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What Does Heat Do To Your Body - And When Does It Become Lethal?

As global warming rages on, we ask: how hot is too hot?


Dr. Katie Spalding

Katie has a PhD in maths, specializing in the intersection of dynamical systems and number theory.

Freelance Writer

A man is hot
If you hated last month's heat wave, we have some bad news. Image: BLACKWHITEPAILYN/Shutterstock

As you may have noticed if you glanced out of your window recently, the whole world is on fire.

And barring some unforeseen divine intervention, it’s going to continue being on fire. Summers are going to get hotter, water is going to get scarcer, and people and infrastructure are going to suffer and die.

A map showing surface air temperatures across most of the Eastern Hemisphere on July 13, 2022.
A map showing surface air temperatures across most of the Eastern Hemisphere on July 13, 2022. Key: red = bad, black = worse. Image: NASA Earth Observatory

But what’s that going to look like? What will life be like in the summers of the future?

Will we even survive?

The future will be increasingly hot and humid

“It is well-established that as a result of climate change, heatwaves in the observational record have become more intense, more frequent, and longer-lasting,” said Dan Vecellio, a postdoctoral fellow at Penn State's College of Health and Human Development.

And as the climate crisis continues to roll on, that’s going to get worse. “With continued future warming, we would expect these changes to their characteristics to continue,” Vecellio told IFLScience.


It can be hard to comprehend what we’re up for in the coming years, so for a rough guide: by 2050, according to a 2019 study published in PLOS ONE, London will feel like Barcelona today, Seattle will be more like today’s San Francisco, and Stockholm – you know, the capital city of Sweden, a country that straddles the Arctic circle – will resemble the central European city of Budapest.

And heat isn’t going to be the only problem. “Given that warmer air has the ability to hold more water vapor in it, the number of humid heatwaves is likely to increase, and these are the types of heatwaves which worry scientists working on extreme heat and human health,” noted Vecellio.

What does heat do to human health?

For many of us, a hot sunny day is an excuse to get outside, kick back at the pool or the beach, and enjoy ourselves – hopefully without forgetting our sunscreen along the way. And that’s not intrinsically a bad thing: the human body has evolved specific mechanisms to cope with higher temperatures, and can even suffer from lack of sunlight.

But with extended exposure to extreme heat, “the systems in the human body that enable it to adapt to heat become overwhelmed,” explained Mike McGeehin, then-director of the CDC's Environmental Hazards and Health Effects Program, in an interview with Scientific American.


“When a person is exposed to heat for a very long time, the first thing that shuts down is the ability to sweat,” he said. “We know that when perspiration is dried by the air there is a cooling effect on the body. Once a person stops perspiring, in very short order a person can move from heat exhaustion to heat stroke.”

Eventually, as your body heats up more and more, the central nervous system and circulatory system start to become impacted – as does the brain. “That's when people begin to get confused,” McGeehin pointed out, “and [you] can lose consciousness.”

But those are just the obvious effects of overheating. “In fact, heat stroke, heat exhaustion, and dehydration account for a relatively small fraction of the total [health risks] associated with days of extreme heat,” explained Gregory Wellenius, professor of environmental health at Boston University and director of the university’s Program on Climate and Health. 

Instead, the adverse effects of heat exposure are likely to include things like kidney problems, skin infections, and even preterm birth, Wellenius told The Brink. And it can even impact our mental health in multiple ways: “hot days were associated with higher risk of emergency room visits for substance abuse, mood and anxiety disorders, schizophrenia, and dementia,” he noted, and have been linked to lower performance on standardized tests, higher risk of judgment errors, and higher risks of occupational injuries.


“And interestingly, it’s not just extreme heat that poses a risk,” he added. “Even moderately hot days can place vulnerable individuals at higher risk.”

When is it “too hot”?

Everybody has different ideas of what a comfortable temperature is – we all have that one friend who we’ve never seen outside of flip flops and shorts whatever the weather, for instance. But when it comes to understanding where “hot” becomes “too hot,” there’s a fairly clear cut-off – and it’s lower than we thought.

“In 2010, it was theorized that the upper temperature limit that humans could thermoregulate up to was 35 Celsius wet-bulb temperature (35°C/95°F air temperature at 100% relative humidity),” Vecellio told IFLScience. 

But when the researchers at Penn State actually put this to the test, they found that the limit was quite a bit lower: “in warm, humid conditions, our subjects stopped being able to thermoregulate at ~31°C wet-bulb temperature (31°C/88°F at 100% relative humidity or ~100°F at 60% relative humidity),” he explained.

If that terminology – the wet-bulb temperature – isn’t familiar to you, then rest assured: it will be. Remember how it’s the humid heatwaves that scientists are more worried about? The reason is because they raise the wet-bulb temperatures – literally, the temperature reading on a thermometer wrapped in a wet cloth, which acts as a proxy for our sweaty human skin – and it’s when the wet-bulb temperatures are high that things get particularly deadly.

