What exposure indicator should be used to study the short-term respiratory health effect of photochemical air pollution? A case study in the Paris metropolitan area (2000-2003)

Many studies report that air pollution has short-term effects on mortality or morbidity. Many of them analyse the exposure-response curve for ozone, but because ozone is what is called a "secondary" pollutant, it is difficult to quantify the short-term links between its daily levels and health. The goal of this study was to determine the most relevant photochemical air pollution indicator for measuring its health effects. For this purpose, a case study of the Paris metropolitan area used 4 different exposure indicators to estimate the links between photochemical air pollution and the number of daily hospital admissions for respiratory diseases. Time series analyses used generalised additive models. The health indicators were adjusted for time trends, seasonal factors, day of the week, influenza, weather and pollen. We found that the best exposure indicator was the daily maximum of 8-hour moving averages of ozone levels, combined with a distinction between hot and cold seasons. Using this exposure indicator we found a significant association between the daily number of hospitalization for respiratory diseases: relative risk of hospital admission increased by 0.84% (95% Cl [0.05; 1.63]) for a 10-mu g/m(3) increase in the levels of the exposure indicator These results confirm previous findings concerning non-lethal effects of ozone. Refinement in the definition of summer season could be improved and may be the subject of further research.