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It’s been hot in London this week. Really hot. A dangerous heat wave has hit Western Europe. Yesterday, the UK recorded its highest ever June temperature at 36.1 °C (about 97 °F). But as the weather app on my phone confirmed, it felt like 39 °C.

It’s frightening that we are seeing such temperatures in the UK in June. According to the Met Office, the country’s national weather and climate service, June temperatures peaked at an average 19 °C (66 °F) in England between 1991 and 2020. Across Europe, the heat wave is likely to cause thousands of deaths. There will be other awful consequences for agriculture, infrastructure, and the health system.

But this week I want to look at what the heat does to our minds and brains. Personally, I’ve found it almost impossible to think straight. The heat is distracting and my mind is foggy. I dread to think about the conditions of people who work outdoors, in even hotter regions.

It’s not just exhaustion and confusion. The effects of heat on the brain can be deadly. And researchers are still trying to figure out why.

Studies have confirmed that as temperatures rise, people seem to get more irritable and more violent. Most of these studies are based on associations, though. It’s difficult to directly study how a heat wave might affect our thinking, says Catherine Thompson, a cognitive psychologist at Liverpool Hope University. 

She has been studying the effects of extreme heat on firefighters instead. It’s easier to measure people’s cognitive skills before and after they undergo scheduled training that involves entering a burning building.  

It’s early days, but the team found that firefighters found it harder to focus and control their attention immediately after heat exposure—something people in heat waves can empathize with, I’m sure. 

The firefighters’ skills returned to normal after 20 minutes or so of cooling down. But they’d experienced just 15 minutes of intense heat exposure. Thompson doesn’t know what the effects of living through a days-long heat wave might be—or how long they’ll last. Figuring that out might involve shipping cognitive test kits to thousands of people during the few days’ notice of an impending heat wave. “My guess [is] that no one’s done it because it’s just so difficult to do,” says Thompson. 

Still, researchers can learn about some of the impacts of heat waves through studies after the fact. And those studies suggest that the heat seems to have more disastrous outcomes for people with mental-health disorders. 

Those outcomes become apparent when temperatures rise above what is considered typical for a given region. “There seems to be a correlation where the hotter it gets, especially during the hottest times of the year, the worse the mental-health outcomes,” says Joshua Wortzel, who directs the Heat-Mind Lab at Hartford HealthCare in Connecticut.

In a study published in 2023, Emma Lawrence at the University of Oxford, who studies the effect of climate change on mental health, and her colleagues reviewed the evidence linking mental-health outcomes to ambient outdoor temperatures. They found that during heat waves, there was a 9.7% increase in the rate of hospital admissions for people with such conditions. 

“People who live with mental-health conditions are among the most susceptible to the physical impacts of heat,” says Lawrence. People with schizophrenia were found to have been three times more likely to die during the record-breaking heat wave that affected Canada in 2021, for example.

In order to protect people, we need a better understanding of the mechanisms underlying these effects. After all, a lot of things change when it’s very, very hot. Some people may end up stuck indoors, avoiding outdoor play and exercise, and it can be difficult to get a good night of sleep, for example. Sleep, socializing, and exercise are all really important for our mental health. 

But whether unusual heat does something specific to our brains is, as Wortzel puts it, “the million-dollar question.”

Research in lab animals suggests that excessive heat can alter the way chemical signals work in our brain. The levels of neurotransmitters like serotonin, for example, seem to increase when rats and mice are exposed to high temperatures, according to multiple studies. The heat may also interfere with the way networks in our brains communicate with each other. It might affect the way oxygen reaches our brain cells.

“There are so many biological reasons why brains may be negatively affected by heat,” says Wortzel.

Emerging research suggests that for whatever reason, children and young people are among the most vulnerable. In research published earlier this week, Wortzel and his colleagues saw a 2.97% increase in the suicide rate among people in the US aged 15 to 24 for every 1 °C increase in average monthly temperature. That’s more than double the increase seen in people over the age of 24 (which is concerning in its own right).

Other work hints that heat exposure might have long-term consequences for children’s brain development. Babies who were exposed to either extreme heat or cold appeared to have altered white matter by the time they were nine to 12 years old—although it’s not clear how these impacts might affect an individual child.

“It seems that extreme temperature exposure for very young children may affect their brain development,” says Lawrence, who spoke to me from Oxford. She was meant to be in London for Climate Action Week, but her event, which focused on extreme heat, ended up being canceled … owing to the extreme heat.

We are living through the effects of climate change. And that brings a new urgency to the question of how heat affects our brains. Children born in 2020 are predicted to experience around seven times the number of heat waves their grandparents did, says Lawrance. “[We] need to be serious about adapting to a warming world.”

This article first appeared in The Checkup, MIT Technology Review’s weekly biotech newsletter. To receive it in your inbox every Thursday, and read articles like this first, sign up here.