When the 2026 London Climate Action Week kicked off in late June, its organisers faced an ironic challenge: it was too hot to assemble in person in many of the venues booked for the event.
A heat dome parked over Europe sent temperatures into the mid-30°C range (above 90°F) and many of the spaces meant to host the gatherings in London lacked air conditioning.
Moving events to virtual spaces helped keep attendees at the climate conference from wilting in the heat, but the benefit to the gathered scientists and policymakers is likely to extend beyond simply keeping cool.
Emerging science is revealing the dangers heat poses to our brains and ability to think, with potential impacts ranging from difficulty concentrating to irreversible declines in brain matter volume.
Just like our bodies, our brains work best within a narrow temperature range. Although, as a 2022 study found, that range can vary somewhat.
The brains of healthy study participants ranged anywhere from 36–41°C (97–106°F), and a quarter of them saw their brain temperatures vary predictably throughout the day.
Nevertheless, our brains don’t take temperature increases above their normal range very well, something that’s becoming more common today as air temperatures continue to rise.
A recent study in the journal Nature Climate Change shows that, compared to 1970, one billion more people now face at least one day of extreme heat stress each year.
The same study also notes that heat is increasing both during the day and at night, leaving many people unable to get any respite from it.
As the world gets hotter, we’ll need ways to protect our most valuable asset: the brains that let us think, work and communicate.
Research shows there are things we can do to help our grey matter adapt to the heat, but they only work to a degree.
Experts say larger-scale changes to the ways we build, work and structure our lives may become increasingly necessary, as it’s almost certain we’ll be facing more broiling days in the years to come.
Here’s what the science says right now about how hot weather affects your brain – and what you can do about it.
Feeling the heat
Overheating can happen well before temperatures begin to feel unbearable, especially if it’s humid or you’re exerting yourself.
When your core temperature climbs, specialised cells in your skin and certain internal organs start to signal parts of the brain, chiefly the hypothalamus, an area responsible for maintaining homeostasis in the body.
The hypothalamus then calls for actions to cool the body, like sweating and sending more blood to your skin to shed heat.
As well as these thermoregulatory responses, as it heats up, your brain also initiates a stress response, says Dr Burcin Ikiz, a neuroscientist and founder of global action tank Neuro Climate Working Group.
Neurochemicals help instigate an inflammatory response in the body, and the communication between neurons begins to change as the brain moves into a state closer to fight-or-flight.
“The brain starts focusing on the essential functions,” she says. “More taxing functions such as complex decision-making, emotional regulation and so on go on the back burner.”
When core temperatures rise even higher, the effects are more dramatic. At internal temperatures above 40°C (104°F), you begin to experience heatstroke, which is often deadly.
The high temperature damages brain cells and causes a breakdown of the blood-brain barrier, which allows neurotoxic chemicals to leak into the brain.
Inflammation and blood coagulation stemming from the body’s protective response can block blood vessels in the brain, starving cells of oxygen. Those who don’t die are often left with lingering consequences.
One study that looked at patients admitted to intensive care units for heatstroke in Paris found that half of those who survived continued to experience neurological symptoms.
The brain-altering effects of heat begin a long way before death’s door, however. Our bodies can only handle temperature swings of a few degrees and even then, only for short periods of time.
Some evidence suggests our brains are even more delicate.
A study published in the Journal of Sport and Health Science in 2024 had 11 participants elevate their core temperature to 39.5°C (103°F) and then sit for an MRI scan.
The researchers found that brain temperatures barely changed even as core temperatures shot up, evidence of how hard their bodies were working to keep the brain cool.
Even so, the researchers found signs of suppressed activity in regions of the participants’ brains responsible for motor control and executive functioning, as well as impaired blood flow in the brain.
The study’s results tally with other research that suggests our brains become worse at a number of core tasks when we get hot, says one of the study’s authors, Dr Jason Lee, director of the Heat Resilience and Performance Centre at the National University of Singapore.
Research shows that elevated internal temperatures are associated with memory deficits, executive control impairments, sensory processing issues, disruptions of functional connectivity and more.
Lee says scientists are still figuring out the full story behind why our brains perform more poorly as temperatures rise, but a few things seem clear.
