Abstract
This thesis describes a series of studies that investigate the validity of using outdoor concentrations and/or traffic-related indicator exposure variables as a measure for exposure assessment in epidemiological studies on the long-term effect of traffic-related air pollution. A pilot study was performed in 14 children attending one school in Amsterdam,
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The Netherlands. The second study involved 54 children attending four different schools, two of which were located within 100 meter of a major road (one ring road and one freeway) and the other two were located at a background location in Utrecht. The third study was assessed in an adult population in Utrecht of which twenty-three participants lived at busy roads (> 10 000 vehicles per 24 h) and 22 lived at urban background locations. In all studies, personal exposure to traffic-related pollutants (PM2.5, soot, NOx and NO2) was measured during four to five 48-hour periods. Simultaneously, school or home outdoor levels of the pollutants were monitored. In the second study, the outdoor concentration of soot was 74% higher at the freeway school compared to its matched background school. Personal exposure to soot was 30% higher. For NOx the outdoor concentration was 52% higher at the freeway school compared to its background school. The personal concentration of NOx was 37% higher for children attending the freeway school. Differences were smaller and insignificant for PM2.5 and NO2. No elevated personal exposure to air pollutants was found for the children attending the school near the ring road. In the study among adults, strong contrasts in home outdoor concentrations were found for soot (68%), NO (127%) and NOx (35%) comparing busy road locations with urban background. For PM2.5 (14%) and NO2 (22%) the contrast was smaller. Slightly elevated ratios were found for personal exposure to soot (1.18, 95% confidence interval (CI): 1.04, 1.35) and NO (1.31, CI: 0.92, 1.87), when comparing adults living at busy roads with adults living at quiet roads. Time spent in traffic was associated with increased personal exposure of soot and PM2.5. In general, differences in outdoor levels of soot and differences in proximity to traffic were reflected in differences in personal soot concentrations. We therefore conclude that outdoor concentrations of soot and living near a busy road or attending a school near a busy road, are valid proxy measures for personal exposure to soot. In addition, the thesis shows that the estimates of the effect of exposure to outdoor air pollution on respiratory and other health effects may be substantially attenuated when exposure measurement used outdoor concentrations in childrens' schools instead of personal exposure. The prevalence ratio of four health outcomes (current wheeze, conjunctivitis, phlegm and elevated total IgE) was estimated using school outdoor measurements, and then adjusted for measurement error using personal exposure data by applying a regression calibration method. The measurement error-adjusted analysis showed effect estimates related to soot and NO2 that were two to three times higher than estimated in the original study.
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