University of California Transportation Center

Intake fraction, which is the fraction of emissions that are inhaled by people, quantifies the ‘‘exposure efficiency’’ of an emission source. We use three methods to estimate intake fractions for vehicle emissions in US urban areas. First, we use a one-compartment steady-state mass-balance model, incorporating meteorological and demographic data. Second, we use an empirical emissions-to-concentration relationship for vehicle carbon monoxide developed for 15 US urban areas. Third, we analyze model results for benzene and diesel particulate matter from the US Environmental Protection Agency’s National-scale Air Toxics Assessment (NATA). The population-weighted mean intra-urban intake fraction for nonreactive gaseous vehicle emissions in US urban areas is estimated to be in the range 7–21 per million, with a best estimate of 14 per million. The intake fraction for diesel particles is 4 per million, based on NATA results. An intake fraction of 4 per million means that 4mg of pollution are inhaled per kg emitted. Intake fraction values for urban vehicle emissions are usually higher in winter than in summer because of seasonal variability in the atmospheric mixing height. The results presented in this work can be used in health risk assessments, cost–benefit analyses, and other investigations that require a summary of the emission-to-intake relationship.