Patrick T. Fallon/AFP via Getty Images
Of all the extreme weather conditions, heat is the deadliest. It kills more people in the US in an average year than hurricanes, tornadoes, and floods combined. The human body has a built-in cooling mechanism: sweat. But that system can only do so much, especially in sky-high temperatures with high humidity.
Here’s a look at what happens to the human body in extreme temperatures, and the three main pathways to fatal consequences.
Organ failure caused by heat stroke
When ambient temperatures approach your internal body temperature, which is about 98.6 degrees Fahrenheit for most of us, your body begins to cool itself through evaporative cooling, better known as sweating. But when it’s very humid, that sweat won’t evaporate as well to cool you down.
When your body is exposed to heat, it will try to cool itself by redirecting more blood to the skin, says Ollie Jay, professor of heat and health at the University of Sydney, where he runs the Heat and Health Research Incubator. But that means less blood and less oxygen going to your gut. If these conditions go on long enough, your gut can become more leaky.
“Then the nasty things like endotoxins that usually reside and stay within the gut start to leak out of the gut, into the circulation. And that sets off a cascade of effects that ultimately results in death,” Jay says.
For example, those toxins can activate white blood cells, says Camilo Mora, a climatologist and professor at the University of Hawaii at Manoa, who has researched how heat can become fatal. “They say, my God, they are attacking us right now. And the white blood cells are going to attack this contamination in the blood, creating coagulation,” or blood clots, Mora says. Those clots can lead to multiple organ failure.
“And at that point, it’s pretty much irreversible,” Jay adds.
The second way people die in high temperatures also has to do with their body pumping more blood to the skin. Your heart has to pump faster, which can make you feel dizzy, to keep your blood pressure up.
“We could have a heart rate of 60 beats per minute, all of a sudden we could be asking the heart to contract 100 times a minute, 110 times a minute. So now you’re asking the heart to do a lot more work,” says Jay.
Those spikes in heart rate can be heart attack triggers, he says, especially for the elderly and those with underlying heart conditions.
Loss of fluids leading to kidney failure.
The third deadly danger has to do with the fluids your body loses in extreme heat. People can sweat as much as a liter and a half an hour, says Jay. And if you don’t replace those fluids, you become dehydrated and your blood volume drops, making it harder to maintain your blood pressure. That can strain your heart and kidneys.
“People with kidney disorders may be at higher risk for a negative health outcome during exposure to extreme heat,” says Jay.
Mora points to another kidney hazard faced by people doing physically demanding jobs in very hot conditions outdoors. Rhabdomyolysis causes muscle tissue to break down, releasing proteins into the blood that can clog the kidneys. This usually occurs in the acute phase of heat stroke. Jay says there’s also some evidence that routinely working outdoors in high temperatures without adequate hydration may increase the risk of chronic kidney disease.
What you can do to stay safe
Watch for the first signs of mild heat exhaustion:
- feel bad in general
If that happens, Jay says, get out of the heat and into the shade or inside ASAP. Drink plenty of water and wet your clothes and skin. Soaking your feet in cold water can also help.
Jay says the goal is to cool down so you don’t progress to severe heat exhaustion, where you could start vomiting or lose coordination, signs of neurological disorders.
If your core body temperature rises to about 104 degrees Fahrenheit, Jay says, that’s where you’re at risk for heat stroke.
How hot is too hot?
Experts say there is no absolute temperature at which extreme heat can become dangerous.
“It depends on the individual,” says Lewis Halsey, professor of environmental physiology at the University of Roehampton in the United Kingdom. “It depends on how acclimated they are to the heat. It depends on how long they are exposed to the heat. It depends on how they are experiencing this heat.”
If sweating is our superpower to keep us cool, then “the kryptonite to that superpower is moisture,” says Halsey.
So a person could start feeling overwhelmed much sooner with higher humidity and lower temperatures than if they were in a dry heat, he says. Direct sunlight will warm us up faster than when we are in the shade. A nice breeze could help evaporate sweat and cool us down.
The elderly and the very young are considered particularly vulnerable to heat. But Mora, of the University of Hawaii at Manoa, points out that heat stress can affect anyone.
It points to the story of a young family who died after dangerously overheating while hiking on a day in August 2021 when temperatures reached 109 degrees Fahrenheit in Northern California. The husband, wife, his one-year-old daughter and even the family dog were found dead two days later.
Mora says those kinds of conditions could kill you in a few hours, even if you’re young and healthy.
“The military has done a lot of research on heat exposure and finds the first symptoms of heat exhaustion, heat stroke, after just a few hours, even among the healthiest of people,” says Mora.