How our bodies adapt to extreme temperatures – until it gets too hot

During recent video calls with my UK colleagues, I have noticed a common theme. Their cheeks are pink, damp hairs stick to their brows and they sigh a lot. The unprecedentedly hot summer in Europe is taking its toll. “How do you cope, living in Australia?” they ask glumly. Earlier this summer, the temperature in the UK reached 38°C (100.4°F), breaking the record for June and many people’s spirits. But that doesn’t sound too bad when you have grown up in Australia, where it usually gets into the 40s in summer. I remember times when it has surpassed 45°C (113°F) in Sydney. I don’t love these scorching days, but they don’t bother me too much, despite working from home in a small un-air-conditioned room. This has sparked several conversations about why the heat affects us differently. I have British and German ancestry, so I don’t have any kind of genetic advantage for tolerating Australia’s heat. One possibility is that my repeated early-life exposure to heat has somehow primed my body to cope better with it. But it turns out that my UK colleagues are just as equipped as me for adapting to heat on a physiological level, and that the advantages of growing up in Australia are merely psychological and behavioural. There are also techniques we can all use to help our bodies adapt to the rising global temperatures (spoiler alert: you may want to switch off your air con). Advertisement Why people react differently to the heat Our bodies function optimally with a core temperature of around 36.5° to 37.5°C (97.7° to 99.5°F), and our organs start to shut down at anything above 40°C (104°F). As a result, we have evolved ways to constantly monitor the temperature of the outside environment and our insides, and cool ourselves down when needed. We sense external heat via thermoreceptors in our skin, which communicate with the brain. “Thermoreceptors are the body’s first responders, detecting and relaying to the brain what is happening at the body’s surface,” says neuroengineer Clarissa Whitmire at the University of Queensland in Australia. If the brain considers the heat to be a threat to the body’s core temperature, it produces a conscious feeling of being uncomfortably hot and bothered, so that we peel off a layer of clothing, rest in the shade or engage in other cooling behaviours. The brain also activates unconscious cooling mechanisms in response to heat. For example, it tells the heart to beat harder and faster to move blood from the body’s core to the skin. “We’re basically trying to bring warm blood from the central circulation to the periphery, where, if air temperature is lower than skin temperature, we can lose heat via convection,” says Julien Périard, an environmental physiologist at the University of Canberra in Australia. This explains why my veins bulge out so much on hot summer days that I look like an anatomical drawing of the circulatory system. We can’t lose heat via simple convection if the outside temperature is the same or hotter than us, like when it recently reached 38°C in the UK, so we have also evolved the ability to sweat. As sweat beads evaporate off our skin into the air, they draw heat away. So far so simple, but things get interesting when you compare people who live in different climates. In hot climates, people become gradually better at self-cooling over the course of summer because their bodies undergo what is known as “seasonal acclimatisation”. Studies show that, at the end of summer, people who are asked to exercise in a hot room redirect blood to their surface faster, start sweating sooner and produce more sweat overall compared with when they were tested before the season. They also have a greater volume of plasma – the liquid portion of blood – to supply more water for sweat. These changes allow them to maintain a lower core temperature, skin temperature and heart rate while in a hot environment. Because of this seasonal acclimatisation, heatwaves tend to result in fewer deaths when they occur at the end of summer compared with the beginning. Another response to regular heat exposure is the expression of heat shock proteins. Our bodies produce these when we get hot to repair and refold other types of proteins in cells that might be damaged by the increased temperature. Their upregulation then helps to protect cells from thermal stress during subsequent heat exposures. The good news is that anyone can undergo this kind of heat acclimatisation, regardless of their genetics or their level of heat exposure growing up, if they spend enough time in a hot climate, says Périard. “We evolved as tropical animals, so we have the systems in place for dealing with heat,” he says. “Everybody can adapt, and the adaptations develop quite quickly.” For example, Périard and his colleagues conducted an experiment in which they brought nine Danish cyclists to Qatar. In Denmark, it had been below 10°C (50°F) for months, but in Qatar, it was in