How hot is too hot for humans? Understanding wet-bulb temperatures

Wet-bulb temperatures prevent the body from cooling down. Modelling suggests that if we don't bring down greenhouse gas emissions, wet-bulb temperatures could regularly exceed dangerous benchmarks in parts of the world this century. 

Humans sweat to keep cool. But when that sweat can't evaporate, heat gets dangerous

A man refreshes himself in a fountain in central Moscow on July 20, 2010, amidst a prolonged heat wave. (Alexander Nemenov/AFP)

As an eastward-moving heat dome continues to envelop Western Canada, resulting in a pressure cooker of a heat wave that has produced the highest temperatures ever recorded in the country, some Canadians might be pondering what future summers could look like.

But in the wake of this dangerous weather event, climate experts are pointing to another concept we might need to worry about: Wet-bulb temperatures.

Wet-bulb temperatures are theoretical measurements that combine temperature and humidity into one value, explains Rachel White, an atmospheric scientist with the University of British Columbia's department of earth, ocean and atmospheric sciences. 

Basically, it's the temperature at which water stops evaporating from a wet thermometer bulb, meaning it can no longer cool.

"With wet bulb temperatures, you can increase them in one of two ways," explains White. "You can either increase the temperature or you can increase the humidity. And obviously it's worse if you do both."

What does that have to do with human survivability? The wet bulb temperature is essentially a metaphor for human sweat. Sweat is the body's cooling mechanism, providing humans with relief when the body heats up. 

But for sweat to actually cool a person's skin, it relies on the process of evaporation to move heat away from the body. 

At theoretical wet-bulb temperatures, evaporation and cooling can no longer take place because the atmosphere is fully saturated with water. And when the wet-bulb temperature reaches 35 C, it crosses a threshold at which humans can no longer lose internal body heat and cool themselves.

This Canadian Tire store in Vancouver was experiencing a shortage of air conditioners and fans this week amid the heat wave cooking Western Canada. Climate experts say that one extreme heat measurement we need to be thinking about is the wet-bulb temperature. (Ben Nelms/CBC)

"Generally, the atmosphere almost very rarely has 100 per cent relative humidity," said White. 

But research shows that even wet-bulb temperatures lower than 35 C can be fatal. That was the case in 2010, when Russia experienced a deadly heat wave where wet-bulb temperatures didn't pass 28 C. 

"That's why people are sort of talking about [wet bulbs], because a very humid heat wave is a lot more dangerous than a very dry heat wave," said White.

Under this week's unprecedented heat wave, cities like Vancouver were experiencing high humidity.

"And it's the very humid heat waves that would have a higher wet-bulb temperature," said White.

In Western Canada, wet-bulb temperatures generally aren't much of an issue. Wet-bulb temperatures are more likely to occur in areas where large bodies of water tend to warm up, White said, such as the Great Lakes region.

But as climate change drives up global temperatures, climate experts are using models to determine future hotspots for wet-bulb temperatures.

Modelling suggests that if society doesn't make a collective effort to bring down greenhouse gas emissions, wet-bulb temperatures could regularly exceed that 35 C benchmark in parts of South Asia and the Middle East in this century. 

A wet-bulb globe temperature (WBGT) measure tests conditions at Shiokaze Park in Tokyo in this 2019 file photo, as part of heat mitigation efforts in the leadup to the Olympic Games. It's something that may become even more common as Earth heats up. (Behrouz Mehri/AFP)

"Climate scientists have predicted for decades that with business as usual, the frequency and intensity of heat waves will increase globally," said Hind Al-Abadleh, who studies atmospheric chemistry and climate change at Wilfrid Laurier University in Waterloo, Ont.

She describes wet-bulb temperatures as similar to the humidex that weather forecasters use to describe how people feel heat on a humid day. In very humid parts of the world, particularly in coastal areas near warm bodies of water, scientists are already seeing evidence of conditions that reach the limits of human tolerance. 

A 2020 study in Science Advances looked at global temperature data from weather stations over the last 41 years and found that both the severity and frequency of extreme humid heat is increasing — and that, the authors say, will represent "a major societal challenge" in the coming decades.

Bringing down greenhouse gas emissions is one way to reduce the likelihood of these dangerous temperatures, experts say. But another is making sure that proper infrastructure exists to accommodate people who can't afford to cool down. 

"Coping with rising wet-bulb temperatures will mean that governments need to issue advisories for people to stay indoors in climate-controlled spaces," said Al-Abadleh.

But, she noted, operating such spaces is energy intensive.

"So transitioning to clean and renewable energy sources and phasing out fossil fuels won't just lower our carbon emissions, but also help people survive heat waves when they occur."

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