Assessing health risks of extremely hot weather and the need for enhanced heat alerts in a City with a subtropical climate.

The Hong Kong Observatory has operated the Very Hot Weather Warning since 2000 to warn the local population of very hot weather, defined as when the daily maximum temperature (Tmax) reaches 33 °C. Recent years have seen an increasing number of days with Tmax reaching 35 °C, hence necessitating a review and update of the warning system. In this study, the health impacts of heat were assessed to inform and support the update of the Very Hot Weather Warning. A retrospective study of heat-health related mortality and emergency calls of the Hong Kong population in 2012 to 2022 was conducted. Generalized linear models and distributed lag non-linear model were performed to estimate relative risk (RR) regarding different levels of Tmax. Sensitivity analyses were performed with other indicators of heat exposure. There was a dip of RR at Tmax starting from around 34 °C for cardiovascular disease and heat stroke emergency calls for those aged 0 to 64, although not reaching statistical significance. Among those aged 0 to 64, the RR for heat-health related mortality and emergency calls increased to 1.148 (95% CI: 0.832 to 1.584) and 1.074 (95% CI: 0.984, 1.171) at Tmax of 35.5 °C, respectively. In terms of lag effect, it was found that the lag effect of high temperatures on emergency call was within 1 day. Among alternative heat exposure indicators, daily minimum temperature (Tmin) consistently demonstrated elevated risk in extreme hot temperature for all the outcomes. This observational study of association between adverse health outcomes and extreme high temperature supports the addition of an advisory message when Tmax reaches 35 °C for Hong Kong. Further enhancement may involve other heat stress indicator.