Compound Heatwaves and Health Inequality: A Nationwide Study on Emergency Admissions and Socioeconomic Vulnerability in South Korea.

Climate change is intensifying the frequency and severity of extreme heat events. Despite the rising frequency of compound heatwaves (CHWs)-concurrent daytime and nighttime heat-evidence regarding cause-specific risks and socioeconomic disparities remains limited. This study aimed to examine associations between CHWs and emergency hospital admissions and to identify vulnerable subpopulations. This study analyzed 2,010,769 emergency hospital admissions in the summer (June-September, 2011-2019) using the South Korean National Health Insurance Service database. Heatwaves were classified as daytime, nighttime, or compound based on district-specific 90th percentile thresholds of the daily maximum and minimum temperatures. A time-stratified case-crossover design with a distributed lag model was applied to estimate odds ratios for emergency hospital admissions associated with each heatwave type. Subgroup analyses were conducted including age, sex, household income, population density, and admission cause. Furthermore, we tested the potential impacts of income by incorporating an interaction term between income and each subgroup. CHWs were associated with emergency admissions, with particularly strong associations for genitourinary diseases. The risks were highest among the elderly, male, residents of highly populated areas, and low-income groups. Furthermore, the association was most pronounced for genitourinary diseases, where the low-income group showed the highest risk. Daytime heatwaves showed higher risks than nighttime heatwaves, which had weaker but disease-specific associations. This study showed the health risks of CHWs that disproportionately affect genitourinary conditions and low-income populations, emphasizing the need for climate adaptation planning that prioritize vulnerable groups and heat-sensitive diseases.