Influence of Urbanicity and County Characteristics on the Association between Ozone and Asthma Emergency Department Visits in North Carolina
Background: Air pollution epidemiology studies, often conducted in large metropolitan areas due to proximity to regulatory monitors, are limited in their ability to examine potential associations between air pollution exposures and health effects in rural locations. Methods: In a time-stratified case-crossover framework, we examined associations between asthma emergency department (ED) visits in North Carolina (2006-2008) collected by a surveillance system, and short-term ozone exposures using predicted concentrations from the Community Multiscale Air Quality (CMAQ) model. Associations were estimated by county groupings based on four urbanicity classifications (representative of county size and urban proximity) and county health. Results: Ozone was associated with asthma ED visits in all-year and warm season (April-October) analyses [Odds Ratio (OR) =1.019; 95% CI: 0.998, 1.040; OR=1.020; 95% CI: 0.997, 1.044, respectively, for a 20 ppb increase in lag 0-2 days ozone]. The association was strongest in Less Urbanized counties, with no evidence of a positive association in Rural counties. Associations were similar when adjusted for fine particulate matter in copolluant models. Associations were stronger for children (5-17 years of age) compared with other age groups, and for individuals living in counties with poorer health status compared with counties that had the highest health rankings, although estimated associations for these subgroups were imprecise. Conclusions: Associations between short-term ozone exposures and asthma ED visits differed by overall county health and urbanicity, with stronger associations in Less Urbanized counties, and no positive association in Rural counties. Results also suggest that children are at increased risk of ozone-induced respiratory effects.