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Cooling limit temperature: When is hot too hot?

Fast NewsCooling limit temperature: When is hot too hot?

The world is getting hotter and hotter – with consequences for our health. But temperature is not the only problem; humidity also plays a role. What is the maximum cooling temperature that people can withstand?

High heat affects the human body in many ways. When it gets hot, the blood vessels dilate and increase their surface area to be able to release as much heat as possible. More blood is pumped into the outer veins to release heat there. We begin to sweat in order to cool the body temperature through evaporation.

In extreme cases, too high temperatures can lead to death: In the recent heat wave in India, about 100 people died. Temperatures of almost 50 degrees prevailed there for days. In heat stroke, the body can no longer cool itself down to the required 37 degrees – the organs fail.

Sultry heat more stressful
It is not that hot in Germany. But tens of thousands of people have died from heat in this country in recent years. But high temperatures are not always equally strenuous for the body. The second decisive factor is the humidity. If the air contains too much water, our sweat hardly evaporates and the cooling of the body no longer works properly.

That’s why experts always look at the so-called cooling limit temperature. This is the lowest temperature that the body can reach through evaporative cooling. It shows more precisely what heat stress acts on the body under what external conditions, because dry heat can be better endured than sultry heat. At a humidity of 100 percent, for example, the value is identical to the temperature in degrees Celsius. At 30 degrees Celsius and 50 percent relative humidity – realistic values for Germany – the cooling limit temperature is around 22 degrees.

Previous value probably too high
For a long time, it was assumed that a cooling limit temperature of 35 degrees could be life-threatening for a healthy person. The reason for this is a study from 2010, in which this theoretical assumption was postulated. But experts now believe that this value is too high.

For a recent study, scientists exposed 24 test persons to various combinations of heat and humidity in a climate chamber. They swallowed a sensor that measured the core body temperature and had to make slight movements, such as walking around.

The result: none of the test persons reached the theoretical limit of 35 degrees; their values became critical well below that. “Our study on young, healthy men and women shows that the upper limit above which life-threatening heat damage is possible is even lower. It is more likely to be at a cooling limit temperature of 31,” the authors write in the magazine “Spektrum der Wissenschaft”.

Children and the elderly are particularly at risk
Such high values are hardly achievable for Germany. But even lower (cooling limit) temperatures can be dangerous for certain people, explains biologist Veronika Huber of LMU Munich. “Above all, the elderly and those with previous illnesses are at risk. Studies show that only a very small proportion of those who die from heatstroke actually do so. More common causes of death are, for example, heart attacks or kidney failure, but these can of course be related to the heat.”

In older people, he says, the body’s cooling system – like many bodily functions – is weaker and reacts more slowly. “In addition, old people often have a different sense of thirst, which makes it more dangerous for them.” (Young) children are also more sensitive, as they have a much lower sweat production.

More and longer heat waves
The problem: Already, the number of hot days – when it gets at least 30 degrees – has almost tripled since World War 2. And due to climate change, the heat will continue to increase. “In the future, we will not only experience higher average temperatures, but also more and longer heat waves,” says Andreas Matzarakis from the Centre for Medical-Meteorological Research of the German Weather Service in Freiburg.

“Their average number will increase from three to more than five per year and they will last longer. Currently, the average duration is about five days, but it will increase to an average of over eight days by the end of the century.” All this could result in more people suffering from heat and possibly dying as a result.

“With a global warming of about three degrees by 2100, which is what most experts are assuming at the moment, heat-related mortality in our country could triple if no effective adaptation measures are implemented,” says biologist Huber. It is important to note that the mortality risk does not increase linearly, but exponentially with temperature. “The difference between 35 and 40 degrees is enormous in terms of mortality,” she says.

Tropical regions particularly affected
As a rich country, Germany is at least in a position to take countermeasures. Many poorer countries – which are, however, sometimes particularly affected by climate change – cannot. Only recently, a study showed that the so-called climate niche is becoming smaller. A climate niche is the temperature range in which people prefer to settle due to favourable climatic conditions. By the end of the century, one third of the entire world population could live outside this niche, i.e. in particularly hot regions.

Particularly in tropical regions, the cooling limit temperatures could rise in the future and lead to serious health problems. According to studies, densely populated and agriculturally intensively used regions in South Asia are likely to be particularly affected. “If climate change continues unchecked, the river valleys there and in China could face serious problems. But also in some regions in the Middle East, temperatures could develop in such a way that they pose an unprecedented danger to the people there,” Huber explains.

Protection against heat
If it gets extremely hot again in Germany this summer, the expert advises above all to drink a lot, avoid the sun and don’t exert yourself too much. The DWD issues heat warnings for such days. These warnings also take into account the perceived temperature: In addition to temperature, humidity, wind strength and other parameters are also taken into account. Because how hot and dangerous it is for people does not depend on the thermometer alone.

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