Policies and practices must account for the synergistic effects of different pollutants The combustion of fossil fuel drives climate change by producing greenhouse gases, and it harms human health through air pollution, which is responsible for more than eight million deaths annually, accounting for nearly 15% of deaths worldwide.1 The linked study by Liu and colleagues (doi:10.1136/bmj-2023-075203) sheds new light on the pervasive harms of air pollution and exemplifies how climate change—and the human activities that drive it—multiply the risk of harm. These authors showed how two common air pollutants, fine particulate matter (PM2.5, airborne particles with a diameter of ≤2.5 µm) and ozone (O3), act synergistically to harm those exposed,2 providing compelling new evidence that both policies and healthcare practices must change to account for new and evolving understanding of the compound effects of climate change, its drivers, and its consequences on human health. PM2.5 is generated

Objective To investigate potential interactive effects of fine particulate matter (PM2.5) and ozone (O3) on daily mortality at global level. Design Two stage time series analysis. Setting 372 cities across 19 countries and regions. Population Daily counts of deaths from all causes, cardiovascular disease, and respiratory disease. Main outcome measure Daily mortality data during 1994-2020. Stratified analyses by co-pollutant exposures and synergy index (>1 denotes the combined effect of pollutants is greater than individual effects) were applied to explore the interaction between PM2.5 and O3 in association with mortality. Results During the study period across the 372 cities, 19.3 million deaths were attributable to all causes, 5.3 million to cardiovascular disease, and 1.9 million to respiratory disease. The risk of total mortality for a 10 μg/m3 increment in PM2.5 (lag 0-1 days) ranged from 0.47% (95% confidence interval 0.26% to 0.67%) to 1.25% (1.02% to 1.48%) from the lowest to hi