RESEARCH ARTICLE Neighbourhood Characteristics and Long- Term Air Pollution Levels Modify the Association between the Short-Term Nitrogen Dioxide Concentrations and All- Cause Mortality in Paris

Background While a great number of papers have been published on the short-term effects of air pollu-tion on mortality, few have tried to assess whether this association varies according to the neighbourhood socioeconomic level and long-term ambient air concentrations measured at the place of residence. We explored the effect modification of 1) socioeconomic status, 2) long-term NO2 ambient air concentrations, and 3) both combined, on the association between short-term exposure to NO2 and all-cause mortality in Paris (France). Methods A time-stratified case-crossover analysis was performed to evaluate the effect of short-term NO2 variations on mortality, based on 79,107 deaths having occurred among subjects aged over 35 years, from 2004 to 2009, in the city of Paris. Simple and double interactions were statistically tested in order to analyse effect modification by neighbourhood characteristics on the association between mortality and short-term NO2 exposure. The data was esti-mated at the census block scale (n=866). Results The mean of the NO2 concentrations during the five days prior to deaths were associated with an increased risk of all-cause mortality: overall Excess Risk (ER) was 0.94 % (95%CI= [0.08;1.80]. A higher risk was revealed for subjects living in the most deprived census blocks in comparison with higher socioeconomic level areas (ER=3.14 % (95%CI=[1.41-4.90], p<0.001). Among these deprived census blocks, excess risk was even higher wher