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 intraurban 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 4 mg 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.
Bibliographical noteFunding Information:
This work was supported in part by a Graduate Research Fellowship from the National Science Foundation, by a Dissertation Fellowship from the University of California Transportation Center, by a fellowship from the University of California Toxic Substances Research and Teaching Program, by the US Environmental Protection Agency National Exposure Research Laboratory through Interagency Agreement No. DW-988-38190-01-0, and by the Lawrence Berkeley National Laboratory through the US Department of Energy under Contract Grant no. DE-AC03-76SF00098. The contents of this paper are solely the responsibility of the authors and do not necessarily represent the views of the funders. The authors thank Dr. Scott Fruin and two anonymous reviewers for helpful comments on an earlier version of this manuscript.
- Box model
- Carbon monoxide
- Diesel particulate matter
- National-scale Air Toxics Assessment
- One-compartment model