Compound Exposure to Extreme Heat and Substantial Landscape Fire-Sourced Air Pollution on Cardiovascular Mortality.

BACKGROUND

Climate change is increasing the frequency and co-occurrence of extreme heat and landscape fire-sourced air pollution; yet, evidence on their combined cardiovascular impacts remains limited.

OBJECTIVES

The purpose of this study was to examine whether compound exposure to heat waves and substantial fire-sourced air pollution (SFAP) is associated with excess cardiovascular disease (CVD) mortality risk beyond single-hazard exposure and assess potential additive interactions.

METHODS

We conducted a multicountry time-series study across 6 countries, analyzing national representative daily CVD mortality data in relation to heat waves, SFAP, and their co-occurrence. Each day was classified into mutually exclusive exposure states: no hazard, heat wave-only, SFAP-only, and compound exposure. We estimated relative risks (RRs) using quasi-Poisson regression and evaluated additive interaction using the relative excess risk due to interaction (RERI), attributable proportion, and synergy index.

RESULTS

Based on 5.9 million CVD death records, compound exposure was associated with an 11.5% increase in cardiovascular mortality (RR: 1.115; 95% CI: 1.100-1.130), exceeding risks for heat wave-only (RR: 1.083; 95% CI: 1.074-1.091) and SFAP-only exposure (RR: 1.014; 95% CI: 1.008-1.021). Modest positive additive interaction was observed (RERI: 0.018; 95% CI: 0.002-0.035), corresponding to 1.598% (95% CI: 0.181%-3.129%) of cardiovascular mortality during compound exposure days. The largest additive interaction was observed for heart failure (HF) mortality, for which compound exposure was associated with an RR of 1.245 (95% CI: 1.191-1.301), with 7.731% (95% CI: 3.293%-12.488%) of HF deaths during compound exposure days attributable to additive interaction (RERI: 0.096; 95% CI: 0.040-0.161). Smaller but positive interaction effects were also observed for ischemic heart disease and stroke. Interaction magnitude varied across countries, with the largest effects observed in Australia. Results were consistent across alternative buffering windows and heat wave definitions, although the magnitude of the estimated effects attenuated modestly with wider buffers.

CONCLUSIONS

Compound exposure to heat waves and SFAP was associated with higher cardiovascular mortality than single exposures, with stronger additive interaction signals for HF. These findings indicate that treating heat waves and landscape fire smoke as independent hazards may underestimate short-term cardiovascular mortality risk.