LONG-TERM TRENDS IN UNITED-STATES HIGHWAY EMISSIONS, AMBIENT CONCENTRATIONS, AND IN-VEHICLE EXPOSURE TO CARBON-MONOXIDE IN TRAFFIC

This paper reviews 16 published studies conducted between 1965 and 199 2 of in-vehicle exposure to carbon monoxide (GO) in traffic on urban r oadways in the United States. Analysis of these studies shows a downwa rd trend in CO exposure, which corresponds to similar trends for CO in motor vehicle emission factors and ambient concentrations. The analys is demonstrates that emission controls on motor vehicles sold in the U nited States have been very effective in reducing commuter CO exposure . It is recommended that future studies of this kind be done routinely in cities nationwide to provide a more robust database for accurate e stimates of commuter exposure. Such studies should relate human exposu re measurements to estimates of emissions at study sites to document t he progress of motor vehicle emission control programs. In addition, f uture studies should use standard protocols to enable comparisons of r esults in time and space. Previous studies have shown that typical in- vehicle exposures vary by study approach (direct versus indirect), cit y, season, roadway type and location, travel mode, and vehicular venti lation. Future studies should carefully account for these factors.