Association of short-term exposure to fine particulate air pollution and mortality: effect modification by oxidant gases

Short term changes in exposure to outdoor fine particulate matter (PM2.5) concentrations are associated with an increased risk of mortality. However, less is known about how oxidant gases may modify the acute health effects of PM2.5. Our objective was to investigate whether associations between acute exposure to PM2.5 and mortality were modified by the oxidant gases O3 and NO2 using their redox-weighted average (Ox). We conducted a multi-city case-crossover study in 24 cities across Canada between 1998–2011 including 1,179,491 nonaccidental mortality events. Interquartile increases in lag-0 and 3-day mean PM2.5 and Ox concentrations were each associated with small increases in nonaccidental and cardiovascular mortality. In stratified analyses, associations between PM2.5 and nonaccidental and cardiovascular mortality tended to be greatest in the highest tertile of Ox with a significant interaction observed between lag 0 PM2.5 and 3-day mean Ox (interaction p-value = 0.04). There was no evidence of effect modification by Ox in the relationship between PM2.5 and respiratory mortality. Overall, the relationship between short-term changes in outdoor PM2.5 and nonaccidental mortality may be greater when oxidant gas concentrations are also elevated. In some regions, reductions in oxidant gas concentrations may also reduce the acute health impacts of PM2.5.