The surprising science behind staying cool in a heat wave

This post first appeared on Canadian Geographic’s Compass blog

Emergency cooling centres are opening across Ontario and Quebec as parts of the provinces wilt under this week’s extreme heat.

Among the usual tips for staying cool are drinking cold water, using wet towels to shed heat and staying in the shade. But according to University of Ottawa professor Ollie Jay, whose research explores the body’s heat-loss mechanisms, not all advice is created equal — and drinking cold water might not be the best way to cool down.

“Obviously staying hydrated is important,” Jay says, “but it’s not going to necessarily save you.”

Jay explains that the body has a separate set of heat sensors in the stomach that, when cooled by cold water, actually signal to the body that it is cooler than it really is, inhibiting sweat production. Normally the body loses heat to the cooler outside environment by radiation, but at temperatures above 35 C, the only way the body can cool itself is by sweating — so throttling back this vital system on a hot day can be dangerous. The heat that is lost to the cool water is not enough to cause a net drop in body temperature, and the feeling of having cooled down might cause people to take fewer, more important precautions, such as seeking shade or limiting activity.

Sweating sheds heat by the endothermic (heat-requiring) process of evaporation. Water absorbs heat to turn into vapour, so when a person sweats, that heat comes from his or her skin, lowering the body temperature. If a person isn’t sweating, there is no way for heat to exit the body when it’s hot outside.

“We need to dissipate heat at the same rate that we’re producing it, otherwise we’re in what’s called a ‘heat imbalance,’” says Jay. “Our core temperature (normally about 37 C) starts going up and up and up. If we start reaching 39 C, 40 C, we start experiencing things like heat exhaustion, or eventually heatstroke if that core temperature reaches 41 C or 42 C.”

Heat exhaustion can cause nausea or dizziness and is a sign that the body is getting dangerously hot. Heatstroke can result in unconsciousness or even death. For elderly people, the most serious danger from heatstroke is that the heart goes into overdrive trying to shed heat by pumping blood to the skin’s surface, and this can expose frailties within the heart. For younger people, death by heatstroke could actually mean death by septic shock as the intestines become permeable like a sieve, leaking toxins into the body.

On a sunny day, temperatures in the sun can be 8 C to 10 C warmer than the reported temperature taken in the shade, and there will likely be plenty of places across Ontario and Quebec that will see temperatures well above the body’s natural core temperature of about 37 C. So find some shade, wear breathable clothing and stay hydrated — but maybe skip the ice-cold glass of water.

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