With the summer, impossible to avoid them. They swarm from the first heat, and turn in circles like drunken mosquitoes. The most common specimens? No doubt the steering wheel honking, ready to trumpet for a late turn signal. Or the uninhibited ones at the campsite, those who hand out “it hits me on the system” slumped on their plastic chair, in shorts and in the shade at all hours. And with them, a string of nervous sweaty, gnashing teeth, tough guys and large thermometer agitated. Does the sun heat up people’s minds, make them more aggressive, more sullen, or even completely insane, as some popular sayings would have it?
Until recently, little research has focused on these questions. They were left fallow, at the mercy of what will be said. But, with global warming and the proliferation of heat waves – such as the one currently affecting the south and east of France – more and more scientists are wondering: are the high temperatures making us lose our minds?
In addition to degrading our physical abilities, temperature spikes also seem to influence our moods, our thoughts and even, in the longer term, our cognitive abilities, according to recent scientific studies. “We already knew that climate change and rising temperatures pose a host of risks to human life, but more awareness is going to be needed about the hidden dangers to psychological health,” warns Rhiannon Thompson, a researcher at Imperial College. , in London.
When the body overheats, so does the mind?
Holder of a doctorate in public health, this young British researcher decided to specialize in the study of the impact of air, noise and climate on mental health. The physiological mechanisms related to heat have been known to scientists for a long time, but it was first necessary to review them to understand how temperatures affect our minds. Elementary biological reminder: in order not to exceed 36.8°C in summer, the blood is sent to cool in the hands, feet and skin. It is normally 1 to 2°C colder there than in the middle of the body.
This cooling circuit only works if the ends of our passenger compartment are cooler than its center. A threatened balance if, as in the South and in the Rhône this weekend, the air rises to 40°C. To compensate, the skin then releases water. As the sweat evaporates, it cools the skin and surrounding blood. A mechanism that has its limits: in French climatic conditions, a human being in perfect health, immobile, protected from UV rays, naked and with water in unlimited quantities sees his cooling system exceeded from 46°C.
This theoretical threshold even drops to 35°C if the air is close to 100% humidity. Because, when the atmosphere is charged with water, the sweat does not evaporate and drips, hot like the rest of the body. Only a few territories in Southeast Asia and the Persian Gulf occasionally experience such “damp heat peaks”. But these moments of torpor have already doubled since 1979. And they are likely to become increasingly common in some places if the atmosphere warms by more than 2.5°C, indicates a study published in Science in 2020.
Psychiatric patients succumb as much as septuagenarians
“Hot strokes”, or “hyperthermia” in medical language, occur at much lower temperatures, if you are not hydrated enough, if you play sports in the sun, or if you are already in poor health or old – aging degrades the ability to sweat and the feeling of thirst. During these potentially life-threatening episodes of overheating, heart and respiratory rates increase. The body desperately tries to send blood to the skin, even if it means depriving the digestive system of it and triggering vomiting and diarrhoea. Eventually, the vital organs wither away. So much for the physical limits.
In Europe, at least 62,000 people lost their lives in a “precipitated” way because of the record heat of 2022, including more than 5,000 in France, according to a study published in NatureMedicine in July 2023, conducted in collaboration with Inserm. By dint of counting the dead, researchers like Rhiannon Thompson have noticed that people followed for psychiatric conditions have a higher risk of dying than the average. Up to three times more, according to the observations ofan italian teamwhich reviewed 48,000 victims of Bologna’s scorching summers, from 2004 to 2017.
Why do patients with mental illness succumb to heat waves as often as the elderly? The antipsychotics, hypnotics and opioids prescribed to them can alter the vascular circulation and in some cases dehydrate. What lessen the defenses against the heat. An interesting track, especially since patients on medication are overrepresented compared to those who take nothing. But this hypothesis does not explain the strange summer suicides, which are recorded where the mercury climbs.
By compiling scientific articles that cross heat waves and hospitalizations, Rhiannon Thompson and her colleagues at Imperial College have discovered that with each additional degree, the suicide rate increased by 1 to 2%. Difficult to see anything other than an effect of temperatures on morale. Especially since hospitalizations for psychiatric reasons jumped 10% during these same heat waves. “We still don’t know how heat affects the mind, but it seems that mood improves when you approach 20-25 ° C and then deteriorates,” says the researcher, whose work was published in July in The Lancet.
According to a study published in 2018 in Nature Climate Change, extreme heat due to climate change could lead to 9,000 to 40,000 more suicides by 2050 in the United States and Mexico – as many as during recessions. Another article, published in 2020 in NatureMedicine, looked at the frequency of road accidents, falls and fatal drownings in the United States between 1980 and 2017: they too increase during heat waves, as do homicides. “An average warming of around 1.5°C, the objective of the Paris agreement, could cause 1,601 additional deaths per year in the country”, estimate the authors.
Sleep deprivation and hormones in shambles
In a thermal emergency, the brain and central nervous system release abnormal levels of cortisol, adrenaline and testosterone, molecules that affect psychological state. Not to mention that, above 20°C at night, as in Grenoble, Nice or Limoges this Sunday, and more and more frequently almost everywhere in France, sleep is no longer restorative. Anxiety, parasitic thoughts and mood disorders increase. In addition to these disorders, there are in some cases societal disturbances linked to climatic disasters, such as isolation and conflicts over access to resources, additional risk factors for mental health.
In the long term, these successive heat waves could even deteriorate the functioning of the brain: “The cumulative exposure to high heat could, in certain cases, deplete our cognitive reserves more quickly than in normal times”, explains Eun Young Choi, researcher at New York University School of Public Health. From 2006 to 2018, she and her colleagues administered questionnaires to 9,500 Americans over the age of 52: those who lacked access to air conditioning and lived in sweatshop neighborhoods saw their brain capacity decline more quickly .
Stress related to extreme heat damages brain cells and generates inflammation. Too frequent and regular because of heat peaks, these events would accelerate cognitive decline and make the effects of aging or certain neurodegenerative pathologies more salient, explain the scientists in their study, published in August in the Journal of Epidemiology and Community Health. “The science is still in its infancy and many questions remain unanswered about the long-term implications of heat, but interest in the topic is growing,” continues Eun Young Choi. Ultimately, this new knowledge could fuel prevention campaigns, which, for the moment, have no word for these leaden screeds and other summer bloodshed.