Long-Term PM10 Exposure and Cause-Specific Mortality in the Latium Region in (Italy): A Difference-in-Differences Approach

BACKGROUND: The link between particulate matter (PM) exposure and adverse health outcomes has been widely evaluated using large cohort studies. However, the possibility of residual confounding and lack of information about the health effects of PM in rural and suburban areas are unsolved issues. OBJECTIVE: Our aim was to estimate the effect of annual PM <= 10 mu m (PM10) exposure on cause-specific mortality in the Latium region (central Italy, of which Rome is the main city) during 2006-2012 using a difference-in-differences approach. METHODS: We estimated daily PM10 concentrations for each 1 km(2) of the region from 2006 to 2012 by use of satellite data, land-use predictors. and meteorological parameters. For each of the 378 regional municipalities and each year, we averaged daily PM10 values to obtain annual mean PM10 exposures. We applied a variant of the difference-in-differences approach to estimate the association between PM10 and cause-specific mortality by focusing on within-municipality fluctuations of mortality rates and annual PM exposures around municipality means, therefore controlling by design for confounding from all spatial and temporal potential confounders. Analyses were also stratified by population size of the municipalities to obtain effect estimates in rural and suburban areas of the region. RESULTS: In the period 2006-2012, we observed deaths due to three causes: 347,699 nonaccidental; 92,787 cardiovascular; and 16,509 respiratory causes. The annual average (standard deviation, SD)( )PM10 concentration was 21.9 ( +/- 4.9) mu g/m(3) in Latium. For each 1-mu g/m(3) increase in annual PM10 we estimated increases of 0.8% (95% confidence intervals (CIs): 0.2%, 1.3%), 0.9% (0.0%, 1.8%), and 1.4% (-0.4%, 3.3%) in nonaccidental, cardiovascular, and respiratory mortality, respectively. Similar results were found when we excluded the metropolitan area of Rome from the analysis. Higher effects were estimated in the smaller municipalities, e.g., those with population <5,000 inhabitants. CONCLUSION: Our study suggests a significant association of annual PM10 exposure with nonaccidental and cardiorespiratory mortality in the Latium region, even outside Rome and in suburban and rural areas.