Numerous factors affect the association between PM2.5 and mortality, including individual and neighborhood socioeconomic status, PM composition and temperature. In a study of 13 million Medicare beneficiaries (older than 65) conducted in the SouthEastern US, researchers at the Harvard ACE Center examined exposures using a hybrid spatio-temporal model that represented more accurately rural populations and those with lower socio-economic status. The risk of death (hazard ratio) increased by 2.1% per each 1 mg/m3 annual increase in PM2.5. The risk decreased with age, but was higher among males, and participants with previous hospital admissions. The risk was higher in neighborhoods with lower socio-economic status, and in more urban neighborhoods. It was also higher as mean summer temperatures increased. The relative concentrations of elemental carbon, vanadium, copper and calcium increased risk, while nitrate, organic carbon, and sulfate decreased the risk.