Independent and interactive effects of wet bulb globe temperature and air pollution exposures on suicide mortality

Individual components of the ambient environment, such as temperature and air pollution, exist as part of a complex mixture and have been associated with suicide; however, their interactive effects remain poorly understood. This study examined the independent and interactive effects of wet bulb globe temperature (WBGT), nitrogen dioxide (NO₂), and fine particulate matter (PM_2.5) on suicide mortality.

We identified 7,551 suicide cases in Utah, USA, from 2000 to 2016 and assigned exposure to daily maximum WBGT (sourced from the European Center for Medium-Range Weather Forecasts) and PM_2.5 and NO₂ concentrations (sourced from a national spatiotemporal ensemble model) using decedent's residential address at the time of death. A case-crossover design with conditional logistic regression was used to estimate the independent and interactive effects of WBGT_max, PM_2.5, and NO₂ on suicide. For exposure windows, we considered single days preceding suicide (lag 0 to 6) and their averages across preceding days (lag 0-1, 0-3, and 0-6). Analyses were stratified by season.

We identified a significant association between WBGT_max and suicide across all seasons (odds ratio [OR] = 1.05, 95% confidence interval [CI]: 1.01, 1.10; per 5 °C increase on lag 0-3 days). The associations were stronger in the warm season (March 22 to September 21), with ORs and 95% CIs ranging from 1.08 (1.02, 1.15) to 1.20 (1.10, 1.30) per 5 °C increase depending on the lag periods. We observed synergistic interactions between WBGT_max and PM_2.5 and NO₂ in the warm season, associated with higher odds of suicide. The associations of WBGT_max with suicide were most pronounced at high NO₂ levels.

We found evidence of synergistic interactions between WBGT_max and PM_2.5 and NO₂ on suicide in the warm season, emphasizing the need for considering the combined effects of heat stress and air pollution in suicide prevention strategies.