How does moisture affect the human body?

Human air conditioning at the limit : Sweating more will soon no longer help in many places

Heidi Klum doesn't sweat. At least not if she doesn't want to. At least that's what she says over and over again in interviews. Apparently she thinks sweating is a human flaw. Actually, visible beads of sweat on the skin or even those unpopular stains in the textile only announce that a body has set its natural and vital air conditioning system in operation. However, this practical cooling function also has its limits. In the course of climate change, they are likely to be exceeded more and more often in some regions of the world - and cause more and more heat deaths.

A thermostat in your head

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When the human body threatens to get too hot, the thermostat in the brain, the hypothalamus, instructs the autonomic nervous system to pour water out of special small sweat glands. Everyone has up to four million of them. What then swells out of the skin consists of over 99 percent water, plus a few salt ions and proteins.

Sweating is an important factor in the energy balance of the human body. The muscles can only convert a small part of the supplied food energy into movement. Most of it just heats up the muscles. Because the body temperature can only fluctuate within narrow limits, the bodies have to release excess heat into their surroundings as quickly as possible. This happens mainly through thermal radiation and through direct heat transfer to the surrounding air. However, the higher the ambient temperature, the less heat a body can give off in this way. And especially if the muscles also supply a lot of heat during physical work or sport, there is a risk of overheating, which is harmful to health.

The sweat evaporates, which costs energy, which makes it colder

But it is precisely for this case that evolution has endowed man with the ability to sweat over almost the entire surface of the body that we only share with a few other animal species. The cooling effect is based on the physical principle of evaporation: the sweat on the skin gradually changes from a liquid to a gaseous state. In the liquid state, the water molecules are slightly connected to one another. When evaporating, some molecules break their bonds and become free gas molecules.

Even this release of molecules requires energy. But this does not yet cover the energy requirements for evaporation. The water vapor produced during evaporation must expand against the existing air pressure. And this increase in volume also devours energy. Overall, the evaporation of sweat removes energy from the environment. Therefore, where sweat evaporates, it gets cooler. And the cooler it gets on the skin, the more heat the body can give off.

Sweating in the sultry is not cool

The sweat glands can shed two to three liters per hour during intense physical activity. But when, as in Schiller's “Song of the Bell”, the sweat only runs “hot from the forehead”, there is still no cooling effect. Dripping or wiping off sweat is a waste of energy. The liquid actually has to evaporate - and the more that evaporates, the better for cooling the body.

But the air can only store a limited amount of water vapor. The more humid it is, the less water vapor it can absorb. People therefore perceive high humidity and high air temperatures as uncomfortably humid: Although they still sweat, the cooling evaporation of sweat into the already moist air is only possible to a limited extent.

Fortunately, however, the air has a property that also supports sweat regulation, especially at high air temperatures: the warmer the air, the more water vapor it can store: with the same number of water molecules, the "relative humidity" drops when the air becomes warmer.

The cooling effect depends on the humidity

But that too has its limits. Especially in southern countries, meteorologists are observing more and more heat waves, in which not only the temperature is high, but also the humidity. In order to be able to assess whether the body cooling through sweat evaporation can still adequately protect a person from overheating, a simple measurement is sufficient: You wrap a wet rag around a thermometer and hold a not so equipped thermometer next to it. Due to the evaporation of the water from the rag, the thermometer shows a lower "cooling limit temperature" than the outside air temperature. You could also call it “wet T-shirt temperature”. Because with sweaty T-shirts, the values ​​on the body surface also decrease accordingly.

The value to which the cooling limit temperature can drop through evaporation depends directly on the "relative humidity". For example, if it is 20 percent, the air contains only a fifth of the amount of water vapor that it could store at this temperature, so a lot of sweat can still evaporate into the air. As a result, the cooling limit temperature that can be achieved through evaporation drops correspondingly low. At an initial air temperature of 40 degrees Celsius, it is 22 degrees. But even at a relative humidity of 40 percent, the cooling limit temperature is already 28 degrees.

High temperature alone is not a problem, it only becomes critical when the air humidity is high

And from here on it gradually becomes critical for the human heat balance. A sufficient amount of heat can only be given off to the environment if the cooling limit temperature generated by evaporation is well below the skin temperature. For a healthy person, this should not exceed 35 degrees Celsius.

As early as 2010, the two climate researchers Steven Sherwood and Matthew Huber described the potentially dangerous consequences of this thermodynamic fact in a publication in the journal "PNAS": High temperatures alone can homo sapiens withstand; However, it is not immune to high temperatures in combination with high humidity. Our body cooling fails above a certain cooling limit temperature. “The cooling limit temperature limit is the point at which you would overheat, even if you were naked, completely wet and standing in the shade in front of a large fan,” Sherwood is quoted in the science magazine “scinexx”.

According to Sherwood and Huber, this limit is 35 degrees. Even for a healthy person, staying in a place with a cooling limit temperature of more than 35 degrees Celsius is fatal after six hours at the latest.

Climate change: In 80 years, entire regions could become uninhabitable

Death from heat stroke in damp heat: As the two scientists warned, this up to now hardly noticed consequence of climate change could make whole areas uninhabitable by the end of the century at the latest - if the temperatures continue to rise as before.

Up to now, the cooling limit temperatures, even in the hottest zones on earth, have rarely climbed above 31 degrees Celsius. But even cooling limit temperatures of 28 degrees Celsius are already difficult to bear. Because even then the body can only give off a little heat; As a result, farmers, for example, run the risk of overheating or even heat stroke when working longer in the field. According to a recent study by Eun-Soon Im and colleagues in the journal “Science Advances”, however, the hotspots of high cooling limit temperatures are, of all places, in densely populated agricultural regions in the valleys of the Ganges in India and the Indus in Pakistan.

In the future, the "cooling limit temperature" should be taken for granted in the weather forecast

However, the most extreme humid heat waves in recent years hit regions around the Persian Gulf. During one of these heat waves, the air temperature in the Iranian city of Bandar Mahshahr rose to 46 degrees Celsius on July 31, 2015. It was moist sea air with a high relative humidity of almost 50 percent at this temperature. Around 4.30 p.m. local time, the cooling limit temperature therefore approached the threshold of 35 degrees Celsius. This is a clear warning that as a result of global warming, the cooling limit temperatures will also rise to values ​​that people can no longer bear or can no longer bear at all.

Whole regions of the world could become uninhabitable, at least temporarily - and maybe earlier than feared. If this forecast is true, the living space for humans will not only shrink due to rising sea levels, but also due to damp heat waves. Presumably, in the second half of the 21st century at the latest, the specification of the cooling limit temperature will be just as much a part of the weather forecast in southern countries as is “normal” air temperatures today. And in the USA, too, people will soon have to get used to warnings of high “wet-bulb temperatures”: As a team of researchers led by Colin Raymond from Columbia University explained in the “Journal of Geophysical Research” in 2017, cooling limit temperatures are Over 28 degrees Celsius, especially in the Mississippi Valley up to the Great Lakes, is no longer a rarity.

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