Long-term exposure to high temperature associated with the incidence of major depressive disorder.

Temperature was reported to have acute effects on mental disorders. However, the effect of long-term temperature exposure is unclear, especially in warmer regions. This research aimed to evaluate the association between long-term exposure to temperature and the incidence of major depressive disorder (MDD). A retrospective population-based study was conducted using Taiwan nationwide Longitudinal Health Insurance Database 2005. Subjects without MDD diagnosis were followed from 2003 to 2013 for MDD occurrence. Weather factors including temperature (°C), sunshine duration (hrs/day), and precipitation (mm/day) were 11-year average of daily data collected from weather monitoring stations of Taiwan Central Weather Bureau. Cox proportional hazard model was performed to examine the association between MDD incidence and weather factors. Covariates including age, gender, and insurance amount were adjusted. A total of 9,539,752 person-years were followed. People residing in regions with long-term temperature of 20-23 °C had the lowest risk of developing MDD. Risk for MDD incidence was higher among residents in regions with average temperature > 23 °C, at 7% increase per 1 °C increment (95% CI = 2-12%). The elderly was more vulnerable to such impact of heat exposure. For the elderly, males were more susceptible to heat-related MDD than females (HR = 1.18 vs. 1.14). However, the younger group showed a reverse finding (i.e. females 1.07 > males 1.04). Long-term residence in regions with extreme cold or heat may increase the risk of MDD incidence, especially higher temperature. Under global warming, these findings have great health implications and warrant further confirmation.