Here’s how much heat your body can take (2023)

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I think I’m running out of ways to say it: it’s hot.

Saturday’s heat was crushing in New York, with temperatures topping 90 °F (32 °C) and air that was absolutely sticky with humidity. While I prayed for a breeze, I found myself thinking again about a story I wrote two years ago on how the human body deals with extreme heat. At the time, I wrote about how climate change is pushing the limits of what we can handle.

Since it’s been a while, I decided to revisit the topic and catch up with one of the researchers I spoke to for that story. So for the newsletter this week, let’s talk about just how hot is too hot.

Why is heat a problem?

Our bodies need to maintain a relatively stable core temperature of around 98.6 °F (37 °C). The thing is, we’re constantly making heat as our cells carry out their jobs in our bodies and burn food for energy. “It’s just a function of being a mammal,” says Zachary Schlader, a physiology researcher at Indiana University Bloomington.

So in order to keep a balanced temperature, we constantly lose heat. We get rid of most of it via our skin, which throws heat into the air around us. Sweating can help speed that process up.

But this heat loss, and therefore the whole balancing act, can get derailed when we’re exposed to extreme heat. If your body isn’t able to cool itself down fast enough, a whole cascade of problems can start, from stressing out your heart to throwing your kidneys and liver into chaos.

How hot is too hot?

As with most things related to humans and bodies and health, it’s not quite as straightforward as a single number. “As much as I hate to say this, because everything is complicated … it’s complicated,” Schlader says.

A whole host of factors can alter exactly how our bodies will keep the teeter-totter of our internal temperatures balanced. Age, health status, medications, and how acclimatized we are to heat (more on this later) help determine how much heat your body is able to lose. People who are very old or very young have more trouble regulating their body temperature. And activity level will determine just how much heat your body is making that it needs to get rid of.

In general, though, researchers typically put the theoretical limits of the human body at 95 °F (35 °C) on a scale called wet-bulb temperature.

Wet-bulb temperature is a weird metric, but basically, it’s an effort to incorporate both heat and humidity into one number. In short, it’s the measure of what a thermometer would read with a wet cloth wrapped around it. In a dry environment, water evaporating off that cloth will cool things down, lowering the temperature. But if the air is already saturated with humidity, there will be less evaporation, and therefore less cooling.

Take two examples of conditions that would reach a 35 °C wet bulb temperature. With mostly dry air, temperatures have to top 130 °F (54 °C) to reach that limit. On the other hand, a temperature of 109 °F (43 °C) and a relative humidity of 50% would result in the same wet-bulb temperature.

It’s a useful metric because it can give you an idea of how much your sweat will be able to cool you down. Above a wet-bulb temperature of 35 °C, your body won’t be able to lose enough heat through the evaporation of sweat. But that’s still a theoretical limit—one that hadn’t been tested much in humans until recently.

Early research has found that the limit might turn out to be more varied, but lower, than theory would suggest. One 2021 study found that even in healthy young adults, heat loss couldn’t keep up at lower temperatures than the theoretical limit, especially in humid environments.

Bottom line: researchers are still trying to understand where our limits lie when it comes to heat, though we do know it’ll depend a lot on specific environmental and health factors. There’s also some interesting research showing that our heat tolerance can change over time—as we age, yes, but even with the amount of heat we’re exposed to.

How can we handle the heat better?

One thing that I found fascinating when I started looking into extreme heat a few years ago is the concept of acclimatization: our bodies can adjust to the heat.

If you’re exposed to heat consistently, your body will go through a few changes, Schlader says. You will start making more plasma, basically pushing up your total volume of blood. That means your heart won’t have to work as hard to move blood around (one of the major ways we lose heat is through blood carrying it to our skin). The process of sweating also changes—you’ll be quicker to sweat, your sweat will increase in volume, and it will get more dilute, so you’ll lose fewer electrolytes. The whole thing is somewhat akin to how you can adjust to a higher altitude.

There’s been a lot of fighting online this week over a Washington Post story that talked about this exact concept. People argued not only about whether this effect is real, but also about whether it’s a big distraction from the need to address climate change.

I have two things to say after reading way too many comments and digging into this a bit more since my initial reporting two years ago. First, as Schlader pointed out, this is a real effect, and our bodies’ ability to adapt to all sorts of things is absolutely wild. Second, bodily adaptation won’t be the silver bullet that helps protect humans from heat caused by climate change.

There’s a limit to how much difference these physical effects can make—over the course of a few weeks, your body might be able to adjust to handle a couple of degrees’ worth of additional heat, Schlader says. That’s not enough to keep people safe in extreme conditions, especially if they have to work in the heat. There’s only so much heat people can endure—that might vary by person or place, but the limits still exist.

As temperatures continue to break records around the world, we’ll have to rely a lot more on other ways to stay safe. This includes using cooling devices like air conditioners and fans, seeking shade, or stopping physical activity when possible. That’s why heat is such an equity issue: not everyone has access to reliable cooling technology, or the ability to shelter inside when temperatures rise.

For more on the limits of our bodies, check out my 2021 story on the topic. Stay safe out there.

Keeping up with climate

I spoke with NPR’s All Things Considered about new materials called desiccants being used in air-conditioning. (NPR)

→ Check out my full story if you missed it last week. (MIT Technology Review)

There’s some interesting data in this opinion piece, where the author argues it’s “time to chill out” about AC and climate change. As he points out, today heating accounts for significantly more emissions than cooling does globally. (Bloomberg Opinion)

→ If you ask me, emissions from heat don’t cancel the need to improve the efficiency of air conditioners, given how much demand is expected to grow by 2050. I’d still call AC a climate antihero, as I wrote in the newsletter last week. (MIT Technology Review)

Tesla has a history of overly optimistic range projections for some of its vehicles. The company reportedly put together a special team to cancel service appointments related to range concerns. (Reuters)

Maine is stepping up its heat pump game. The state just blew past its goal of installing 100,000 of the devices by 2025 and re-upped the target to 175,000 by 2027. (Canary Media)

Scientists have previously shown that a major ocean current is weakening, but according to a new study, that collapse could come as soon as 2025. The risk is serious, but there are questions about this research and whether it’s overstating the near-term dangers. (E&E News)

→ I’d highly recommend this deep dive that my colleague James Temple took into this topic in late 2021. (MIT Technology Review)

Companies have sold millions of dollars of credits in programs that promise to capture and store carbon in soil on farmland. But some researchers say that the benefits of new agricultural practices aren’t so clear cut. (Science)

File this under weird climate change impacts: acidifying oceans (the result of more carbon dioxide in the atmosphere) are wiping out crabs’ sense of smell. As ocean chemistry gets more wacky, it could affect how crustaceans and other creatures can sense food and predators. (Hakai)