At what temperature does it become life-threatening?

But at some point it becomes dangerous – the point is closer than many think.

One of the first films I saw in the cinema was Beverly Hills Cop. I was nine years old and I probably wasn’t officially allowed to go to the cinema, but I guess nobody was interested at the time. Great movie. I was very impressed how fast Eddie Murphy can talk, became his biggest fan and immediately bought the soundtrack.

Which is why I’m thinking of Eddie Murphy this summer. Which in turn is due to “The Heat Is On”, by Glenn Frey, track number 6 on the soundtrack. Because I am, was, red-haired. My skin is about the color of milk and when other people tan I look like a lobster. Every summer I ask myself anew: Can you actually sweat yourself to death?

is an author and columnist and lives in Hamburg. Among other things, he works for the magazine “Geo” and the research network Zoonotic Infectious Diseases, funded by the Federal Ministry of Education and Research. Kohlhöfer also writes screenplays and designs communication concepts. He was shot at for one story in the Pacific, for another he marched through the rainforest for days. In 2021 he published the bestseller “”. pandemics. How viruses are changing the world

You can certainly die from the heat. “For Germany, the effect of heat on mortality could be quantified,” says a study by the Robert Koch Institute from April. In the three years from 2018 to 2020, almost 20,000 people died from heat.

On the other hand, not all heat is the same. If you die from it, a lot comes together, the temperature, of course, pollutants, humidity, water intake, the health of the person. But is there a temperature below which, under ideal circumstances, a healthy person can no longer live?

I’ll make it short, without tension: It’s 46 degrees Celsius. According to a 2014 study by the University of Sydney, this is the highest temperature a person can live in continuously – if they drink constantly, do not exert themselves, wear little clothing, actively cool themselves and the humidity is low. No, the handful of people who live in Death Valley don’t count because they don’t stand in the blazing sun.

Physics helps us for a while

They can’t either, because then it slowly becomes dangerous. We are warm-blooded mammals and as such we have a body temperature in which there is a constant balance between heat loss and heat gain. The biochemical processes in our body work best at around 37 degrees Celsius, which is why it does everything it can to keep the temperature between 36 and 38 degrees.

This allows us to survive in an environment where the air temperature is higher than our body temperature – as long as we drink. In order to maintain its core temperature in hot environments, the body primarily uses one means: sweat. When water evaporates from a surface, the surface remains cooler.

Doesn’t look great wearing a shirt, but works to a degree. You can’t sweat yourself to death.

Sweating does not work indefinitely, an untrained adult can sweat around two liters per hour, athletes and people who have adapted to the heat around twice as much – and it only works as long as the humidity is not higher than 75 percent. Above it is so humid that there is already a lot of water vapor in the air. The sweat can no longer evaporate as quickly, sweating no longer cools down as much. And the finely balanced system of the body gets out of step.

That’s probably the reason why I don’t like lying on the beach: too boring, but above all too hot. I tend to take a siesta at midday in the summer, but I can’t really sleep well even in the heat, which is why I’m constantly overtired.

Grilled by the sun

It’s no wonder, because the blood vessels dilate when it’s hot. The heat dissipation to the environment increases. That’s good at first, but it’s just trying not to die, the body prioritizes. The increased blood flow to the skin means that the rest of the skin gets less blood.

The heart helps itself by beating faster, but the blood supply to the organs is reduced and blood pressure drops. Some people get diarrhea when it’s hot, and it’s also not good for top intellectual performance because the brain gets less oxygen.

The large muscles are less supplied with blood, the general performance decreases. Direct sunlight grills the brain – and that’s exactly right, because if you don’t wear a hat for a long time, the meninges and brain tissue are attacked: sunstroke.

Acute overheating causes the brain to swell, the organs to fail, and the enzymes to go haywire. Extreme heat can cause serious kidney and heart problems. When the body is overheated and there is no way to cool it, multi-organ failure occurs. Heatstroke.

However, whether 46 degrees Celsius is really the maximum that a person can survive is almost irrelevant. On the one hand, it doesn’t matter what we do: around 20 percent of our kinetic energy goes into the muscles, the rest is converted into heat. The more exercise, the more heat the body has to get rid of. On the other hand: In order to be able to give a reasonable estimate, scientists usually use the cooling limit temperature.

The English term wet-bulb-index is more apt, because it’s about the temperature a thermometer measures when wrapped in a wet cloth – it’s used because then you can estimate what the skin temperature would be you sweat constantly.

The wet-bulb index reflects not only the heat, but also the water content in the air. It doesn’t really have anything to do with the temperature in the weather forecast, it’s a dry bulb temperature because it’s measured with a dry instrument.

The world becomes more uninhabitable

Years ago I was in Madagascar, in the rainforest. I had to walk through it for a week. It was the hardest thing I’ve ever done. It was very hot and very, very humid. And that’s no exaggeration: I almost died.

Because the limit of the wet bulb index is not even that high. 35 degrees Celsius is pretty much the end of human tolerance. At a higher temperature, the body is no longer able to dissipate heat efficiently enough to maintain its core temperature. We don’t die immediately, but suffer serious damage to the brain and organs.

About a third of all people now live in areas that experience a life-threatening combination of heat and humidity for at least twenty days of the year. Without wanting to spread a bad mood: this will increase – even if we as humanity meet all climate targets, which it doesn’t look like.

Extreme heat events will become more frequent, more intense and last longer. This also applies to Germany, we are not an island of the blissful, the maximum air temperature is also shifting in the direction of extreme heat in this country. Climatological parameters are “hot days” and “tropical nights”, as the German Weather Service calls them. Both are increasing and both are not really healthy. However, it’s also true: you don’t die from it.

There is also hope

On the other hand, some climate models predict that we will reach a wet bulb index of 35 in parts of the world by the middle of the 21st century. Which isn’t that long away. In some places in the subtropics such conditions have already occurred, never for long, but forever longer.

This simply means: These areas will become uninhabitable. And because the people who live there understandably won’t die voluntarily, but rather leave the area, that will bring some geopolitical instability with it… But don’t do that, otherwise the text will be too long and my mood too bad.

Heat and everyday work: a dangerous combination

Also, less extreme conditions are often deadly, and we don’t even need to go to the tropics for that. That’s because when it’s hot, people don’t just lounge in the pool and drink non-stop, they go to work, are in hot cities, and are either too young or too old to have good temperature regulation. The good thing is: heat tolerance can change over time. People in warm countries have it easier.

I also know that from my own experience, job in Tel Aviv. All wore light blue business shirts without a single sweat stain. I looked like I just got out of the shower with all my clothes on. Which was still strange, because in general, people who are more used to the heat sweat just as much, but differently. Your sweat is thinner. You lose fewer electrolytes, which protects the body from heart and kidney problems. Heat waves in early summer are therefore usually more deadly than those in late summer – because people have gotten used to the high temperature by then.

But the rule here is that the border cannot be moved. We cannot develop further than physics and our bodies allow. And that applies always and in every case.