The temporal change in heat-related mortality in relation to the Heat Health Action Plans in the five major cities of Australia.

Heat-Health Action Plans (HHAPs) are essential public health interventions to reduce heat-related mortality and morbidity, yet how heat-related health risks have changed following their implementation remains scarce. This study aimed to examine the temporal changes in heat-related mortality in relation to the implementation of the HHAPs across five major cities of Australia, including Melbourne, Sydney, Brisbane, Adelaide, and Perth. Daily mortality and temperature data from 1999-2019 were analysed using distributed lag non-linear models to compare the heat-related mortality attributable fractions (AF%) between pre- and post-HHAP periods. The results showed an overall reduction in heat-related mortality after HHAP implementation, with a notable 0.12% (95% CI: -0.28 - 0.04) decrease in mortality attributable to extreme heat across five cities. The reduction was more evident in individuals aged 75 and older (-0.18%, 95% CI: -0.35 - 0.01), while the 0-74 age group saw slight increases (+0.13%, 95% CI: -0.13 - 0.39). Heatwave-related mortality showed general reductions or slight increases across cities, with AF differences in period 2 and 1 ranging from -0.20% to +0.01%. Brisbane, Perth, Sydney, and Adelaide witnessed notable AF decreases or minimal increases in period 2 across different definitions of heatwaves (-1.38% to -0.01% in Brisbane, -2.01% to +0.05% in Perth, -0.19% to 0.00% in Sydney, and -0.71% to 0.17% in Adelaide). In Melbourne, AF increased notably in period 2 for low-intensity heatwaves (≥2 days + 90th/92th percentile) by +0.25% and +0.38%, primarily due to the increase among those aged 0-74. Meanwhile, Sydney showed AF increases by +0.08% to +0.43% among the elderly during low-intensity heatwaves despite overall AF reductions. These findings suggest that HHAPs have reduced heat-related mortality in Australia's major cities, particularly during extreme heat days. However, extreme heat continues to pose significant health risks, highlighting the need for enhanced heat-health strategies.