The potential for directly monitoring public exposure to air pollutants has been realized with the development of personal exposure monitors. The microenvironmental approach to exposure assessment involves monitoring the concentrations within microenvironments thought to contribute significantly to a population's total exposure. This paper focuses on the commuting microenvironment, because of the suspected large contribution that commuting makes to the total population exposure to carbon monoxide (CO). In particular, this paper presents typical CO concentrations, to which automobile, bus, and rail commuters of the Washington, D.C. metropolitan area were exposed, on 15 hypothetical routes during winter 1983. In addition, the paper assesses the relative importance of several factors that explain variability in CO levels inside automobiles during rush-hour periods.The study found that automobile commuters were exposed to average CO concentrations that typically ranged from 9 to 14 ppm over trips that typically took between 40 and 60 minutes. Average CO levels for bus commuters typically ranged from 4 to 8 ppm for trips lasting between 90 and 110 minutes, and those for rail commuters typically ranged from 2 to 5 ppm for trips of 30-45 minutes. The most important factors influencing CO concentrations inside automobiles were identified as link-to-iink variability, day-to-day variability, and the interaction between link and commuting period. Variability in CO levels by route, driver, and factors specific to a particular commute were moderately important. Between and within monitor variation were the least important sources of variation in CO levels. Increasing automobile speed from 10 to 60 mph reduced average CO exposure by 35 percent regardless of commuting period. The study suggests that automobile commuters, who begin their homeward trips from highly polluted downtown parking garages, may carry residual garage concentrations with them as they travel along downtown streets.
This paper presents statistical models of passenger exposure to carbon monoxide (CO) inside a motor vehicle as it traveled a coastal highway in Honolulu, Hawaii during morning periods between November, 1981 and May, 1982. The 3.85-mile study site was divided into three links. The models predict the average CO concentration inside the vehicle's passenger cabin on the third link as a function of several variables: the average CO concentrations inside the cabin on previous links; traffic, temporal, and meteorological variables; motor vehicle CO emission factors; and ambient CO concentrations. Based on data for 80 trips, the three most powerful models (adjusted R 2 =0.69) were nonlinear combinations of four variables: the average CO concentration inside the cabin for the second link; wind speed and direction; and either the travel time, vehicle speed or CO emission factor for the third link. Several nonlinear models were based on data for 62 trips for which nonzero, ambient CO concentrations were available. For this database, the most practical models (adjusted R 2 =0.67) combined three variables: the ambient CO concentration; the second-link travel time; and either the travel time, vehicle speed or CO emission factor for the third link. Two factors of third-link CO exposure varied seasonally. Relatively lighter traffic flows and stronger winds lowered cabin exposures during late fall, while heavier traffic flows and calmer winds elevated cabin exposures during winter and spring. This study confirms the importance of seasonal effects on cabin exposure, as observed by a California study, and adds new insights about their effects.
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