The United States Environmental Protection Agency's National Exposure Research Laboratory has initiated a project to improve the methodology for modeling motor vehicle emissions from source through the pathway to human exposure. A real-time microscale automobile emission factor model for CO (MicroFacCO) virtually capturing all the information in the real world had been developed for United States vehicles. The goal is to use this CO model as a surrogate for other tailpipe air toxic emissions. The emission model is being used in conjunction with roadway dispersion models (e.g., CALINE4), and being evaluated in the roadways around Research Triangle Park, North Carolina in a range of traffic fleet and meteorological conditions. Modeled concentrations are being compared with measured concentrations inside a moving vehicle and parked vehicle along the roadside. This paper discusses the new emission model, demonstrates the use of the emission model in modeling roadway air concentrations through an example, and discusses the issues and research needs for improving the methodology of modeling human exposures to mobile source emissions.