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

Traditional heatwave monitoring generally relies on daily maximum or minimum temperatures, or on the number of consecutive days above a defined threshold. These measures remain valuable for tracking broad trends, but they can conceal how heat develops through the day. A location may avoid being classified as having an exceptionally hot day while still experiencing several hours of hazardous temperatures, particularly during the afternoon, evening or overnight. By examining temperature conditions at an hourly scale, the researchers reveal a more continuous and extensive form of heat exposure.

The analysis focuses on hourly heat extremes, or HHEs, during summer and evaluates both the occurrence of these events and the number of people exposed to them. This distinction is important because risk depends not only on whether a region experiences an extreme temperature, but also on how long people remain exposed, when the heat occurs and whether it interrupts nighttime cooling. Hourly conditions can influence sleep, physical work, energy demand and the ability of homes and bodies to recover between hot periods.

The researchers report that HHEs are already emerging worldwide and are projected to accelerate sharply under a high-emissions scenario. By the end of the century, the global frequency of hourly heat extremes could be approximately four times higher than today. At the same time, the number of people exposed to these conditions is projected to increase sixfold. The difference between the growth in events and the growth in exposure reflects both the expansion of extreme heat and the distribution of populations across regions where warming and demographic change overlap.

Daily statistics also appear to underestimate the total duration of heat stress. According to the study, each day already identified as hot is expected to gain about four additional hot hours in the future. Even more strikingly, approximately three additional hot hours will occur on days that daily metrics would not classify as hot. These overlooked hours could create a hidden layer of risk, allowing dangerous conditions to affect workers, children, older adults and people with chronic illnesses without triggering conventional heatwave warnings.

The findings also expose a stark economic divide. Low- and middle-income countries account for more than three-quarters of current and projected human exposure to hourly heat extremes. Many of these countries are located in regions expected to experience rapid warming, while also having fewer resources for air conditioning, resilient housing, reliable electricity, public cooling centers and emergency health systems. Outdoor workers and people living in densely populated urban settlements may face particularly intense exposure, although the study’s central message applies across rural and urban environments.

Heat exposure is not distributed equally across generations either. Successive generations are projected to experience substantially more hourly heat extremes over their lifetimes than people born earlier in the century. The contrast is especially severe in low-income countries, where younger cohorts may spend nearly their entire lives under climatic conditions that were exceptional for previous generations. This intergenerational pattern links today’s emissions choices directly to the environmental risks inherited by children and those not yet born.

The study’s results suggest that heat preparedness must move beyond counting hot days. Forecasting systems and public-health alerts may need to incorporate the timing, duration and accumulation of extreme temperatures within individual days. Hourly data could support more precise warnings for workplaces, schools, hospitals and households, while helping electricity providers anticipate surges in cooling demand. Urban planning, building design and occupational safety standards could likewise be adapted to reduce exposure during the most dangerous hours rather than responding only after a formal heatwave has been declared.

The authors emphasize that reducing greenhouse-gas emissions remains essential for limiting the scale of future exposure, but adaptation will also be unavoidable. Equitable responses will require directing resources toward populations facing the greatest risks, particularly in low- and middle-income countries. By making invisible hours of heat visible, the research challenges the way societies define extreme weather and provides a sharper measure of the climate pressures that people will experience throughout their daily lives. In a warming world, the most dangerous heat may not arrive as a single record-breaking day, but as a steadily expanding presence woven into ordinary summer hours.

Subject of Research: Global and intergenerational exposure to hourly heat extremes during summer under climate change.

Article Title: Globally and intergenerationally unequal exposure to hourly heat extremes

Article References: Liao, W., Zhang, X., Liu, X. et al. “Globally and intergenerationally unequal exposure to hourly heat extremes.” Nature Climate Change (2026). https://doi.org/10.1038/s41558-026-02724-8

Image Credits: AI Generated

DOI: https://doi.org/10.1038/s41558-026-02724-8

Keywords: hourly heat extremes, climate change, heatwaves, extreme heat exposure, global warming, public health, inequality, low- and middle-income countries, intergenerational climate risk, adaptation