Short-term exposure to ambient temperature and the mortality burden of suicide in Japan

Background. Previous studies suggest that temperature may trigger suicide, with higher ambient temperatures associated with increased risk globally. However, quantification of the attributable suicide burden due to non-optimal temperatures, particularly across specific subgroups like sex, age, and method of suicide, is limited. This study aimed to quantify this attributable burden across Japan and assess variations in these subgroups to enhance understanding and identify high-risk populations. Methods. Data on daily suicide counts and mean temperatures were collected for all 47 prefectures in Japan (1973-2015) for a comprehensive, nationally representative analysis. Using time-stratified case-crossover analyses and a distributed lag non-linear model (DLNM), we estimated temperature-suicide associations, accounting for long-term trends, seasonality, and day-of-the-week effects, and capturing both nonlinear and delayed impacts of temperature. Overall attributable fractions and fractions attributable to cold (0th-10th percentile), cool (10th-50th), warm (50th-90th), and hot (90th-100th) temperatures were computed. Subgroup analyses were conducted by sex, age (0 - 64 years and >= 65 years), and method of suicide (violent or nonviolent). Results. Approximately 19.9% of suicides (95% empirical confidence interval (eCI): 18.8, 20.9) could be attributed to non-optimal temperatures, representing the proportion of cases that would not have occurred under optimal temperature conditions, identified at the temperature corresponding to the minimum risk of suicide. The highest fraction (9.9%, 95% eCI: 9.4, 10.4) was observed for warm temperatures (50th-90th percentile). Higher burdens were observed in females (23.7%), individuals aged >= 65 years (31.9%), and violent suicides (22.4%). Conclusions. Nearly 20% of suicides in Japan are associated with non-optimal temperatures, particularly during warmer conditions. These findings highlight the importance of integrating temperature considerations into suicide prevention strategies, particularly for vulnerable populations such as the elderly. While focused on Japan, these results advocate future research to explore similar burdens globally, enhancing our understanding of environmental impacts on suicide and informing targeted public health interventions.