At what point does heat stress become dangerous?

Climate change is increasing heat stress on humans, plants, and animals. Experts at the Technical University of Munich set out to investigate the precise temperatures at which weather becomes dangerous for all of Earth’s living organisms.

According to the study, the most optimal temperatures range from 17 to 24 degrees Celsius, or 62.6 to 75.2 degrees Fahrenheit.

“We have studied which temperatures are preferable and which are harmful in humans, cattle, pigs, poultry, and agricultural crops and found that they are surprisingly similar,” said study co-author Professor Senthold Asseng.

The investigation showed that in weather conditions with high humidity, mild heat strain for humans begins at about 23 degrees Celsius, and at 27 degrees Celsius in low humidity. 

“If people are exposed to temperatures above 32 degrees Celsius at extremely high humidity or above 45 degrees Celsius at extremely low humidity for a lengthy period of time, it can be fatal,” said Professor Asseng. 

“During extreme heat events with temperatures far above 40 degrees Celsius, such as those currently being observed on the U.S North West Coast and in Canada, people require technical support, for example in the form of air-conditioned spaces.”

Professor Asseng points to a variety of strategies, including increasing natural shade from trees, to mitigate heat. Furthermore, cities and buildings can be designed with lighter, reflective roof and wall colors to reduce heat strain.

In cattle and pigs, heat strain occurs at 24 degrees Celsius with high humidity and at 29 degrees Celsius with low humidity, according to the study. 

The milk yield from cows can decrease by up to 20 percent when they are exposed to heat stress, and the fattening performance in pigs is also reduced. 

The comfortable temperature range for poultry is 15 to 20 degrees. Chickens were found to experience mild heat strain at 30 degrees Celsius. Beginning at a temperature of 37 degrees Celsius, chickens experience severe heat stress and lay fewer eggs. 

In addition, the experts report that heat stress reduces growth in cattle, pigs, chickens and other livestock, which means both lower yields and reproductive performance. 

“There are examples of evolutionary adaptations to warm weather in terrestrial mammals. Transylvanian naked chickens are more heat tolerant than other varieties of chickens because of a complex genetic mutation that suppresses feather growth. They are naturally air-conditioned because they lack feathers on their necks,” explained Professor Asseng.

“In crops, the optimal temperature zone and temperature thresholds seem to be more diverse due to differences between species and varieties.”

For example, cold-temperate crops such as wheat do better at cooler temperatures, while warm-temperature crops such as corn are sensitive to frost but can tolerate warmer temperatures. 

“By the end of the century, 45 to 70 percent of the global land area could be affected by climate conditions in which humans cannot survive without technological support, such as air conditioning. Currently, it’s 12 percent,” said Professor Asseng. 

In other words, up to 75 percent of the human population will be chronically stressed by heat in the coming decades. A similar increase in heat stress is expected for agricultural crops, livestock, and other plants and animals. 

“Genetic adaptation to a changing climate often takes many generations. The time available is too short for many higher forms of life. If current climate trends persist, many living things could be severely affected or even disappear completely from Earth due to temperature change.”

The study is published in the journal Lancet Planetary Health.

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By Chrissy Sexton, Earth.com Staff Writer