Influence of Exposure Error and Effect Modification By Socioeconomic Status on the Association of Acute Cardiovascular Mortality with Particulate Matter in Phoenix
Using ZIP code-level mortality data, the association of cardiovascular mortality with PM2.5 and PM10-2.5,measured at a central monitoring site, was determined for three populations at different distances from the monitoring site but with similar numbers of deaths and therefore similar statistical power. The % risk and statistical significance for the association of mortality with PM2.5 fell off with distance from the monitor, as would be expected if exposure error increased with distance. However, the % risk for PM10-2.5 increased in going from the population in Central Phoenix, where the monitoring site was located, to a population in a Middle Ring around Phoenix and fell off in an Outer Ring population. The % risks for the Outer Ring were low for each of the six lag days (0-5) and for the 6-day moving average. The lag structures for PM2.5 and PM10-2.5 also differed for the Central Phoenix and Middle Ring populations. These differences led us to examine the socioeconomic status (SES) of the populations. On the basis of education and income, the population in Central Phoenix had a lower SES than the Middle Ring. Thus, the differences between Central Phoenix and the Middle Ring may be due to effect modification by SES and differences in exposure error. However, the effect modification by SES may be different for thoracic coarse particulate matter (PM) than for fine PM.