MOBILE4.1 is the latest revision to EPA's highway vehicle mobile source emission factor model. Relative to MOBILE4, it contains numerous revisions and provides the user with additional options for modeling highway vehicle emission factors. It will calculate emission factors for hydrocarbons (HC), carbon monoxide, (CO), and oxides of nitrogen (NOx) from highway motor vehicles. It calculates emission factors for eight individual vehicle types, in two regions of the country (low and high altitude). The emission factors depend on various conditions such as ambient temperature, fuel volatility, speed, and mileage accrual rates. It will estimate emission factors for any calendar year between 1960 and 2020 inclusive. The 25 most recent model years are considered in operation in each calendar year. EPA is requiring that states and others preparing emission inventories for nonattainment areas for CO and ozone to use MOBILE4.1 in the development of the base year 1990 emission inventories required under the Clean Air Act of 1990.