Temperature tolerance threshold is likely to be lower for humans
Climate Change Extreme heat Wet-bulb temperature

Temperature tolerance threshold is likely to be lower for humans

Dr. Lindy Whitehouse
Dr. Lindy Whitehouse

In 10 seconds? Summer is on in the Northern Hemisphere (at the time of writing), and a recent study has found that the “wet-bulb temperature” limit at which we can cope with heat stress is much lower than thought - even for younger, fit people.

Wet what temperature? Wet-bulb temperature! It is calculated by measuring the temperature using a standard thermometer with the bulb covered in a wet cloth. Simples, right? This gives a measure of both temperature and humidity and is a useful tool for measuring bodily heat stress. Previous research had suggested that humans would struggle to cope with a wet-bulb temperature above 35°C (95°F), but this had not been tested using empirical data. This study examined the wet-bulb temperature limit in young, healthy people and found that it is around 30 – 31°C (86-87.8°F), even for them, which is much lower than the previous threshold.

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So, why is it a hot topic? Well, two words: climate change. Have you heard of the blistering heatwave of 2022 in Pakistan and India, where in some cities the mercury hit 51°C (123.8°F)? A group of published scientists claims that climate change made it 30 times more likely. Hauling from Imperial College London, the Indian Centre for Atmospheric Sciences, and Princeton's Department of Geosciences, among others, the authors used peer-reviewed methods, although their paper has not appeared yet in a renowned scientific journal at the time of writing. The wet-bulb temperature limit of 35°C (check out this Digest) has been used as a threshold to predict the future liveability of regions around the world under different climate change scenarios.

How widespread is this problem? Billions around the world will be affected, although the predicted changes in wet-bulb global temperatures are not expected to be uniform. The areas expected to be most impacted include northern India, China, northern Australia, Africa, Central America, and Southeast Asia. The study demonstrates that the tolerance threshold should be lowered to 30 – 31°C which will greatly expand the regions that will become unliveable. Not only does this have health impacts for humans around the world, but also socioeconomic impacts which will significantly affect developing countries, unless they can find ways to adapt. And so, it is likely to affect the developed world as well.

Map of locations with extreme heat and humidity (hottest 0.1% of daily max. wet bulb temperatures) from 1979-2017. Source: National Oceanic and Atmospheric Administration, 2020.
Map of locations with extreme heat and humidity (hottest 0.1% of daily maimum wet bulb temperatures) from 1979-2017. Source: National Oceanic and Atmospheric Administration, 2020.

How did the researchers discover this? The team measured the core body temperature of the participants while performing a low-intensity task. The participants were all between 18 to 34 years of age and during the experiment they pedaled fixed cycles at a pace that mimicked the energy required to carry out daily life activities. During this trial, the dry bulb temperature was increased at a rate of 1 degree every 5 minutes until the researchers recorded a significant body core temperature increase in the participants. The participants were given gastrointestinal temperature telemetry capsules (developed in the 1980s and used for example, by NASA astronauts) to swallow before the experiment to enable the researchers to accurately record their core body temperature and they were also given dry bulb thermometers to wear to measure their skin temperature.

It sounds a bit of an obvious question, but what can we do to prevent it? This study highlights the urgent need for action on climate change as we get dangerously close to the reality that vast areas of the Earth could become inhabitable much sooner than we previously thought. In addition, increasing temperatures will impact human health, causing an increase in heat-related deaths and malnutrition as a result of famine. To combat this, we need to rapidly introduce climate mitigation strategies to prevent the Earth from warming any further. As well as climate mitigation, we also need to examine ways that populations can adapt to climate change to ensure that those living in areas where the temperature regularly exceeds the wet-bulb temperature limit of 30 – 31°C can cope or we can expect to see a large impact on human life.

This all sounds a bit alarming, are there any limitations of this study? Indeed – and I need to highlight that it only focused on healthy adults between the ages of 18 and 34. But extreme temperatures and heat waves will increase the risk of heat-related illnesses in vulnerable populations including children, the elderly, and communities living in poverty even more. In addition, climate change will result in more than just increasing temperatures. We can expect to also observe increases in pollution, disease transmission, ultraviolet radiation exposure, and likely a shortage of food. Together, these cause a physiological strain on the body which could further reduce the increase in heat that people can tolerate.

You may not know this but …

... the recent IPCC report stated that the last decade is likely to have been the hottest period in the last 125,000 years!

If the Earth continues to warm at its current rate, then three billion people could be living in areas that have a mean annual temperature above 29°C by 2070.

However, it's not all bad news. If we significantly cut emissions now, then scientists think we can reduce the impact that global warming will have on the Earth and in a few decades global temperatures will stabilize.

Lindy has distilled 10 research papers, saving you 33 hours of reading time.

The Science Integrity Check of this 3-min Science Digest was performed by Dr. ASM Mainul Hasan.

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