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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.

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Results of the comprehensive analysis of emissions from a pressurized fluidized-bed combustion unit (the Exxon Miniplant) are described as an illustration of the methodology for comprehensive analysis. The results are discussed in the context of the overall environmental assessment of the process being conducted by the U.S. Environmental Protection Agency. The comprehensive analysis of the fluidized-bed combustion emissions and process streams involved approximately 740 measurements on about 90 samples, using more than 40 different inorganic, organic, and physical analytical methods. A brief discussion on the sampling methods and analytical techniques is also included.