Flood exposure and suicide risk in Japan, 2008-2022: a nationwide study

Flooding is a major climate-related hazard whose frequency and severity are increasing globally, yet evidence linking flood exposure to suicide remains limited. We aimed to evaluate the association between prefecture-level flood exposure and suicide in Japan. In this nationwide ecological longitudinal study, we analyzed monthly panel data from all 47 prefectures in Japan (2008-2022). Suicide counts were obtained from the Vital Statistics of Japan and flood exposure (number of flood days) from the Dartmouth Flood Observatory. Associations between flood days and suicide risk over the subsequent 0-18 months were estimated with a two-stage distributed lag non-linear model (prefecture-specific quasi-Poisson models pooled by multivariate random-effects meta-analysis). Among 8460 prefecture-months, 380,224 suicides were recorded, and 42 (89.4%) of 47 prefectures were flood-affected. One flood day versus none was associated with an increase in suicide risk over 0-18 months (cumulative relative risk 1.112, 95% CI 1.011-1.222), with significant elevation from lag 6 to 14 months and a decline thereafter. Flood exposure corresponded to an attributable fraction of 0.54% of suicides (95% CI 0.06-1.02%), an annual average attributable number of 105 (95% CI 11-200), and an attributable number rate of 0.11 per 100,000 person-years (95% CI 0.01-0.20). These exploratory findings suggest flood-related suicide risk may persist beyond the acute phase and warrant further investigation.