A large and growing body of research has provided increasingly compelling evidence that exposure to particulate matter (PM) air pollution contributes to cardiovascular disease (CVD) morbidity and mortality.1 Overall, the evidence suggests that fine, combustion-related particulate pollution, measured as PM_2.5 (PM with an aerodynamic diameter of less than 2.5 µm), is most strongly associated with adverse CVD health outcomes. Studies have evaluated day-to-day changes in mortality risk associated with short-term (one to a few days) changes in PM air pollution as reviewed elsewhere.1 One of the earliest of these studies2 evaluated daily mortality counts in London for the winters of 1958–1972, a period of time with episodes of extremely high concentrations of air pollution. Elevations in air pollution were associated with increased risk of mortality and the exposure–response relationship was non-linear across the full range of exposures: relatively steep at lower levels of exposures and flattening out at higher levels of exposures.2

Over the subsequent decades, many studies have evaluated associations between short-term changes in air …