What are ‘wet-bulb’ temperatures — and why can they be so deadly?

A heat dome in the western U.S. continues to smash temperature records, the latest in cases of significant heat around the world so far this year.

In Canada, temperatures are climbing, with Ottawa soaring past 30 C on Wednesday. But with more heat on the way people should be aware there can be a temperature that’s too hot for humans.

When people check the weather they often look at the temperature, also known as the “dry-bulb temperature,” and the humidity to get a sense of how hot it will feel.

Some meteorologists say people should also be aware of the “wet-bulb temperature,” which can signal deadly heat.

It gets its name from the act of covering a thermometer with a water-soaked cloth. When water evaporates and lowers the temperature on the device, it mirrors how the human body cools down with sweat.

But the more humidity there is in the air, the harder it is for that moisture to evaporate and cool the thermometer — or you — down.

“If you’re at 100 per cent humidity then you can’t evaporate any water and so that’s the reason why humidity is such a big impact, because it does really increase our heat stress,” Kent Moore, an atmospheric physics professor at the University of Toronto Mississauga, told Global News.

“Although the temperature might be the same, we can’t cool ourselves off.”

Humans lose about 80 per cent of heat through sweating.

But when both the humidity and air temperatures are high, it’s harder to shed that heat.

A study in 2010 found a wet-bulb temperature of 35 C that lasts longer than six hours could have serious health impacts or even death.

But research published in 2022 in the Journal of Applied Physiology found that limit may be lower.

At 100 per cent humidity, that threshold could be 31 C, while at 38 C, just 60 per cent humidity could be too much for the body.

Humidity in Ottawa on Wednesday sat at 51 per cent with temperatures hovering around 30 C, according to Environment Canada.

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Moore also points to the humidex values, a Canadian invention that measures how hot we feel, as an indication of what could be dangerous, noting anything above a humidex value of 40 could be seen as such. Ottawa’s humidex value on Wednesday was 36.

So for example, he says, 30 C at 70 per cent humidity means the humidex could sit at 41 or 45 at 100 per cent.

“Even at a 60 per cent relative humidity, what that means is that your body’s maybe only 40 per cent as efficient as taking heat away from you,” he said. “Your body’s only so efficient at cooling itself.”

Once higher wet-bulb temperatures are reached, your body will find it difficult if not impossible to cool down because your sweat will not be able to evaporate.

“A person’s normal physiologic mechanisms for coping with extreme heat start to not be able to cope with that heat,” Dr. Samantha Green, a family physician with Unity Health Toronto, told Global News.

When people get too hot they can experience heat exhaustion, which can include symptoms of nausea, lightheadedness or dizziness.

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Green said it can quickly progress to heat stroke, which is a medical emergency and sees people get confused and have different levels of consciousness.

She said to call 911 and immediately try to cool the person down if you believe they are experiencing heat stroke.

Underlying chronic conditions like respiratory illnesses and heart disease can also be exacerbated by extreme heat, Green added.

This summer is likely to see above-normal temperatures in Canada and potentially multiple heat domes in the U.S., raising the question of how to stay safe.

Moore said if your job requires you to be outside, make sure to take breaks to cool down and stay hydrated. When the temperature is high, limit your amount of time doing strenuous work and if you like to exercise, do so in cool areas indoors or find cooler times of day to do it.

Being indoors, though. is not a surefire solution, Green says, noting many of the deaths from the 2021 heat dome in B.C. were people indoors.

“The answer is not necessarily to go indoors if you don’t have air conditioning, so I think that really what we need to inform people of is they need to get to a cool space,” she said.

Drinking lots of water to stay hydrated is also recommended, and even taking cool showers or baths can help.

The most important thing, Green said, is that people should make plans for extremely hot days because if you’re in a heat wave or heat dome that has heat lasting multiple days and not cooling in the night, you need to know how you’re going to stay cool regardless of the conditions.

— with files from Reuters