“The main way by which we cool ourselves is via the evaporation of sweat from our skin,” Vecellio explained. “Our bod[ies]… move heat from our core to the periphery of the body which then can be lost to the ambient environment.”

But if that ambient environment is already saturated with water vapor, that sweat has nowhere to go – meaning that with a high enough wet-bulb temperature, the body will continue to overheat regardless of how much water or shade it’s given. 

A wet-bulb temperature of 35°C (95°F) is not unheard of, but they are very rare, Vecellio told IFLScience. But “The ~31°C (87.8°F) wet-bulb threshold that we empirically found has been observed more frequently,” he explained, and as the planet continues to warm, “we expect the threshold to be crossed more frequently in more places across the globe… and perhaps for longer durations during the day.” 


They’re still pretty rare in objective terms, he explained, and never last for extended periods of time – yet. But don’t assume you’re in the clear: places in Oklahoma, Texas, Florida, and elsewhere recorded wet-bulb temperatures that far exceeded 31°C and even 35°C in the heat wave two weeks ago.

Who is most at risk from extreme heat?

Vecellio’s experiments were run on young, healthy subjects, doing minimal activity – which means that for pretty much anybody else, the situation is even worse. 

“Very hot days put everyone at risk, but there is no question that some groups and some communities are at greater risk than others,” said Wellenius. “Outdoor workers – such as agricultural workers, construction workers, and landscapers – are at particularly high risk of heat-related illness.”

Even simple geography can raise your risk of heat-related illness: “In some communities, the lack of trees and parks makes those neighborhoods even warmer than the surrounding areas,” Wellenius noted. “People that live in urban heat islands are also thought to be at higher risk.”


And we’ve not even started on the medical factors. If you’re elderly, or pregnant, or have certain preexisting diseases like diabetes or hyperthyroidism, you’re more likely to suffer from heat sensitivity – and for some people, the problem isn’t even the disease, but the treatment. 

There are a whole host of medications that can increase your sensitivity to heat – or reduce your body’s ability to cope with it. Many of these drugs are prescribed for mental conditions – certain antidepressants and antipsychotics are a prime example, as is lithium – but some are fairly routine blood pressure meds. Even things like Benadryl or ibuprofen can be a problem.

“A lot of people take medications, particularly elderly people, but also a lot of other groups in the community,” Kimberly Humphrey, an emergency medicine physician and climate change and human health fellow at the Harvard T.H. Chan School of Public Health, told Consumer Reports. “As we have more and more hot days, we’ll see more and more people who are affected, and are affected because of the medications that they are taking.”

What can we do about these increasing temperatures?

If hotter, more humid temperatures are going to become more likely – and evidence suggests they are – then we’re going to have to adapt to cope with it. And by “we,” we mean individuals, sure – but change will also be needed on a society-wide level.


“The [2022] heatwave has certainly made the headlines. It’s not the first this year, and probably won’t be the last. We’ll get more next year, and the year after, and so on,” said Nigel Arnell, a professor of meteorology at the University of Reading, in the BBC’s Science Focus magazine.

“We’re not really geared up to deal with more frequent heatwaves, other than through enacting emergency plans, and we can’t really run in crisis mode each and every summer,” he pointed out. “We urgently need to improve our existing housing stock to better insulate against both hot and cold weather – and that will help reduce our exposure to rising energy bills too.”

Experts are cautioning that existing infrastructure is seriously at risk from the effects of climate change: “transport is… vulnerable, particularly due to so many facilities like roads and bus terminals being in flood zones,” said David Levinson, a professor of transport engineering at the University of Sydney’s School of Civil Engineering. 

“New Jersey Transit lost $20 million in damage to buses after Hurricane Sandy,” he pointed out in 2019, and “tunnels like those in the New York Subway were severely damaged from flooding.”


And extreme heat can cause roads to buckle – or melt – and bridges to break down well before time, all of which will require significant investment and planning to mitigate.

But let’s face it, most of us don’t have our city’s civil engineering budget at our discretion – so what can we do on a personal level to keep ourselves safe?

“The most important thing for everyone to do is to be aware that the health risks of extreme heat are real and important,” advised Wellenius. “Individuals should stay out of the sun as much as possible, drink a lot of water, and find places to cool off when needed.” 

One thing is for sure: our lives are set to look quite different from what we’re used to. “This threshold [being] crossed… doesn't mean that everyone is going to instantly die,” stressed Vecellio. “The risk of illness and death do go up, but the biggest change would probably be lifestyle choices in regions where the threshold is crossed with more regularity.”


In these areas, he told IFLScience, “more time will have to be spent indoors with mechanistic cooling strategies” – although, he noted, “not all peoples will have this luxury and climate mobility will become a needed reality.”

But above all else, there’s one thing that should really be on all of our agendas: tackling the levels of damage we’re inflicting on our planet.

“The biggest thing we can do… is to drastically reduce our greenhouse gas emissions as quickly as possible,” said Vecellio. “[We should] invest and move to renewable sources of energy to prevent more future climate warming.”


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