tetrachloroethane in the air exhaled by teachers and school-children were significantly greater than in a control group. The study demonstrated the applicability of biologic monitoring, where the exposure derives from the environment instead of the workplace.

Levine et al. (1985) monitored air at a hazardous-waste remedial-action site. This study demonstrated that, when workers remained in fixed job locations, "occupational inhalation exposure monitoring must consider contaminants generated upwind of the job location." Biologic monitoring would have been useful in this population of workers.

The selection of individual monitoring versus area or population monitoring of exposure is a matter of continuing concern. In particular, overreliance on "central-site monitoring" for assessment of exposure to air pollutants may be undesirable because of the possibility that personal exposures may differ from those estimated by central-site monitoring. Unrecognized variation among individuals in true air-pollution exposure contributes to variability among individuals in estimated slopes. While important findings have derived from air-pollution studies that make use of ambient-air monitoring data, investigations of more-precise questions such as determinants of variations in response may be improved through the use of personal monitors that are now becoming available.

The improved accuracy of personal monitoring of exposure generally comes at the expense of a substantial increase in cost and may therefore have its greatest value in the validation of other, less-expensive models of exposure. Freeman et al. (1991) have developed a location and activity log for assessing personal exposure to air pollutants and conducted pilot studies to validate the approach. Schwab et al. (1991) have suggested that the use of self-reported exertion levels in time-activity diaries has useful application to exposure assessment, particularly with respect to estimating the relation between exposure assessment and dose assessment. More studies of this nature are needed. The value of time-activity logs has been illustrated in detail by Sexton and Ryan (1989).

New instruments with the sensitivity and specificity necessary to conduct personal air-monitoring exposure assessments are also needed, and there have been a number of new developments of sampling instruments. Many of these instruments appear to be reasonable in cost (NRC, 1991b).

Hasabelnaby et al. (1989) used microenvironmental monitoring to characterize the exposure of preadolescent children to fine-particle air pollution. The authors concluded that microenvironmental monitoring is useful for estimating personal exposure in preadolescent children because they generally do not smoke and are not exposed occupationally. They also spend statistically significant periods at home, in school, or outdoors near their homes. These authors used data from the Six Cities Study to obtain more-accurate measures of exposure to passive smoking. They

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