An ecological time-series study of heat-related illness in Honolulu County, Hawaiʻi: implications for heat preparedness and public health response

Abstract

Heat-related illness (HRI) is a growing public health concern. HRI risk in Hawaiʻi has not previously been reviewed to determine the risks of heat on public health and well-being. This study summarizes HRI in Honolulu County and evaluates the relationship between local daily heat metrics and HRI by residency status. Syndromic surveillance was used to collect clinical visits to hospital emergency departments, urgent care centers, and outpatient clinics for HRI in Honolulu County from January 2022 to December 2024. Associations between daily maximum heat metrics (dry bulb, wet bulb, and heat index) and HRI were estimated using Poisson regression with distributed lag non-linear models (DLNMs). Models incorporated a 3-day cumulative lag and were adjusted for temporal trends and day of the week, with relative risks estimated by comparing the 75th to the 25th percentile of each heat metric. During the study period, the syndromic surveillance system captured 969 HRI cases in Honolulu County; 698 (72.0%) occurred in residents and 271 (28.0%) in visitors. Daily maximum dry bulb, wet bulb, and heat index temperatures showed seasonal variation. In DLNMs, dry bulb temperature was not significantly associated with HRI. In contrast, higher wet bulb temperature was associated with increased HRI risk for both residents (RR = 1.60, 95% CI: 1.21–2.12) and visitors (RR = 2.54, 95% CI: 1.66–3.90), and higher heat index was also associated with increased risk for residents (RR = 2.13, 95% CI: 1.36–3.34) and visitors (RR = 2.68, 95% CI: 1.33–5.41). HRIs in Honolulu County were strongly associated with heat metrics that incorporate humidity. Consistent with previous studies, these results highlight the importance of accounting for both temperature and humidity when assessing the risk of heat stress, as humidity-inclusive metrics may better guide heat-related public health prevention and preparedness efforts in humid climates such as Hawaiʻi. Because meteorological exposures were derived from a single monitoring station representing county level conditions, these findings should be interpreted at the county level and may not fully capture localized microclimatic variability across Oʻahu.