June 15, 2025, 6:02 am | Read time: 3 minutes
German summers are getting increasingly hotter, with heatwaves posing significant challenges for many people. This weekend, temperatures in Germany are expected to reach up to 86 degrees Fahrenheit. But at what outdoor temperature does it actually become dangerous for our bodies? Researchers provide an answer to this question.
According to the German Weather Service, the summer of 2022 was one of the four warmest summers in Germany since weather records began in 1881. The Weather Service assumes that this was not an exception and that very warm and dry summers are likely to become the norm.1 In 2023, one heat wave follows another. And this year, high temperatures also seem to be on the horizon.2 This makes it all the more important to understand what happens in the body during heat and at what outside temperature it becomes dangerous for humans. British researchers from the University of Roehampton in London have investigated this question and are providing answers for the first time.
Overview
Heat Above 40 Degrees Celsius Is Critical for the Body
On July 4, 2023, the highest average temperature on Earth was recorded. The temperature was 17.18 degrees Celsius, as reported by the Washington Post.3 Most people are aware that global warming has negative consequences for the environment. But what are the direct effects of heat on our bodies? This is what Prof. Lewis Halsey and his research colleagues at the University of Roehampton in London have been investigating. The researchers presented their findings in 2023 at the biologists’ meeting at the ‘SEB Centenary Conference‘ in Edinburgh, Scotland.
They found that temperatures above 40 degrees Celsius are critical for the body. The researchers based their findings on a previous study they published in 2021 on this topic.4 They discovered that the resting metabolic rate is higher when people are in a hot and humid environment. “A lot of research has been done on temperature ranges in which different animal species prefer to live in terms of minimal metabolism and thus low energy consumption. But strangely, much less information is available for humans,” says Prof. Halsey in a press release.5
What Exactly Happens in the Body During Heat?
According to the researcher, the effect is similar to being in a sauna: As the temperature rises, the metabolic rate increases. This makes it more challenging for the body to maintain its normal functions. If the temperature gets too high or if we are exposed to high temperatures for too long, the circulatory system collapses.
In addition to the heat itself, other environmental factors during hot weather pose risks. The brain, circulatory system, and kidneys are particularly affected, as overheating and the loss of fluids and electrolytes put a significant strain on these organs. Pre-existing conditions of the heart, lungs, or kidneys can be exacerbated. In particularly hot years, a significant increase in mortality can also be observed. Given the already measurable increase in heat waves since the 2000s, the risk to public health is considered (very) high.
Besides the heat itself, other environmental factors during hot weather pose additional risks. Ozone, for example, can irritate the respiratory tract and eyes and, like heat, cause headaches. Intense UV radiation can lead to cell damage in the skin and eyes, potentially causing conditions such as sun allergies, cataracts, conjunctivitis, or even skin cancer.6
What Happens in the Body During Heat—and When It Becomes Dangerous
Prevent Muscle Loss After 40! This Training Plan Will Help
Sunburn in Children? Far from Rare
How to Protect Yourself in High Heat
- Drink enough water: The most important thing is to stay well-hydrated, especially when sweating heavily.
- Don’t drink alcohol: Alcohol can dehydrate you, so it’s best to avoid it in hot weather.
- Protect your skin: This means not only applying sunscreen but also wearing breathable clothing. Your skin should also be protected with fabric and a hat.
- Avoid strenuous activities: When it’s hot outside, take it easy. It’s best not to exercise to avoid excessive sweating.