Why high humidity makes heat waves harder to take
As Ottawa steamed on Wednesday, Nicholas Ravanelli cranked up the heat and humidity in a sealed room where a volunteer sat, and for good measure, Ravanelli pumped away some of the oxygen.
It’s all in the name of science. Ra- vanelli is a PhD student looking at how both heat and cold affect our bodies, and what we can do about it.
He has already published one surprise this year: Fans cool you down even in very hot weather. That’s contrary to a lot of advice from health authorities.
The belief has been that if you’re in a very hot room (mid-30s or so), then a fan just blows more heat at you. It’s like a convection oven blowing hot air at a turkey.
Ravanelli doesn’t buy that comparison. “The difference between a human and a turkey in the oven is that we sweat.”
His research in the Journal of the American Medical Association shows that the flow of air, even hot air, evaporates sweat from a person’s skin. And that has a major cooling effect.
In hot weather, scientists say, cooling down is all a matter of sweat and humidity.
When you sweat, the air usually makes those droplets evaporate. This evaporation cools your skin — unless the air is too humid, and can’t absorb moisture from your skin.
Just as laundry won’t dry in high humidity, your skin can’t dry off. That means you stay hot.
At the University of New Hampshire, climatologist Matthew Huber explains why this matters so much.
“About 80 per cent of heat loss (in warm weather) is through evaporation of sweat,” he wrote in an email.
“If you take a walk on a really hot day with a dog you’ll notice that they get tired very quickly and get very unhappy with the thought of doing anything. That’s because they can’t sweat and you can. Their bodies are telling them, ‘keep this up and you’ll cook.’ And they are right.
“We’re lucky because normally we can sweat away most of our heat. That’s why people can run marathons in Death Valley. On the other hand, if humidity is high, our ability to evaporatively cool is minimized.”
Back at the University of Ottawa’s School of Human Kinetics, Ravanelli is measuring heart rate, blood flow in the skin and core temperature (volunteers can choose a rectal thermometer or a down-the-throat kind). He also does blood tests. Heat can change the chemical balance in blood, and even make it thicker.
As the heat rises, eventually the body’s internal temperature starts to climb and the heart starts beating faster. This is the point where you’re getting into trouble.
Step one is heat exhaustion, your body’s warning flag.
“That point where you start to feel woozy and a bit sick and you don’t feel good,” he said. You may be pale and headachy. “You’re still sweating, though.”
The next stage is called heat stroke. The body is in shock, and it stops the heat-loss mechanisms such as sweating. It can start leaking toxins from the gut into the blood, causing sepsis ( blood infection.)
Ravanelli’s final tip: You can make artificial sweat. Pour water over your head. You get the cooling effect without sweating, which means you won’t dehydrate as much.
Thursday’s forecast high is 28 C. Environment Canada has extended its heat warning but says cool air will arrive later in the day.