Correcting for the effects of location and atmospheric conditions on air pollution exposures in a case-crossover study

A limitation of most air pollution health effects studies is that they rely on monitoring data averaged over one or more ambient monitors to represent daily air pollution exposures for individuals. Such data analyses therefor e implicitly require the assumption of a homogeneous spatial distribution f or particulate matter ( PM). This assumption may be suspected in the Pacifi c Northwest because of its hilly topography and local variations in wood bu rning. To examine the bias from substituting regional PM ( i.e., the averag e of three ambient monitor measurements) for individual PM exposure, we con ducted an exposure substudy to identify the influence of location factors, specifically urban versus suburban classification and topographic features ("upstream" versus "downstream"), on local ambient measurements. Using neph elometer measurements collected over I year in four locations, we developed regression models to predict local PM as a function of regional PM, atmosp heric stagnation, temperature, and location. We found a significant interac tion between atmospheric stagnation and topography, with the most upstream site having reduced PM levels on high stagnation days alter controlling for regional PM. We also found a significant interaction with temperature at o ne downstream site thought to be heavily exposed to wood smoke in the winte r. These results are consistent with the physics of surface radiation inver sions. The interactions reordered the index versus referent exposures in a case-crossover analysis of out - of- hospital primary cardiac arrest for su bjects living in specific locations, but did not meaningfully change the as sociations with PM from the analysis using regional PM as the exposure. Til e lack of change in these results may be due to limitations in the data use d to correct the exposure estimates or to the absence of a PM effect among persons without prior heart disease who experienced a primary cardiac arres t.