At the top of the list of tools needed to forecast reliably any potential adverse health outcomes from proposed highway projects are, predictably, models of mobile source emission factors. A comparison of the two current regulatory models is provided: Emfac2007, developed by the California Air Resources Board for applications in California, and MOBILE6.2, developed by the U.S. Environmental Protection Agency for regulatory applications elsewhere in the United States. Pollutant emission factors computed by the models are evaluated. Although the two models share the same fundamental principle-that is, emissions from motor vehicles are based on testing with correction factors utilized to account for on-road vehicle use-the models produce strikingly divergent results. MOBILE6.2 tends to predict higher emission factors for carbon monoxide, volatile organic compounds, nitrogen oxides, and benzene compared with Emfac2007. In contrast, Emfac2007 predicts higher emission factors for particulate matter (PM) of size ¢â€°Â¤2.5 ‚µm and diesel PM. The reasons for these calculation differences are discussed. The implications pertaining to the evaluation of highway project alternatives are also discussed.