The US Environmental Protection Agency (EPA) emission factor document called “AP-42” provides emission factors for emissions of particulate matter from re-entrained dust on roadways. The way the calculations work, the amount of particulate matter (dust/silt loading) continues to increase with each passing vehicle. There is no upper limit on the amount of dust that can be created, even when there is only a finite amount of dust available on the roadway. Neither the national mobile source emission factor model, (MOBILE6.2) nor the California model (EMFAC) account for re-entrained dust, so the options available to analysts are to either conduct expensive field monitoring to develop site specific emissions or use AP-42 emission factor calculations. The objective of this study was to identify and develop guidelines for the use of more accurate calculation methods (or a more accurate modification of the existing AP-42 method) to calculate PM2.5 and PM10 emission factors for re-entrained road dust for both paved and unpaved roads that will be acceptable to the USEPA and states.