Surviving in a warmer climate, what can the body cope with?

The often-used temperature limit for human survival in extreme heat must be adjusted downward. This is what Coen Bongers of the Radboudumc and scientists from Australia and the US write in an article in Nature Communications. Especially in older adults and especially older women, the limit is significantly lower than previously assumed.

Many heat records were broken again this year. Not only in England (40°C), but also in Portugal (47°C) and India (49°C). With these rising temperatures, the question arises of how much heat a human body can handle. The air temperature (T) we use in everyday life is not a good measure of this. The wet bulb temperature (Tw) is more suitable because it takes into account the humidity of the air and thus the possibility of sweat evaporation, resulting in the cooling of the body (see *) for explanation).

Lower limit elderly

For more than a decade, a critical wet bulb temperature (Tw) of 35°C has been used as an upper limit for human survival. However, this does not include variations in human physiology (such as age, gender, and physique), argues an international group of researchers with Coen Bongers of the Radboudumc's Department of Medical Biosciences. If you do, the Tw threshold for human survival turns out to be lower. Sometimes even significantly lower. Especially for the elderly and especially for older women.

The research, published in Nature Communications, uses a physiology-based approach for young and older adults to improve the estimation of survival. Bongers: "In older people, the ability to sweat decreases, with women having a lesser maximum sweat capacity than men anyway. That means they can cool their bodies less by losing heat through sweating. As a result, the threshold temperature at which the elderly can survive is also a lot lower than that wet bulb temperature (Tw) of 35°C. That limit is 0.9 to 13 degrees lower."

Survival and livability

The researchers not only looked at the survival limit, but also introduced the term livability. By this they mean the temperature limit at which people can still safely carry out their daily activities. After all, survival is different from livability. Bongers: "That livability decreases with sun exposure and humidity, but most dramatically with age. Perhaps remarkable, but reductions in that safe activity for younger and older adults indicate a stronger influence of aging than of warming. With continued aging in the coming decades, that is an important criterion to include in models looking at effects of climate change. In the tail end of the COP28 climate conference in Dubai, they may be able to take the results of our study into account nicely."

*) Ratio of temperature and wet bulb temperature

Usually in everyday life we use the air temperature (T). This is the temperature also mentioned in the weather report and it is always measured in the shade. The wet bulb temperature (Tw), which is measured by a thermometer covered with a wet cloth in the air stream, is almost always lower because that measurement includes evaporation (of water). Through evaporation, our body can get rid of heat. That evaporation depends on humidity. The higher the humidity, the less evaporation and the closer T and Tw become to each other. At 100 percent humidity, no more evaporation takes place and T and Tw are equal to each other.

Combinations of temperature and humidity with the corresponding law bulb temperature:

Air temperature(T)                 Humidity           Wet bulb temperature (Tw)

35°C                                      100%                                     35°C

35°C                                      50%                                       26.6°C

35°C                                      20%                                       19.3°C

48°C                                      40%                                       35°C

38°C                                      81%                                       35°C

40°C                                      70%                                       35°C

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Publicatie in Nature Communications: A physiological approach for assessing human survivability and liveability to heat in a changing climate - Jennifer Vanos, Gisel Guzman-Echavarria, Jane W. Baldwin, Coen Bongers, Kristie L. Ebi & Ollie Jay

https://www.nature.com/articles/s41467-023-43121-5

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