A 20-minute run in 40 C. What happens to the body in extreme heat

Around the world, heat records keep being shattered. And it's proving deadly.

More than 100 people have died from heat-related causes in Mexico so far this year, along with dozens across multiple U.S. states. In Arizona — a desert state which has faced several weeks of temperatures above 40 C, breaking (and re-breaking) a five-decade record high — residents keep dying from heat stroke or facing life-threatening burns after falling on scalding pavement.

Canada isn't faring much better, with heat warnings throughout the Western provinces and several suspected heat-related deaths in B.C., where a "heat dome" killed more than 600 people in 2021.

So why is extreme heat so dangerous to the human body?

Scientists say it's because high temperatures have a negative impact on multiple organs, putting strain on the heart, muddying your memory, causing rapid dehydration and eventually leading to death if your body can't cool down fast enough.

And those impacts are felt from head to toe.

As temperatures rise, your brain can struggle to process information. That drop in cognitive function can impair judgment, and puts people at risk of falling or injuring themselves. In extreme cases, high heat can even cause dangerous brain inflammation.

At that point, someone's nervous system is "really not functioning well because the brain is getting so little blood," said Stephen Cheung, a senior research fellow at Brock University in St. Catharines, Ont., whose lab studies the impacts of environmental stressors — like extreme heat — on human physiology.

Heat also affects mental health, research suggests. Higher temperatures have been linked to higher suicide rates in the U.S. and Mexico, while a study in Bangladesh showed links between a variety of climate-related stressors and the burden of anxiety and depression.

Heat is "probably exacerbating people's existing mental health conditions," said researcher Amruta Nori-Sarma, an assistant professor in the Environmental Health Department at Boston University's School of Public Health.

One of her own studies, published by the Journal of the American Medical Association last year, showed days of extreme heat in the U.S. were associated with higher rates of mental health-related emergency department visits for conditions including anxiety, self-harm and substance abuse.

When the temperature spikes, your skin plays an important role in cooling the body — since humans need to maintain their core body temperature within a "pretty narrow range," said Larry Kenney, a professor of physiology and kinesiology at Penn State University. 

"The two main ways that we're able to control our rising body temperature is by pumping a lot of blood flow to the skin — which is a system unique to humans — and by sweating over most of the body surface area and evaporating that sweat," he said.

But that process can go awry when there's extreme heat, particularly when it's coupled with high humidity. Those factors in tandem disrupt the evaporation of sweat so it doesn't have any cooling impact on the body, while also making you more dehydrated. (Scientists call the concept the wet-bulb temperature, the point where water stops evaporating from a wet thermometer bulb. At 35 C, humans can no longer cool themselves.)