Compound hot-humid extreme events and their mortality associations during summer and shoulder months in a subtropical coastal city

Abstract

Humid heat may pose greater health risk than heat alone, however, epidemiological studies have been limited in identifying humidity effects. This study assessed mortality impacts of compound hot-humid events to capture the joint effects of heat and humidity in subtropical Hong Kong. We investigated 22 definitions using specific and relative humidity under absolute and seasonally-determined thresholds. Generalized Linear Models and Distributed Lag Non-linear Models were used to estimate non-external mortality risk of compound events, further stratified by season, age, sex, and disease subgroups. Summer season compound hot-humid events were associated with increased mortality under seasonally-determined definitions of Hot_wet (all ages: Relative Risk RR: 1.026, 95 % CI: 1.003, 1.050, Attributable Number AN: 822) and Hot_wet95 (RR: 1.056, 95 % CI: 1.022, 1.091; AN: 901). Higher mortality risk was found for older adults, males, and pneumonia-related mortality. Pre-summer season analysis additionally found increased mortality with VHWW_wet and VHWW_wet95, when local weather warning is hoisted in 33 °C and above. Overall, compound hot-humid events using specific humidity and a seasonally-determined threshold captured the increased risk of high humidity on mortality. Under climate change, heat-health warning systems, heat action plans, and adaptation strategies need to be prepared for the impacts of compound hot-humid events and those early-in-season.