One is that the brain gets less oxygen when we’re hot, because our bodies divert more blood to our skin to dissipate heat.
“There’s competition now between the limited blood flow to the vital organs, including your brain, to the muscles if you’re working, and the skin for heat dissipation,” he says.
With less oxygen available, neurons can’t function as well, leading to a deterioration of brain activity.
Neurotransmitters likely also play a role. When our brains begin to get hot, they respond in two ways, explains Ikiz: by releasing a range of neurotransmitters and changing the way those chemicals are taken back up.
The chief culprits among these heat-associated neurotransmitters are serotonin, dopamine and noradrenaline, though others are likely to be involved as well.
Heat-related perturbations to these chemicals likely affect our memory, cognition and emotional state, Ikiz says, and might explain why the heat is associated with irritability.
For example, a higher ratio of serotonin to dopamine, linked by some studies to increasing core temperatures, likely increases central fatigue in our brains when we exercise in the heat, making us feel more tired and acting to restrict our activity.
Still, the complex cascades of neurotransmitters and brain activity in response to heat are largely unclear, according to Ikiz.
Most evidence comes from animal studies, and the evidence in humans for how heat and neurotransmitters are linked is varied and, at times, even contradictory.
Factors such as how hot a person gets, how long the heat lasts and how acclimated they are may all change the neurochemistry of heat in the brain for any one person.
“We’re still trying to understand exactly what’s [happening] and why it leads to lower test scores or problems with decision-making or more aggression,” says Ikiz.
Read more:
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Hot and bothered
In addition to the physical changes in our brains, heatwaves bring a range of effects that can impact our brain function, says Dr Emma Lawrance, an expert on climate change and mental health at the University of Oxford.
“We don’t sleep as well [and] we can’t do the things that we normally do to keep ourselves physically and mentally healthy,” she says.
And the longer a heatwave lasts, the more these effects begin to stack up.
“When the heat goes on for days [as research indicates may become increasingly likely] and especially when you don’t get a chance to cool down overnight, these lagged or cumulative effects can worsen,” Lawrance says.
The effects can be even more severe for people with cognitive or mental health issues.
In a 2023 review of existing research on temperature and mental health, published in The Lancet Planetary Health, Lawrance and her co-authors found a 1°C (1.8°F) increase in temperature caused an increase in suicide attempts of between 1.5 and 1.7 per cent, and that heatwaves were associated with nearly 10 per cent more hospital admissions for mental illness.
On top of that, numerous indicators of wellbeing, ranging from anxiety and depression to mental health service use, also took a turn for the worse during heatwaves, the study says.
A separate study in JAMA Psychiatry found that days with extreme heat resulted in an 8 per cent increase in emergency department visits for mental health conditions.
Lee notes that many people with mental health conditions face multiple risk factors that can put them at exceptional risk from heat, including a decreased ability to sense heat, impaired responses and a potential lack of thermal capacity to deal effectively with being hot.
In addition, many medications used to treat mental health issues may also decrease the body’s ability to cope with high temperatures, compounding the risks.
Lawrance says the physiological factors that link increased body temperatures and mental health are still being studied, but agrees that findings like hers stem from a confluence of many factors related to heat.
“You’ve got this stress response in your body, you’ve got altered neurotransmitter activity [and] altered blood flow through your brain, and that can all contribute to strain and mood disturbances,” she says.
Cumulative effect
The reality is that more and more of us are being forced to confront is that not only are heatwaves becoming more frequent, but they’re also lasting longer.
We already know that just a few days of high temperatures can cause problems, but heatwaves are an almost unavoidable part of our long-term future. So, what happens to our brains when we’re exposed to year after year of excessive heat?
“We understand that if… you cross the [brain’s temperature] threshold, then bang: there are consequences,” Lee says. “But little is known about the chronic [effects of heat].”
It’s a question that has no clear answers as of yet, but the initial indications are worrisome.
In one of the largest studies of the question to date, scientists from Tsinghua University in Beijing, China, used data on over 41,000 people in the UK Biobank to link chronic heat and brain structure.
