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.

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.

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.