Ambient temperature, suicide, and urbanicity: A nationwide time-stratified case-crossover study in South Korea.

Several studies have examined the nationwide or multi-region relationship between ambient temperature and suicide mortality; however, evidence of comprehensive roles of urbanicity that can affect temperature, suicide, and risk-risk populations was limited. To reduce the gaps in knowledge, this study examined the nationwide ambient temperature-suicide association and heterogeneous high-risk populations by urbanicity levels and sex/age groups, based on a time-stratified case-crossover design with national mortality data (2015-2019). In the total population (65,645 suicides), we found an inverted J-shaped relationship between ambient temperature and suicide, and across all populations, higher temperature was associated with an increased risk of suicide mortality. Nevertheless, the association differed by urbanicity level, sex, and age group. First, in the total population, metropolitan and less-urban areas showed the stronger ORs (odd ratios between the maximum suicide temperature and 20th percentile of the temperature; metropolitan OR: 1.471, 95% CI: 1.141-1.898; less-urban OR: 1.631, 95% CI: 0.968-2.747) than mid-urban areas. In metropolitan areas, stronger ORs were observed in individuals aged 65 years or older (1.926, 1.175-3.157) and 0-44 years (1.575, 0.951-2.608) than in those aged 45-64 years. In mid-urban areas, all subgroups showed no evident association except for people aged 65 years or older. Whereas, less-urban males showed a marginally higher OR (1.667, 0.911-3.051) than less-urban females, although we could not observe sex differences in other areas. Our findings provide evidence for establishing more precise urban health policies and social interventions to reduce the risk of suicide related to ambient temperatures.