The AP 42 Compilation of Air Pollutant Emission Factors, was first published by the U.S. Public Health Service in 1968. In 1972, it was revised and issued as the second edition by the U.S. Environmental Protection Agency (EPA). In 1985, the subsequent fourth edition was split into two volumes. Volume I includes stationary point and area source emission factors, and Volume II includes mobile source emission factors. Volume I is currently in its fifth edition and its main table of contents is available online at www.epa.gov/ttn/chief/ap42/index.html. Volume II is no longer maintained as such, but software programs for estimating emissions from onroad vehicles and from non-road vehicles and mobile equipment are available at www.epa.gov/otaq/models.htm.