Heat Exposure Following the Rainy Season Is Associated With an Increased Risk of Cardiovascular Emergency Among the Elderly in Japan

BACKGROUND: Despite the impact of heat exposure caused by global warming, few studies have investigated the hourly effects of heat exposure and the risk of cardiovascular disease (CVD) in the elderly. We examined the associations between short‐term heat exposure and the risk of CVD in the elderly in Japan and evaluated possible effect‐measure modifications by rainy seasons that occur in East Asia. METHODS AND RESULTS: We conducted a time‐stratified case–crossover study. The study included 6527 residents in Okayama City, Japan, aged ≥65 years who were transported to emergency hospitals between 2012 and 2019 for the onset of CVD during and a few months after the rainy seasons. We examined the linear associations between temperature and CVD‐related emergency calls for each year and for hourly preceding intervals before the emergency call during the most relevant months. Heat exposure during 1 month after the end of the rainy season was associated with CVD risk; the odds ratio (OR) for a 1° C increase in temperature was 1.34 (95% CI, 1.29–1.40). When we further explored the nonlinear association by using the natural cubic spline model, we found a J‐shaped relationship. Exposures 0 to 6 hours before the case event (preceding intervals 0–6 hours) were associated with CVD risk, particularly for the preceding interval 0 to 1 hour (OR, 1.33 [95% CI, 1.28–1.39]). For longer periods, the highest risk was at preceding intervals 0 to 23 hours (OR, 1.40 [95% CI, 1.34–1.46]). CONCLUSIONS: Elderly individuals may be more susceptible to CVD after heat exposure during the month after the rainy season. As shown by finer temporal resolution analyses, short‐term exposure to increasing temperature can trigger CVD onset.