VARIATIONS IN PM-10 CONCENTRATIONS WITHIN 2 METROPOLITAN-AREAS AND THEIR IMPLICATIONS FOR HEALTH-EFFECTS ANALYSES

Temporal variations of PM-10 levels at multiple sites between 1985 and 1990 in two major metropolitan areas, Cook County, IL, and Los Angele s County, CA, were characterized, and sensitivity of mortality-PM-10 a ssociations to the choice of alternate sites was examined. In both cit ies, the correlation of PM-10 levels among multiple sites decreased as their distance increased. While averaging PM-10 concentrations over m ultiple sites generally improved the significance of PM-10-mortality a ssociations, the highest PM-10-mortality association in Cook County wa s found for an individual site. In Cook County, the magnitude of the m ortality association for the average of 6 PM-10 sites, as expressed as ''relative risk'' per 100 mu g/m(3) PM-10, was similar (RR = 1.06, 95 % CI 1.01-1.10) to those reported in other PM-10 studies with similar model specifications. However, the significance of regression coeffici ents for individual PM-10 sites varied considerably it ratios range -0 .62 to 3.30). Furthermore, every-6-days subsamples of the daily data a t a site in Cook County showed a wide range in the significance of reg ression coefficients (t ratios range -0.17 to 3.44). This variability of significance among the six sites may be partly due to their small s ample sizes (n approximate to 300), which raises concern regarding the potential for compromised statistical power of health effects analyse s in ''short'' study periods (<6 yr) at the current every-6-days sampl ing frequency used for most PM-10 monitors in the United States. Also, the qualitative site information available, such as land use, locatio n setting, and monitoring objective, did not show any coherent influen ce 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.