Regional variation in the role of humidity on city-level heat-related mortality

The rising humid heat is regarded as a severe threat to human survivability, but the proper integration of humid heat into heat-health alerts is still being explored. Using state-of-the-art epidemiological and climatological datasets, we examined the association between multiple heat stress indicators (HSIs) and daily human mortality in 739 cities worldwide. Notable differences were observed in the long-term trends and timing of heat events detected by HSIs. Air temperature (T_air) predicts heat-related mortality well in cities with a robust negative T_air-relative humidity correlation (C_T-RH). However, in cities with near-zero or weak positive C_T-RH, HSIs considering humidity provide enhanced predictive power compared to T_air. Furthermore, the magnitude and timing of heat-related mortality measured by HSIs could differ largely from those associated with T_air in many cities. Our findings provide important insights into specific regions where humans are vulnerable to humid heat and can facilitate the further enhancement of heat-health alert systems.