the high 30s. “Their initial performance on an outdoor time trial was atrocious, but after training outside every day for two weeks, their performance got much better,” says Périard. “So even people from Denmark, who are not used to really hot summers, can adapt in a short time.” When I relay this information to my UK colleagues, however, they are unconvinced. They swear that even after enduring months of baking weather, they still don’t feel like they are getting used to it. But this could just be a trick of the mind. Our emotional response to heat According to Carolyn Broderick, who is a sports medicine physician at the University of Sydney, if people in the UK were to do an exercise test in a hot room right now, they probably would show signs of being more physiologically heat-adapted than before the hot summer – for example, they would likely produce a greater volume of sweat to more efficiently cool themselves down. However, they might not consciously feel any better because there is a difference between physiological adaptation and thermal perception. Research shows that even when people become physiologically acclimatised to heat during summer, they often don’t report feeling any more comfortable when placed in a hot room than they did before summer, for reasons that are unclear. There is also an expectation effect, says Broderick: “If you grow up in a hot climate, you see it as normal.” In Australia, everyone knows that summer is going to be blisteringly hot, so they are psychologically prepared. From a young age, you also learn ways to cool yourself down, like running under a sprinkler or sleeping under a towel soaked in cold water. Conversely, when I spent a year in Montreal in my early twenties, the sub-zero winter came as a shock because I had never known anything like it. It took time for me to work out that it was unwise to go out in ballet flats and that late-night poutines were vital for warming my insides and laying down insulative fat tissue. In these psychological and behavioural senses, therefore, early-life exposure to hot or cold may be important for coping with it later on, says Broderick. Children growing up now in the UK and other traditionally cool countries will probably become better at dealing with the heat than their parents because they will have greater exposure, she says. The urgency of the climate crisis could also be affecting our psychological responses to peaks in temperature. Notably, one of the previous hottest years in the UK’s history – 1976 – is often remembered as its happiest ever, despite the temperature reaching 35.9°C (96.6°F) and surpassing 32°C (90°F) for 15 consecutive days (although it was less humid than this summer’s heatwave). “There was that long, hot summer… Everyone in the country was happier and more relaxed,” recalled writer Will Self when describing 1976 to the Guardian newspaper. Photos of the time show children gleefully splashing in paddling pools and shirtless adults sunning themselves in parks. But now that climate change is front of mind, our perception of heatwaves has changed, and we tend to find them worrying and ominous rather than enjoyable. How to get better at being hot Since summers are only going to get hotter, I ask Périard and Broderick how we can improve our heat resilience. They both say that the most helpful thing we can do is regular exercise. This is because physical activity raises our internal heat, which naturally stimulates heat acclimatisation. Exercising outdoors on hot days is even better because the external heat boosts the acclimatisation effect, says Broderick, who is chief medical officer of Tennis Australia and the Australian Open, as well as the medical director of the Australian Olympic team. “You should go out into the heat – start with small doses of low-intensity activity and build up a bit more each day,” she says. People over the age of 65 need to be careful because they are at risk of developing heat stroke if they do strenuous exercise in the hot sun, but walking in shaded areas is usually safe. A recent study found that during pregnancy, brisk walking or moderate-intensity jogging or cycling is also safe when the outside temperature is 32°C or lower. For people who can’t exercise easily, there is also growing evidence that regular sauna bathing builds up similar physiological mechanisms for coping with heat, like expanding plasma volume to support more sweating. While exercising in the heat, it is important to stay hydrated to replace fluids lost through sweating and to quickly implement cooling measures if you feel like you are overheating, says Broderick. “For example, you can put a cold towel around your neck where the large blood vessels are or use a misting fan.” Air conditioning is also useful for cooling down, but Périard cautions against spending too much time in artificially cooled environments, since we need heat exposure to promote acclimatisation. This makes me think of some British friends who have