MedUni Vienna: Heat can trigger or intensify mental illnesses

Heat waves are not only a threat to physical health, but also have negative effects on the psyche. Anxiety disorders and depression can be caused or aggravated by the enormous stress caused by prolonged temperatures above 30 degrees Celsius. The likelihood of aggressive behaviour and fatigue both increase on high temperature days. In the light of the continuing rise in the number of hot days to up to 80 per year by the end of the century, the psychological consequences of the climate crisis should not be underestimated, appeals environmental physician Hans-Peter Hutter of MedUni Vienna.

The danger to mental health comes not so much from singular events such as individual hot days, but rather from repeated and frequent negative events such as those caused by the climate crisis. The prevailing feeling that the stressful condition cannot be improved or changed is particularly hard on the psyche. "In situations of helplessness, the stress hormone cortisol is produced in greater quantities, which, when released over a longer period of time, has a number of detrimental effects on both physical and mental health," explains Hans-Peter Hutter from the Centre for Public Health at MedUni Vienna.

The psychological consequences of prolonged heat are reflected in an increase in anxiety disorders and depression, but also in rising alcohol and substance consumption. For those already affected, the symptoms of their mental illness can get worse. Research also found that global warming is causing suicide rates to rise. "A one degree Celsius increase in average temperature is scientifically estimated to be associated with a one percentage point increase in the suicide rate," says Hutter. Data from the USA and Mexico, for example, show that the suicide rate increases by 0.7 per cent and 3.1 per cent, respectively, for a 1°C increase in the monthly average temperature. Particularly affected by the negative consequences of heatwaves are older adults or people with chronic health problems and socially disadvantaged people who often cannot escape the heat. "Studies by MedUni Vienna have shown that during a heatwave anxiety and depression increase, especially amongst these groups," says Hutter.

Between aggression and exhaustion

Heat also has serious effects on human behaviour. It increases the potential for aggression, which is also reflected in an increase in violent crime (e.g. increase in domestic violence). At the same time, high temperatures have dampening effects on the psyche: exhaustion due to heat stress manifests itself, for example, in lethargy and apathy, depressed mood and reduced mental capacity. This can be observed especially in urban and metropolitan areas, where cooling down and thus recovery are increasingly absent even at night. In Vienna, for example, before 1991 an average of one or two tropical nights, with overnight temperatures above 20 degrees, was to be expected annually. In the summer of 2015 and 2022 there were twenty three and thirty tropical nights recorded respectively.  In the summer of 2015 (23) and 2022 (30) the number of tropical nights increased 10 fold compared to 1991. Records show a total of 23 tropical nights in 2015, whereas in 2022 a total of 30 tropical nights were counted.

Number of annual heatwave days on the rise

In Austria, a heat wave occurs when there are at least three consecutive hot days with maximum temperatures of more than 30 degrees. According to Geosphere Austria (formerly the Central Institute for Meteorology and Geodynamics), between 1961 and 1990 there were between three and twelve heat days in the provincial capitals of Austria per year, with a maximum of 20 heat days. Whereas between 1991 and 2020 there were between nine and 23 heat days recorded in provincial capitals per year (average), with records mostly over 40 heat days. Currently, the highest value of recorded 40 heat days per year in Austria will become the norm by 2100, if greenhouse gas emissions continue unchecked worldwide. The records would then lie in, a currently completely unimaginable, range of 60 to 80 heat days per year.

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