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

• Published in: Sustainable cities and society 2025-01, Vol.118, Article 106031
• Main Authors: Ho, Janice Y., He, Yueyang, Chong, Ka Chun, Ren, Chao
• Format: Article
• Language: English
• Subjects: Compound event; Heat-related mortality; Hot humid; Humidity; Pre-summer season; Shoulder month
• ISSN: 2210-6707
• DOI: 10.1016/j.scs.2024.106031

•22 compound hot-humid definitions assessed for mortality impact.•Hot_wet (specific humidity & seasonal threshold) linked to higher mortality risk.•Pre-summer high humidity increased risk via Hot_wet & VHWW_wet definitions.•Study identified difference between sexes in response to heat & humidity.•Humidity must be integrated into warning systems, action plans & adaptation. 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.