Five unsubstituted PAHs were included for comparison. Results from the measurements were used to estimate benzo­[a]­pyrene equivalent toxicity (BaPeq) of individual compounds in order to investigate the significance of a single compound in contributing to the overall toxic equivalency (TEQ) of air mixtures. To determine which compounds merit further investigation, BaPeq values of individual compounds were compared to the measured BaP toxicity. Our results showed that both unsubstituted and alkylated PAHs were more abundant in the urban area (38 and 30%, respectively). Benzo­[a]­pyrene levels at the urban location exceeded Ontario’s 24 h guideline (40% of the events), and on average, it was 5 times higher than that at the semi-urban area. Gas-phase two- and three-ring compounds contributed up to 39% (urban) and 76% (semi-urban) of the TEQ of all compounds analyzed. Some alk-PAHs such as 7,12-dimethylbenzo­[a]­anthracene had a huge impact on the toxicity of urban air, and its BaPeq was on average 8 times higher than that of BaP. We emphasize that the toxic impact of alkylated and gaseous PAHs, which is not routinely included in many air monitoring programs, is significant and should not be neglected.