Hot and humid conditions can kill people 'far more easily than previously believed'

As the world warms up thanks to climate change, scientists are racing to understand the highest temperatures human beings can endure.

It's well-known that humans are less able to endure temperatures when it's humid, but now Penn State researchers believe that the limit may be just 31 degrees Celsius in a wet-bulb temperature.

'Wet-bulb temperature' refers to temperatures taken with a thermometer covered in a wet cloth, which are normally slightly cooler than 'dry-bulb' temperatures.

Wet-bulb thermometers allow researchers to work out whether humans can sweat: if the water evaporates, the thermometer cools down, so that wet-bulb temperatures are lower than dry-bulb temperatures.

In high humidity, the water will not evaporate, and the wet-bulb temperature will be the same as the dry-bulb temperature.

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Human beings can survive very high temperatures (well over 50C) when humidity is low, but in high humidity humans cannot survive high temperatures for long periods, because there is no way to cool down by sweating.

Even the fittest people often die within hours in such conditions.

In their study, the researchers found that the actual maximum wet-bulb temperature is lower – about 31C wet-bulb or 100% humidity – even for young, healthy subjects.

The temperature for older populations, who are more vulnerable to heat, is likely even lower.

W Larry Kenney, professor of physiology and kinesiology, said: "If we know what those upper temperature and humidity limits are, we can better prepare people – especially those who are more vulnerable – ahead of a heat wave.

"That could mean prioritising the sickest people who need care, setting up alerts to go out to a community when a heatwave is coming, or developing a chart that provides guidance for different temperature and humidity ranges."

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According to the researchers, while previous studies have theorised that a 35C wet-bulb temperature was the upper limit of human adaptability, that temperature was based on theory, not real-world data from humans.

Kenney said: "If you look at heat wave statistics, most of the people who die during heat waves are older people.

"The climate is changing, so there are going to be more – and more severe – heat waves. The population is also changing, so there are going to be more older adults. And so it's really important to study the confluence of those two shifts."

For this study, the researchers recruited 24 participants between the ages of 18 and 34.

Kenney said: "Young, fit, healthy people tend to tolerate heat better, so they will have a temperature limit that can function as the 'best case' baseline.

"Older people, people on medications, and other vulnerable populations will likely have a tolerance limit below that."

Prior to the experiment, each participant swallowed a tiny radio telemetry device encased in a capsule that would then measure their core temperature throughout the experiment.

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Then, the participant entered a specialised environmental chamber that had adjustable temperature and humidity levels.

While the participant performed light physical activity, such as light cycling or walking slowly on a treadmill, the chamber either gradually increased in temperature or in humidity until the participant reached a point at which their body could no longer maintain its core temperature.

After analysing the data, the researchers found that critical wet-bulb temperatures ranged from 25C to 28C in hot-dry environments and from 30C to 31C in warm-humid environments.

Kenney said: "Our results suggest that in humid parts of the world, we should start to get concerned – even about young, healthy people – when it's above 31 degrees [Celsius] wet-bulb temperature.

"As we continue our research, we're going to explore what that number is in older adults, as it will probably be even lower than that."

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