Humans metabolize benzene via two pathways toxico-kinetic studies, which relied primarily upon data from humans exposed to concen- trations ≥1 ppm, would have failed to detect it (Bois et al. 1996; Travis et al. 1990). Although our results favor the presence of two benzene-metabolizing pathways in humans, they provide little insight into the identities of the particular enzymes repre- sented. Because CYP2E1 is the primary enzyme responsible for mammalian metabo- lism of benzene (Koop et al. 1989; Nedelcheva et al. 1999; Powley and Carlson 2000), it is reasonable to assume that the low-affinity path- way in our model (pathway 1) is dominated by CYP2E1. This conjecture is supported by previous analyses of 386 male and female sub- jects from our study, which showed that the homozygous variants of CYP2E1 1054C–T (rs2031920, ascribed to RsaI) had significantly lower levels of urinary muconic acid, phenol, and hydroquinone than did homozygous wild types (Kim et al. 2007). Furthermore, the ratios of the metabolite levels between homozygotes (i.e., variant/variant:wild type/ wild type) became much smaller as benzene air concentrations increased above 0.1 ppm, suggesting that CYP2E1 1054C–T is active primarily at higher concentrations. Regarding the identity of the high-affinity pathway 2, we speculate that this could repre- sent another CYP enzyme(s). Of the various candidates that have been shown to oxidize benzene in mammalian in vitro systems, only CYP2E1, CYP2B1, and CYP2F1 appear to be active in humans, and these have apparent Km values in the order 2F1 < < 2E1 < 2B1 (Koop et al. 1989; Nedelcheva et al. 1999; Powley and Carlson 2000). Thus, although CYP2B1 could contribute to metabolic pathway 1 in our model, CYP2F1 could contribute to metabolic pathway 2, which operates at low benzene concentrations. Powley and Carlson (2000) concluded that CYP2F1 is expressed in human microsomes from both liver and lung and is particularly active in the lung, where benzene is absorbed after inhalation. Another promising candidate CYP enzyme that is worth considering in this context is CYP2A13, which is highly expressed in the human lung and has recently been shown to catalyze several known CYP2E1 substrates, namely, toluene, styrene, chlorzoxazone, and p-nitrophenol, with much higher affin- ity compared with CYP2E1 (Fukami et al. 2008). Thus, CYP2F1 and CYP2A13 should be investigated as potential contributors to the enhanced benzene metabolism observed at air concentrations < 1 ppm. Because concentrations of benzene in ambient air tend to be < 0.01 ppm through- out the world (Kim et al. 2006a), our results suggest that the high-affinity pathway 2 is responsible for most metabolism of this air- borne carcinogen in the general population (e.g., about 73% in nonsmoking females; Figure 2). Given the potential importance of benzene as a source of leukemia and other hematopoietic malignancies in the general population, it is important that our results be independently verified and that the puta- tive high-affinity pathway 2 be identified. Nonetheless, based on the prediction of 184 µM of benzene metabolites per part per mil- lion benzene under our two-pathway model versus a prediction of 68.6 µM/ppm under our one-pathway model, it is reasonable to conclude that current risk assessments would likely underestimate leukemia risks at ambi- ent air concentrations of benzene by a factor of about 3 for nonsmoking women. In summary, this study of benzene expo- sures and metabolite levels among 263 non- smoking women provides strong statistical evidence that an unknown high-affinity path- way is responsible for most metabolism of benzene at sub-part per million air concentra- tions. 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