How extreme heat impacts the human body and why it eventually kills you

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 dehydrationand eventually leading to death if your body can't cool down fast enough.

And those impacts are felt from head to toe.

The brain

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 affectsmental 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 conditionsincluding anxiety, self-harmand substance abuse.

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The skin

When the temperature spikes, your skin plays an importantrole 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 thewet-bulb temperature, the point where water stops evaporating from a wet thermometer bulb. At 35 C, humans can no longer cool themselves.)

Seniors and infants can be at particular risk if their body temperature creeps dangerously high, though Kenney stressed anyone can be affected. "In warm, humid conditions, with humans just going about our activities of daily living, even young healthy men and women would start to see continuous increases in body temperature," he said.

That can lead to the most serious heat-related illness: heat stroke. When the body's ability to sweat fails, the internal temperature keeps rising and can hit more than 40 C in just 10 to 15 minutes which can lead to permanent disability or death.

The heart

As body temperature rises, it begins putting strain on the entire cardiovascular system.

That's because more blood is trapped out in the skin as your body tries to cool itself, said Cheung. "What happens is that it places greater strain on your heart because now it has less blood coming back, and it has to work harder to pump that blood out to the rest of the body."

When the heart becomes deficient in oxygen and nutrients, it can lead to potentially fatal conditions like a heart attack, says Daniel Gagnon, a researcher at the Montreal Heart Institute.

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"There are a lot of studies that show an association between heat extremes, or extremely hot temperatures, and cardiovascular outcomes," Gagnon said.

One new study out of China, published in the journal Circulation, suggestedthe particular combination of extreme heat and air pollution hiked the risk of having a deadly heart attack, particularly among older individuals, by as much as nearly 75 per cent during a four-day heat wave.

The lungs

Breathing in hot, humid air can be tough on the lungs, flaring up existing conditions and putting people at a higher risk of other respiratory issues.

"Extremes of heat are particularly hazardous to those with respiratory disease," reads a 2018 editorial in The Lancet medical journal. "Exacerbations of chronic conditions such as chronic obstructive pulmonary disease (COPD) and asthma increase drastically in line with high temperatures, often aggravated by increased air pollution in cities, as well as seasonal allergies."

One observational study in the U.S. looked at the impact on seniors, and found each 5.6 C increase was associated with a roughly four per cent increase in same-day emergency hospitalizations for respiratory diseases.

The kidneys

As your body gets dehydrated,it takes a major toll on your kidneys the organs responsible for filtering your blood by removing wastes and extra water that are eventually flushed out as urine.

"Dehydration can cause things like kidney failure or electrolyte imbalance which can also lead to hospital admissions or mortality," said Gagnon.

"It's a triad of increased stress on the heart, increased body temperature, and fluid strain or dehydration that can cause increased stress on the kidneys."

One meta-analysis of previous studies, published in The Journal of Preventive Medicine and Public Health, found a 30 per cent increase in kidney disease morbidity in high temperatures. Researchers also noticed spikes in certain conditions, including kidney stones.

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Protecting yourself

It all paints a grim picture throughout the entire human body: Extreme heat can damage multiple organs, spike your internal temperature to dangerous levels, and ultimately kill you unless you're rapidly able to reverse that process.

Cheung says if you start to feel clammy, disorientedand overly thirsty and if your sweat isn't actually cooling your body down you may be entering the danger zone.

"Get indoors, into the air conditioning if you can, and if things get really bad, the best way to cool down is to get into cold water," he said.

That means plunging yourself into an icy bath, or standing under cold water in the shower. If those aren't options, he said soaking towels and wrapping them around your body can also help.

"The best thing you can do is to cool down as best as you can," he said.