A new international study warns that severe and prolonged heatwaves may lead to increased hospital admissions for mental and behavioral disorders, as large parts of the United States and Europe are experiencing repeated sharp rises in temperatures.

The study, led by researchers from Monash University in Australia and published in the journal Nature Health, said that sustained exposure to extremely high temperatures during heatwaves was associated with a significant increase in the risk of hospitalization for mental health problems.

The researchers analyzed more than 2.6 million hospital admissions during warm seasons across 852 locations in Brazil, Canada, Chile, and New Zealand between 2000 and 2019, in the first study to compare the burden of mental disorders associated with heatwaves across multiple countries.

A heatwave is defined as a period lasting several days during which temperatures reach exceptionally high levels compared to the usual averages in the region, leading to increased heat-related illness and death.

The study's co-author, Yuming Guo, a professor at Monash University, said the association between heatwaves and hospital admissions for mental and behavioral disorders was more pronounced among the elderly and residents of areas with low population density.

He added that the findings indicate that prolonged exposure to extreme heat can rapidly increase demand for mental health services in hospitals, highlighting the need for preparedness plans targeting the most vulnerable groups during severe heatwaves.

The study not only estimated the overall risk but also compared results by age, sex, population density, average per capita GDP, prevalence of air conditioning, travel time to health facilities, and various causes of mental and behavioral disorders.

Heat stress and sleep disorders

According to the study researchers, heatwaves may acutely exacerbate symptoms of mental disorders by disrupting sleep, increasing physiological stress on the body, and altering its response to stress.

High temperatures affect the body's ability to regulate its temperature, and can cause dehydration, fatigue, rapid heart rate, and disruption of nervous system functions, factors that can increase anxiety, agitation, and confusion, especially in people who already have mental disorders.

Patients taking certain psychiatric medications may be more sensitive to heat, according to the study, as some of these drugs can affect sweating, thirst sensation, or the body's ability to cool itself. However, the study did not directly test the effect of specific drugs.

Lack of sleep during very hot nights can lead to mood instability, increased irritability and impulsivity, and worsening of symptoms of anxiety, depression, and some psychotic disorders.

The researchers said that heat-related shortness of breath, along with behavioral changes that may occur during heatwaves, can contribute to symptom deterioration and increased need for emergency care or hospitalization.

Elderly

The results showed that the elderly were among the most affected groups, which may be related to decreased efficiency of body temperature regulation with age, higher rates of chronic diseases, and use of multiple medications that can increase sensitivity to heat or dehydration.

The increased risk in low-population-density areas may reflect, according to the researchers, longer distances to medical facilities, scarcity of mental health services, low prevalence of air conditioning, along with potential social isolation.

Study co-author Shanshan Li said that climate change has become an urgent factor affecting mental health globally, accompanied by extreme weather events, resource scarcity, and environmental disruptions that increase psychological stress and the risk of mental disorders.

She added that the increasing frequency and intensity of heatwaves represent one of the most prominent climate-related threats, noting the heatwaves witnessed in the United States and European countries in recent times.

Hospital preparedness

The findings support calls to include mental health in heatwave emergency plans, which typically focus on heatstroke, dehydration, and cardiovascular and respiratory diseases.

The study researchers believe that health authorities may need to increase the readiness of emergency departments and psychiatric services during heatwaves, while monitoring high-risk patients and ensuring they can access cool places and obtain water and medications safely.

Proposed measures may include issuing early warnings to patients and caregivers, providing cooling centers, enhancing telephone and community support services, and reviewing the impact of medications on patients who have difficulty regulating their body temperature.

The researchers classify the study as "observational," therefore it shows an association between heatwaves and increased hospital admissions, but does not alone prove that heat was the direct cause in every case. The data were also limited to four countries, meaning the magnitude of risk may differ in hotter countries or those less able to provide health care and cooling.

The results, according to the study researchers, provide multi-country evidence that the effects of heatwaves are not limited to physical diseases but also extend to mental and behavioral health, noting that these effects are likely to worsen as global temperatures continue to rise.