lived in Australia for over 10 years but still suffer badly in summer. They cope by blasting their home air con, but perversely, this may be one reason why they have never properly acclimatised to the outdoor climate. “Air con is good at night because we want to be able to sleep, but in the day, it might be better to set it at 24°C [75°F] rather than 18°C [64°F],” says Périard. Personally, I have air conditioning in my bedroom for occasional nighttime use, but not in the rest of the house. Tellingly, even though climate change is making the planet hotter, people in the US are actually experiencing cooler temperatures on average nowadays because of the widespread adoption of air conditioning in homes and workplaces. Modelling shows that the average temperature experienced by Americans is 1.4°C cooler than in 1960. “It’s kind of alarming because we’re spending more time inside in air conditioning and exercising less, so it’s not the ideal scenario for helping us adapt to climate change,” says Périard. Unfortunately, heat acclimatisation is also temporary, so you have to build it up again each summer. Research shows that it gradually decays over a period of around six weeks once heat exposure ceases, like when winter sets in. This suggests that if I was to travel to the UK right now, having come from winter in Australia, I would perform worse in a heat stress test than locals, who have undergone seasonal acclimatisation over summer. The limits to adaptation What’s more, there is a limit to how much heat the human body can acclimatise to and survive. If a person’s core temperature exceeds 43°C (109°F), death is almost certain. Core temperature starts to rise to this fatal level if the air temperature is very high, because the skin can’t lose heat via convection. High humidity makes things worse because the moist air hinders the evaporation of sweat. According to modelling led by Jennifer Vanos, an environmental health researcher at Arizona State University, a young healthy adult could survive for a maximum of 6 hours in the shade if the relative humidity was 25 per cent and the air temperature was 50°C (122°F). If the relative humidity increased to 50 per cent, they could only withstand 43°C for 6 hours. “Our survivability model identifies conditions beyond which even the fittest young adults, at best, will perish over 3 or 6 hours without access to cooling resources,” Vanos and her colleagues write. Older people are more vulnerable to heat because, as we age, our hearts become less efficient at pumping blood to our skin and we sweat less. Vanos and her team estimate that regardless of humidity, a person over the age of 65 couldn’t withstand anything above 46°C (115°F) for more than 5 hours. Older people can also succumb to temperatures in the 30s if they are repeated over many days, because of the prolonged pressure on their hearts, which is why thousands are estimated to have died in the European heatwave in June. If current rates of global warming continue, some places on Earth will go above the survivable limits of heat and humidity modelled by Vanos and her colleagues, says David Romps, who studies the climate at the University of California, Berkeley. “Homo sapiens has been around for 300,000 years and over that time, the Earth has been 100 per cent habitable, meaning I could put you anywhere on the planet, in any one of those 300,000 years, at any time of day, and you would be fine with the heat, humidity or cold, so long as I gave you adequate shade, water or clothing. You wouldn’t necessarily be happy, but you would survive,” he says. But if things continue as they are, he says, “parts of the Earth will become uninhabitable for the first time in human history”. These no-go regions could include parts of India, China, Australia, Africa, South America, the Middle East and the US Midwest, says Romps. In these places, people might be able to live inside artificially cooled buildings, but going outside for extended periods would be fatal. On our current climate change trajectory, the first place on Earth to become unsurvivable will likely be the Persian Gulf, says Romps. Modelling has found that Kuwait City could experience temperatures above 60°C (140°F) by around 2070. The outlook for Australia is also alarming, with some studies forecasting that Sydney and Melbourne will experience 50°C days by 2060 if there is 2°C of global warming above pre-industrial levels. With 2.5°C of warming, even the UK could experience 45°C days by 2056. I might be able to bear 40°C days, but 50°C or 60°C sounds unimaginable. We all have the ability to adapt to a bit more heat than we are used to now, but ultimately there is a limit to what we can stand. As Romps says, “the human body has adapted to deal with the worst conditions the planet has historically doled out to us, but we’re not that far away from conditions in which the human body runs out of tricks.” At that point, no amount of exercise or sauna bathing will be able to prepare us.