Abstract
BACKGROUND: The hazardous impact of heat on health has been extensively reported, with many studies showing long-term adaptation to heat. However, short-term acclimatization, particularly within a season, remains unclear. METHODS: Using a two-stage time-series approach and nationwide medical claims data (2011-2019, May-September) for South Korea, we estimated the association between ambient temperature and emergency department (ED) visits as well as its within-summer variation, focusing on individuals aged ≥65 years. FINDINGS: Within the study period, 4,637,075 ED visits were recorded. For the overall population, the relative risk (RR) comparing the 95th versus the 50th percentile of summer temperature was 1·076 (95% confidence interval [CI]: 1·070-1·082). Individuals aged ≥75 years, females, low-income groups, and those with renal-related ED visits exhibited greater vulnerability to extreme heat. RRs for heat were higher in early summer (RR: 1·124 [95% CI: 1·098-1·150]) than in late summer (RR: 1·054 [95% CI: 1·043-1·066]). This within-summer variation was generally consistent across sex, age, income level, and renal- and mental disorders or neurological diseases-related ED visits. However, groups more vulnerable to extreme heat, such as individuals aged ≥75 years, females, low-income groups, and those with renal-related ED visits, showed smaller variation, suggesting persistent vulnerability throughout the summer period. INTERPRETATION: Heat-related ED visits for extreme heat were generally higher in early summer than in late summer, suggesting the necessity to consider temporal changes in heat risk when developing nuanced and targeted national heat adaptation policies. FUNDING: Korea Environment Industry & Technology Institute.