Heat index impacts psychiatric emergency visits: results from a large-scale study

Increasing ambient temperatures have been associated with higher psychiatric emergency department (ED) visits. Previous studies reported a 2–5% increase in visits per 1 °C rise, mainly for mood, anxiety, psychotic disorders, and suicidal behavior. However, the impact of the heat index (HI), which integrates temperature and humidity, on psychiatric emergencies has not been studied in France. We analyzed 69,764 psychiatric emergency visits (2016–2023) using distributed lag non-linear models to examine the association between daily HI and psychiatric emergency visits in France. Model robustness was evaluated through sensitivity analyses varying lag structures and spline degrees of freedom. The HI showed a non-linear association with Psychiatric ED visits, peaking at 28.4 °C with a 11% increase (RR = 1.11, 95%CI: 1.03–1.19) compared to -1 °C. Effects were stronger in women (+ 16%, RR = 1.16, 95%CI: 1.05–1.35) than in men (+ 8%, RR = 1.08, 95%CI 0.95–1.25), but the sex × HI interaction was not statistically significant. High HI was also associated with increased hospitalization (+ 12%, RR = 1.12, 95% CI:1.01–1.26) and involuntary hospitalizations (+ 20%, RR = 1.20, 95% CI: 1.02–1.42). The heat index is an important environmental determinant of psychiatry emergencies. Integrating meteorological indicators into mental health surveillance and prevention systems could enhance preparedness for heat-related psychiatric crisis in changing climates.