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Too hot to handle: How to survive amid extreme heat and humidity


A few summers ago, Gordon Giesbrecht was doing work on a lakeside dock when he says he started feeling “a little funny.” Minutes later, he was vomiting — a symptom that he immediately recognized as heat exhaustion.

As an expert in how extreme temperatures affect human bodies, Giesbrecht is the first to admit he should have seen the warning signs.

“After I finished throwing up, I said, ‘That’s it for the dock’ … I went and got a lawn chair and stuck it in the shallows, and just sat in the lake … and cooled off,” said Giesbrecht, a professor of thermophysiology at the University of Manitoba.

Amid warnings of heatwaves across much of Canada this week, Giesbrecht’s experience is a reminder of how summer heat can affect our bodies without us realizing — and the need to watch out for potentially deadly symptoms.

In Europe, more than 1,100 people have died from the heat in Spain and Portugal in recent days. There are also fears for people elsewhere in Europe and in the United Kingdom — places where few have air-conditioning to cope with temperatures soaring well above 30 C.

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Adding humidity to that heat makes it harder for us to cool down by sweating — and humans can literally be cooked alive.

“Our body can actually survive a decrease in core temperature of 10, even 20 degrees, but our body can only survive an increase in core temperature of five [to] seven degrees — and then you can be in big, big trouble,” Giesbrecht said.

The hotter and more humid it is, the greater the risk of heat exhaustion. Symptoms include headache, heavy sweating, clammy skin, dizziness or confusion, cramps, rapid breathing, nausea and vomiting, among others, according to Health Canada.

It’s not (just) the heat, it’s the humidity

The point at which a combination of heat and humidity becomes especially dangerous, or even deadly, is explained by scientists as “wet-bulb temperature” — the lowest temperature at which an object can cool down due to evaporating moisture.

Imagine a thermometer wrapped in a wet cloth: the water will keep evaporating from the cloth up until a certain level of humidity, when the air contains too much moisture for evaporation to continue. Because of the evaporative effect, the temperature of the thermometer will be lower than the air around it — that is, until evaporation stops.

This chart shows how air temperature and humidity combine for a wet-bulb temperature, which explains the points at which it becomes difficult or impossible for water or sweat to evaporate. This causes the human body’s core temperature to rise. (CBC News)

Our bodies work the same way: at a wet-bulb temperature of 35 C (such as when it’s 40 C outside with 70 per cent humidity), sweat will no longer evaporate from our bodies, and scientists estimate that a human can only survive about six hours in those conditions.

“No matter how much water you have, it’s not going to help because you cannot maintain a survivable body temperature,” said Tapio Schneider, a climate scientist and a professor at the California Institute of Technology in Pasadena.

As your core temperature rises, your organs — including your heart, kidneys and brain — begin to suffer damage, and the protein in your cells breaks down. A person with heat stroke can suffer seizures, go into a coma, and die.

Lower wet-bulb temperatures can also be deadly, especially for the elderly, young children, and people with health issues. For example, during the European heatwave of 2003, which killed more than 20,000 people, wet-bulb temperatures remained below 28 C.

People cool off in the Dreisam river in Freiburg im Breisgau, Germany, on Tuesday. Few European households have air-conditioning, leaving their occupants sweltering in the current heatwave. (Arnd Wiegmann/Reuters)

Even at that level of wet-bulb reading, it can be dangerous to do anything that might further raise your body’s core temperature, such as exercising or the kind of manual labour Giesbrecht was doing when he suffered heat exhaustion.

“If you’re an outdoor worker, or working in agriculture or construction on days that are hot and humid … you are at risk of heat stroke, cardiac stress and the like,” Schneider said. “Heat can be deadly well before you reach this absolute limit of survivability.”

Keep cool and stay aware

With more hot and humid days ahead this summer — and many more to come as the planet grows hotter — experts say people should limit the time they spend outdoors in the heat, and use air-conditioning to keep living spaces cool.

People relax on a beach in Ottawa on Tuesday, as the temperature topped 31 C. Experts say people should not spend too much time in the sun, especially when it’s humid, to avoid heat illnesses. (Francis Ferland/CBC/Radio-Canada)

Fans only help if the air temperature is cooler than your 33 C skin temperature; if the air is warmer, the fan will only increase the temperature further, Giesbrecht said.

Staying hydrated will help you sweat, while misting your body or clothing with water also has a cooling effect on your skin as the water evaporates.

If someone is suffering heat stroke, call 9-1-1 and immediately move them to a cool place — ideally an air-conditioned building or vehicle. Symptoms include high body temperature, confusion and lack of co-ordination, dizziness or fainting, no sweating, and very hot, red skin.

Instead of removing a person’s shirt to cool them down, Giesbrecht says you should pour water on their clothing: remember the wet-bulb effect?

People rest in the shade on the bank of river Thames during a heatwave in London on Tuesday (Henry Nicholls/Reuters)

“The water in the shirt will evaporate and cool the skin off. If you don’t have a shirt on any part of their skin, that water’s just going to roll off onto the ground,” said Giesbrecht.

“I’ll take a litre [of water] and pour it on this person, and wait till it starts to dry out, and then I’ll put some more on them, so I can keep using that cooling effect of evaporation.”


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