They were able to correlate changes to brain structure based on MRI scans with a person’s exposure to elevated temperatures, in this case above 27°C (81°F), over the past 20 years.
They found a clear link between exposure to high temperatures and decreases in brain volume across multiple areas of the brain including the cerebellum, accumbens, hippocampus and thalamus.
The more days a person experienced above 27°C (81°F) the greater the declines in brain volume, the authors found, indicating subsequent exposures may compound harm.
As in other studies, the authors note the effects of heat aren’t uniform.
Their study found a link between mental health issues and greater exposure to high temperatures, most notably depression, and that older people seem particularly vulnerable to heat.
Brain volume losses increase with age, they say, amounting to an acceleration of ageing that could leave heat-affected brains looking years older.
Other research has pointed to potential impacts at the other end of life.
In a study published in Nature Climate Change, researchers in China found evidence that prenatal and postnatal exposure to high heat increases the risk of neurodevelopmental delays in children.
Exposure during pregnancy increases risk by 35 per cent and exposure from birth to three years old increases risk by 53 per cent, the authors say, with risks rising more steeply at very high temperatures.
Chronic heat exposure “is a hugely important area of work to understand across the life course,” Lawrance says. “It just pushes the need for adaptation and mitigation so much.”
Pairing long-term heat exposure with other factors like air pollution could further worsen outcomes.
There’s some evidence to suggest that particulate pollution and heat together magnify the negative impacts on our brains, which could be particularly dangerous in places like India, which has high levels of both.
Lawrance says researchers are investigating the intersection of heat and pollution at the moment, though good evidence about their effects is yet to come.
“What we really need to start thinking about isn’t just heat as a singular factor,” Ikiz says. “We have to really understand how these factors play together in impacting our brain health.”
Icy does it
Though heatwaves can be deadly, Lee says deaths are not his chief concern.
“It’s very hard to die of heat,” Lee says. But “we can’t assume that when nobody dies, it means everything is going well.” The heat can significantly compromise lives and wellbeing even when the effects fall far short of death.
While higher temperatures bring new challenges, there are solutions that may help people and societies adapt. And when it comes to cognitive and mental health, the best way to keep your brain cool is to keep your body cool.
If you find yourself overheating, the best quick fix may actually come from an ice-cream truck. Ice slushies are scientifically proven to cool the body faster than cold water or cooling vests.
The reason comes down to a physics concept known as the latent heat of fusion. Put simply, ice absorbs extra energy as it turns from a solid to a liquid, so it sucks even more heat from your body than ice-cold water alone.
(A similar concept explains why sweating is so effective at cooling our skin, as the water absorbs extra energy as it turns into a gas).
In his 2024 study that elevated participants’ body temperatures to 39.5°C (103°F), Lee tested the ice slushy method and discovered it prevented the cognitive impairments found in participants who hadn’t been given an icy drink.
There are also steps we can take to prepare our bodies for the heat. One is acclimatisation – gradually exposing ourselves to higher temperatures so our bodies can adapt.
Some preliminary studies show heat acclimatisation improves cognition in the heat, though the effects often don’t hold for all cognitive tasks studied and may depend on age.
Lee says that while acclimatisation is useful, the best way to prepare for a hot summer is to work out.
A robust cardiovascular system is the best defence against the loss of blood and oxygen to internal organs that occurs as our bodies pump blood to our skin to cool off.
“I use the analogy that a small pot boils the quickest,” Lee says.
Those with a smaller capacity for exertion, which might include the young and the elderly, will overheat more readily as their bodies struggle to shed heat by redirecting blood flow to the skin.
For those who need to be outside when it’s hot, Lee recommends taking adequate breaks and moving work hours to cooler times.
Lawrance says community-level actions may help people cope with heat in the future, including green spaces, rooftop gardens, and buildings with better ventilation and passive cooling features.
Establishing heat action plans featuring early warning systems and coordination with first responders and medical systems is another way for communities to prepare.
Unfortunately, a recent report for United for Global Mental Health found just 40 countries surveyed had heat action plans.
Broader efforts to adapt to the heat are underway and will hopefully come soon enough to benefit the billions of people whose brains may be at risk from high temperatures around the world.
Until then, ice slushies it is.
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