Ozone and short-term mortality in nine French cities: Influence of temperature and season

BackgroundSeveral epidemiological studies have observed significant short-term associations between ozone and daily mortality. In a context of climate change, it is important to understand how this association is influenced by the meteorological conditions.ObjectivesWe investigated how season and temperature modified the short-term effect of ozone on mortality by cause in nine French urban areas during the 1998–2006 period.MethodsThe relationship between daily max-8 h ozone and daily mortality was analysed in each city using a time-stratified case-crossover model for the whole year, by season, and by temperature strata. Sensitivity of the results to the statistical modelling strategy, to the choice of the temperature terms, and to the introduction of PM2.5 was examined.ResultsA 10 μg m−3 increase in daily ozone level was significantly associated with an increase in non-accidental (+0.3% [95% CI 0.1; 0.5]), cardiac (+0.7% [0.2; 1.1]) and cardiovascular mortality (+0.4% [0.0; 0.7]). The estimates were larger during summer (+0.8% [0.5; 1.2], +1.3% [0.6; 1.9] and +1.1% [0.3; 1.9] respectively) and for the warmest temperature strata (+0.9% [0.4; 1.3], +1.3% [0.6; 2.1] and +1.2% [0.3; 2.1] respectively). A significant interaction was found between ozone and warm days for non-accidental mortality. Results were robust to the sensitivity analyses.ConclusionsThis study provides evidences of a larger impact of ozone when the temperatures are warmer for non-accidental mortality and cardiovascular mortality.

► The study investigated if temperature modifies the mortality impacts of ozone.
► Nine cities and six causes of mortality were investigated per seasons and temperature.
► A significant interaction was found between ozone, warm days and mortality.
► The non-accidental and cardiovascular causes were the most impacted.