Combined exposure to substantial fire-sourced air pollution (SFAP) and heat waves was associated with a higher excess risk of cardiovascular mortality than exposure to either hazard alone, according to a multicountry time-series study published July 29 in JACC. The findings suggest that evaluating landscape fire smoke and heat waves as separate hazards may underestimate short-term cardiovascular mortality risk.

Bo Wen, PhD, et al., analyzed nationally representative cardiovascular mortality data from 2000 to 2019 across six countries: Australia, Brazil, Canada, Chile, Mexico and Thailand. To assess the individual and combined effects of SFAP and heat waves, each day was classified as no hazard, heat-wave only, SFAP-only or compound exposure. Researchers then calculated relative risk (RR) and relative excess risk due to interaction (RERI).

Based on nearly 6 million cardiovascular death records from 755 locations, compound exposure was associated with a 12% increase in cardiovascular mortality, exceeding the risk observed with heat-wave-only exposure (RR, 1.083) or SFAP-only exposure (RR, 1.014).

Approximately 2% of cardiovascular deaths occurred on compound exposure days. HF-related mortality showed the greatest increase (RR, 1.3), with 8% of HF deaths on compound exposure days attributable to the additive interaction (RERI, 0.096).

Smaller, but still positive, additive interactions were observed for ischemic heart disease and stroke.

The magnitude of these interactions varied by country, with the strongest effects observed in Australia. Results were consistent across buffering windows and heat wave definitions, although estimated effects were modestly attenuated with wider buffers.

"Our findings suggest that cardiovascular risk may be shaped not only by individual environmental hazards but also by the convergence of multiple climate-related extreme events occurring within the same short time window," the authors write, adding that the results have "important implications for cardiovascular risk management under climate change."

In an accompanying editorial, Lingzhi Chu, PhD, writes that "the substantial geographic heterogeneity in interaction estimates across countries is compelling," and that "understanding which contextual factors drive this variation would help in translating these scientific findings into locally actionable strategies."