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 (PM(2.5)) concentrations are associated with an increased risk of mortality. However, less is known about how oxidant gases may modify the acute health effects of PM(2.5). Our objective was to investigate whether associations between acute exposure to PM(2.5) and mortality were modified by the oxidant gases O(3) and NO(2) using their redox-weighted average (O(x)). 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 PM(2.5) and O(x) concentrations were each associated with small increases in nonaccidental and cardiovascular mortality. In stratified analyses, associations between PM(2.5) and nonaccidental and cardiovascular mortality tended to be greatest in the highest tertile of O(x) with a significant interaction observed between lag 0 PM(2.5) and 3-day mean O(x) (interaction p-value = 0.04). There was no evidence of effect modification by O(x) in the relationship between PM(2.5) and respiratory mortality. Overall, the relationship between short-term changes in outdoor PM(2.5) 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 PM(2.5).