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 to 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 %-0.00 % in Sydney, and -0.71 %-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.