Warm season hourly temperature and psychiatric emergencies: A case-crossover study of a vulnerable population in Boston, 2005–2019

Background: Previous studies have demonstrated associations between extreme heat and mental health; however, the acute temporal dynamics of this relationship, including diurnal variation and lag, remain unexplored, particularly among high-risk groups such as those who are unstably housed. In this time-stratified case-crossover study, we investigated the association between hourly ambient temperature exposure and psychiatric emergency department (ED) visits in a vulnerable population in the United States. Methods: Using data from the Boston Emergency Services Team, we identified warm-season (May–September) psychiatric ED visits from 2005 through 2019, including publicly insured and uninsured individuals in Massachusetts. Hourly ambient temperature at 1 × 1 km spatial resolution was estimated for each individual’s residential location for each of the 24 hours preceding each ED visit. Results: Among 63,691 psychiatric ED visits, cumulative hourly temperature increases were associated with elevated psychiatric emergency risk over 24 hours. At the 99th percentile (31.96°C), risk increased 1.13-fold (95% confidence interval: 1.03, 1.25) compared with the minimum morbidity temperature (8.57°C). Hyper-acute exposure (0–6 hours) showed imprecise protective associations, while delayed exposure (15–18 hours) demonstrated substantially increased risk. Notably, sustained extreme heat exposure was associated with stronger risk (e.g., 14 hours of extreme temperatures yielded a 1.43-fold increased risk [95% confidence interval: 1.05, 1.94]). Conclusions: Our results demonstrate that heat impacts are acute (i.e., within a single day) but not immediate, manifesting most strongly after several hours of sustained exposure. These findings suggest diurnal temporal patterns should be considered for heat-health interventions and provide insights for clinical resource allocation during extreme heat.