Changes in susceptibility to heat on suicide mortality within summer in Japan, 1973-2020: A two-stage time-series...

We examined the changes in the risk of heat-associated suicide during the summer in Japan from 1973 to 2020.

A two-stage time-series analysis was conducted separately for daily maximum, mean, and minimum temperatures to examine the association between heat and suicide. In the first stage, we performed prefecture-specific analyses using a distributed lag non-linear model (DLNM) to estimate the exposure-lag-response association between temperature and suicide mortality and its within-summer variation. In the second stage, we used multivariate meta-regression to combine temperature-suicide associations across 47 prefectures and estimated the relative risks (RRs) for the 90th percentile of temperature compared with the minimum value.

A total of 383,115 suicides were included. The RRs of suicide for maximum, mean, and minimum temperatures were 1.23 (95% confidence interval (CI) 1.14-1.31), 1.18 (95% CI 1.10-1.27), and 1.10 (95% CI 1.03-1.17), respectively. When comparing early and late summer, the RRs for maximum temperatures did not differ between the two periods (RR = 1.24 [95% CI = 1.15-1.33] and RR = 1.24 [95% CI = 1.09-1.39], respectively). However, the RR for minimum temperatures was higher in early summer (RR = 1.14 [95% CI = 1.07-1.22]) and lower in late summer (RR = 1.05 [95% CI = 0.94-1.17]).

We found a modest late-summer decline in susceptibility to heat-associated suicide from minimum temperatures, suggesting adaptation to nighttime heat, whereas the risk associated with maximum temperatures, reflecting daytime heat, remained stable across summer.