Trends in Passenger Exposure to Carbon Monoxide Inside a Vehicle on an Arterial Highway of the San Francisco Peninsula over 30 Years: A Longitudinal Study.

This paper describes a long-term trend study of passenger exposure to carbon monoxide (CO) inside a vehicle travelling on an arterial highway in Northern California. CO exposure was measured during four field surveys on State Route #82 (El Camino Real) on the San Francisco Peninsula in 1980-1981, 1991-1992, 2001-2002, and 2010-2011. Each field survey took at least 12 months. Fifty trips from each survey-for a total of 200 trips-were matched by date, day of the week, and starting time of the day to facilitate comparisons over three decades. The mean net CO concentration of each trip was obtained by subtracting the background CO level from the average CO concentration for the entire trip. The mean net CO concentration (0.5 ppm) for 2010-2011 was only 5.2% of that (9.7 ppm) for 1980-1981. For the 50 trips, the average travel time for the 1980-1981 period (39.6 min) was only 8.3% higher than during the 2010-2011 period (36.3 min). The estimated round-trip distance on the highway was held constant at 11.8 miles. The reduction in the mean net CO concentration was attributed to more stringent CO emission standards on new vehicles sold in California since 1980. The state's cold-temperature CO standard implemented in 1996 appeared to reduce high CO concentrations that were observed during the late fall and winter of 1980-1981. In addition, the observed standard deviation in concentration fell from 3.1 ppm in 1980-1981 to 0.2 ppm in 2010-2011, and the range of the 50 mean net CO concentrations narrowed from 14.9 ppm in 1980-1981 to 1.1 ppm in 2010-2011; but the relative variability, as indicated by the geometric standard deviation, remained the same. These results have important scientific implications for regulatory policies designed to control air pollution from motor vehicles.