Attributable Risks Calculation From Case-Crossover Study of Air Pollution and Asthma Attacks

Abstract

ISEE-411 Objective: Case-crossover studies allow studying the short-term effects of air pollution for shorter study periods and smaller populations (eg, US census tracts) than time-series studies. To our knowledge, attributable risks of air pollution have never been calculated from case-crossover studies. Material and Methods: A case-crossover study of the relationship between daily PM10 concentrations modeled by small census units (IRIS, comparable with US Block Group) and 4683 emergency calls to mobile emergency practitioners for asthma attacks was carried out in an eastern France conurbation including 450,000 inhabitants (years 2000–2005). We obtained an overall odds ratio of 1.034 (95% CI: 0.996–1.074) for a 10 μg/m3 increase in PM10 (adjusted for meteorology, pollens, and influenza). This odds ratio was used to estimate the annual number of calls attributable to PM10 in the conurbation, using the method recommended by the French National Institute of Health Surveillance (InVS) for time series. Computation of this estimate integrated the pluri-annual (2000–2005) mean PM10 concentration of 22.5 μg/m3 observed in the conurbation. Results: The annual number of calls for asthma attacks attributable to the exceeding of the “baseline” PM10 concentration of 10 μg/m3 in the conurbation was 9 (95% CI: 0–15) for 100,000 inhabitants. This estimate assumes a causality link between ambient PM10 concentrations and asthma attacks, which is likely according to literature data. On the basis of these elements, we can at least assume that the number of calls attributable to PM10 should be inferior to the upper confidence interval of the estimate. Conclusions: Our results suggest that, in our study setting, the annual number of calls to mobile emergency practitioners for asthma attacks attributable to the exceeding of the “baseline” PM10 concentration of 10 μg/m3 is less than 15 for 100,000 inhabitants.