Variations in PM-10 Concentrations Within two Metropolitan Areas and Their Implications for Health Effects Analyses

Abstract

Temporal variations of PM-10 levels at multiple sites between 1985 and 1990 in two maJor metropolitan areas, Cook County, IL, and Los Angeles County, CA, were characterized, and sensitivity of mortality-PM-10 associations to the choice of alternate sites was examined. In both cities, the correlation of PM-10 levels among multiple sites decreased as their distance increased. While averaging PM-10 concentrations over multiple sites generally improved the significance of PM-10-mortality associations, the highest PM-10-mortaiity association in Cook County was found for an individual site. In Cook County, the magnitude of the mortality association for the average of 6 PM-10 sites, as expressed as “relative risk” per 100 μg/m³ PM-10, was similar (RR = 7.06, 95% CI 1.01–1.10) to those reported in other PM-10 studies with similar model specifications. However, the significance of regression coefficients for individual PM-10 sites varied considerably (t ratios range -0.62 to 3.30). Furthermore, every-6-days subsamples of the daily data at a site in Cook County showed a wide range in the significance of regression coefficients ft ratios range -0.17 to 3.44). This variability of significance among the six sites may be partly due to their small sample sizes (n < 300J, which raises concern regarding the potential for compromised statistical power of health effects analyses in “short” study periods (< 6 yrJ at the current every-6-days sampling frequency used for most PM-10 monitors in the United States. Also, the qualitative site information available, such as land use, location setting, and monitoring obJective, did not show any coherent influence on the site's PM-10–mortality association's significance. Overall, it was found that the choice of PM-10 sites and sampling frequency can make a substantial difference in the calculated significance of such health effects time-series analysis.