A new multinational study has found that sustained extreme heat is associated with a measurable rise in hospitalizations for mental and behavioural disorders, adding to growing evidence that heatwaves affect far more than the cardiovascular and respiratory systems. An analysis of more than 2.6 million warm-season hospitalization records from Brazil, Canada, Chile and New Zealand found that the risk increased both during prolonged heat events and over the days that followed. The findings suggest that severe heat may create a short-term surge in mental-health-related demand for hospitals, particularly among older adults and people living in areas with low population density.
Published in Nature Health, the study examined 2,618,307 hospitalization records collected between 2000 and 2019 across 852 locations. The researchers focused on the warm seasons in each country and investigated whether hospital admissions for mental and behavioural disorders were more common during periods of unusually high temperatures. Rather than comparing different groups of people, the analysis compared individuals with themselves across different days, allowing the researchers to examine whether hospitalization risk changed when a person’s location experienced a heatwave.
The primary definition of a heatwave was deliberately strict. It required daily mean temperatures to remain above the location-specific 97.5th percentile for at least four consecutive days. In practical terms, this threshold identified temperatures that were exceptionally high for a particular place, taking local climate into account. A temperature that is extreme in one region may be relatively ordinary in another, so using location-specific percentiles allowed the analysis to measure heat stress against the conditions to which local populations are normally exposed.
Under this definition, the relative risk of hospitalization was 1.033 on the same day as the heatwave condition. This corresponds to an estimated 3.3% increase in hospitalization risk compared with non-heatwave control days, with a 95% confidence interval ranging from 1.007 to 1.059. The confidence interval indicates the range of values compatible with the statistical analysis, and because it remained above 1, the researchers classified the association as statistically meaningful. Although a 3.3% increase may appear modest at the individual level, even a small percentage change can translate into a substantial number of additional admissions when extreme heat affects large populations.
The consequences became more pronounced when the researchers considered delayed effects. From the same day through the following eight days, the cumulative relative risk reached 1.056, corresponding to an estimated 5.6% increase in hospitalization risk. The confidence interval for this cumulative estimate was 1.011 to 1.103. This lagged pattern suggests that the mental-health effects of sustained heat may not end when temperatures begin to decline. Physiological strain, sleep disruption, dehydration, medication-related vulnerabilities and the psychological stress of prolonged heat could continue to influence health after the most intense conditions have passed.
To identify these patterns, the researchers used a time-stratified case-crossover design. This method is commonly used to study short-term environmental exposures, such as air pollution, temperature extremes or infectious disease outbreaks. Each hospitalization day was compared with control days for the same location and the same day of the week within a defined time period. By making these comparisons within individuals or closely matched time windows, the approach automatically controls for characteristics that do not change rapidly, such as genetics, long-term socioeconomic status and chronic baseline health. The time-stratified structure also helps reduce bias caused by seasonal trends and weekly hospitalization patterns.
The analysis showed that the association was stronger among older adults. Aging can increase sensitivity to heat through reduced thermoregulatory capacity, a higher prevalence of chronic illness and the use of medications that affect hydration, sweating, blood pressure or sleep. Older people may also be more vulnerable to social isolation during extreme weather, particularly if they live alone or have limited access to cooling environments. The researchers also observed stronger associations in low-population-density areas, where hospitals and emergency services may be farther away, cooling infrastructure may be less available and public-health monitoring may be more limited.
The findings do not establish that every heatwave directly causes a mental-health hospitalization, and the study does not identify a single mechanism responsible for the observed increase. Mental and behavioural disorders encompass a broad range of conditions, while hospitalization is influenced by healthcare access, clinical decisions, family support and the availability of community services. Nevertheless, the consistency of the association across multiple countries and hundreds of locations strengthens the evidence that prolonged extreme heat can act as an acute environmental stressor with consequences for mental health. The use of a large international dataset also helps demonstrate that the relationship is not restricted to one climate zone or healthcare system.
The study arrives as heatwaves are becoming more frequent, intense and persistent in many parts of the world. Climate change is increasing the likelihood of days that exceed historical temperature extremes, raising concern that hospitals will face overlapping surges in heat-related illness and mental-health demand. The researchers’ results point to the importance of including mental and behavioural health in heatwave preparedness plans. Early warnings, welfare checks for vulnerable residents, accessible cooling spaces, medication guidance and coordination between emergency departments and community mental-health services could help reduce the burden. As extreme heat becomes a recurring feature of modern summers, the study suggests that protecting public health will require treating mental-health preparedness as an essential part of heatwave response.
Subject of Research: The association between sustained extreme heat and hospitalizations for mental and behavioural disorders across Brazil, Canada, Chile and New Zealand.
Article Title: Mental health hospitalizations associated with sustained extreme heat in multiple countries
Article References: Liu, Y., Xu, Z., Huang, W. et al. Mental health hospitalizations associated with sustained extreme heat in multiple countries. Nat. Health (2026). https://doi.org/10.1038/s44360-026-00166-2
Image Credits: AI Generated
DOI: https://doi.org/10.1038/s44360-026-00166-2
Keywords: heatwaves, extreme heat, mental health, behavioural disorders, hospitalizations, climate change, public health, older adults, case-crossover analysis, environmental epidemiology
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