Compound exposure to heat wave, SFAP elevates risk of CV deaths

Individuals with multiple exposure to extreme heat and substantial fire-sourced air pollution (SFAP) are at greater risk of cardiovascular mortality than those with single exposures, a study has found.

This study conducted across six countries analysed national representative daily cardiovascular disease (CVD) mortality data in relation to heat waves, SFAP, and their co-occurrence.

The researchers classified each day into mutually exclusive exposure rates: no hazard, heat wave-only, SFAP-only, and compound exposure. They also estimated relative risks (RRs) using quasi-Poisson regression and examined additive interaction using the relative excess risk due to interaction (RERI), attributable proportion, and synergy index.

Analysis of 5.9 million CVD death records revealed an 11.5-percent increase in cardiovascular mortality (RR, 1.115, 95 percent confidence interval [CI], 1.100‒1.130) in individuals with compound exposure, exceeding risks for those with heat wave-only (RR, 1.083, 95 percent CI, 1.074‒1.091) and SFAP-only exposures (RR, 1.014, 95 percent CI, 1.008‒1.021).

A modest positive additive interaction was also noted (RERI, 0.018, 95 percent CI, 0.002‒0.035), which corresponded to 1.598 percent (95 percent CI, 0.181‒3.129) of cardiovascular mortality during compound exposure days.

Notably, heart failure (HF) mortality showed the largest additive interaction, in which compound exposure correlated with a 24.5-percent risk increase (RR, 1.245, 95 percent CI, 1.191‒1.301), with 7.731 percent (95 percent CI, 3.293‒12.488) of HF deaths during compound exposure days attributed to additive interaction (RERI, 0.096, 95 percent CI, 0.040‒0.161).

Smaller, positive interaction effects were also seen for ischaemic heart disease and stroke. Interaction magnitude varied across countries, with the largest effects noted 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,” the researchers said.

“These findings indicate that treating heat waves and landscape fire smoke as independent hazards may underestimate short-term cardiovascular mortality risk,” they added.