It was 118 degrees after 5 p.m. in Phoenix, Arizona on July 18, 2023. More than 80 million people have heat warnings as unrelenting temperatures continue to bake across the western and southern US

PATRICK T. FALLON/AFP via Getty Images

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PATRICK T. FALLON/AFP via Getty Images

It was 118 degrees after 5 p.m. in Phoenix, Arizona on July 18, 2023. More than 80 million people have heat warnings as unrelenting temperatures continue to bake across the western and southern US

PATRICK T. FALLON/AFP via Getty Images

Of all extreme weather conditions, heat is the deadliest. It kills more people in the United States 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 high temperatures with high humidity.

Here’s a look at what happens to the human body in extreme temperatures and the three main pathways that lead to fatal consequences.

Organ failure caused by heat stroke

When surrounding temperatures approach core body temperature, which is around 98.6 degrees Fahrenheit for most of us, your body begins to cool itself through evaporative cooling, better known as perspiration. But when it’s very humid outside, even that sweat won’t evaporate and make you cold.

When your body is exposed to heat, it will try to cool itself by redirecting more blood to the skin, she says Ollie Jay, professor of heat and health at the University of Sydney, where he heads the Heat and Health Research Incubator. But that means less blood and less oxygen will get to your intestines. If these conditions go on long enough, your gut can become more leaky.

« Then, nasty stuff like endotoxins that usually reside and stay inside the gut start leaking out of the gut, going into the circulation. And that triggers a cascade of effects that ultimately lead to death, » says Jay.

For example, those toxins can activate white blood cells, she says Camillo Moraclimate scientist and professor at the University of Hawaii at Manoa who has wanted how heat can transform fairyL. « They say, Oh my God, we’re attacking each other right now. And the white blood cells are going to attack this contamination in the blood, creating clotting » — or blood clots, says Mora. Those clots can lead to multiple organ failure.

« And at that point, it’s pretty irreversible, » adds Jay.

Cardiovascular collapse

The second way people die from high heat also has to do with your body pumping more blood to your skin. Your heart has to pump faster, which can make you feel lightheaded, to keep your blood pressure up.

“We might have a heart rate of 60 beats per minute, all of a sudden, we might 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,” Jay says.

Those spikes in heart rate can be triggers for a heart attack, she says, especially for the elderly and those with underlying heart conditions.

Fluid loss leading to kidney failure

The third deadly danger has to do with the fluids your body is losing in extreme heat. People can sweat up to a gallon and a half an hour, says Jay. And if you don’t replenish those fluids, you become dehydrated and your blood volume shrinks, which makes it harder to maintain blood pressure. This can strain your heart and kidneys.

« People with kidney disorders may be at greater risk for an adverse health outcome during exposure to extreme heat, » says Jay.

Mora notes another kidney hazard faced by people who do physically demanding jobs in high temperatures outdoors. Rhabdomyolysis causes muscle tissue to break down, releasing proteins into the blood that can clog the kidneys. This usually occurs in acute phase of heatstroke. Jay says there’s also some evidence that routinely working outdoors in hot temperatures without adequate hydration can increase your risk of chronic kidney disease.

What you can do to stay safe

Look out for the first signs of mild heat exhaustion:

• heachache
• dizziness
• lethargy
• feeling bad in general

If that happens, Jay says, get out of the heat and into the shade or indoors as soon as possible. Drink lots of water and wet your clothes and skin. Dipping 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 might start throwing up or lose coordination — signs of neurological disorders.

If your core body temperature rises to around 104 degrees Fahrenheit, Jay says, that’s where you risk heatstroke.

How hot is too hot?

Experts say there is no absolute temperature at which extreme heat can become dangerous.

« It depends on the individual, » he says Lewis Halsey, a professor of environmental physiology at the University of Roehampton in the UK « 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 for staying cool, then « the kryptonite of that superpower is humidity, » says Halsey.

So a person may start to feel overwhelmed much sooner in high humidity in cooler temperatures than in dry heat, she says. Direct sunlight will warm us up faster than when we are in the shade. A pleasant breeze could favor the evaporation of sweat and refresh us.

The elderly and the very young are considered particularly vulnerable to the heat. But Mora of the University of Hawaii at Manoa notes that heat stress can affect anyone.

Cue the story of a young family who died after dangerously overheating while hiking one day in August 2021, when temperatures reached 109 degrees Fahrenheit in Northern California. The husband, wife, their one-year-old daughter and even the family dog ​​were found dead two days later.

Mora says these kinds of conditions could kill within hours, even if you’re young and healthy.

« The military has done a lot of research on heat exposure and they find early symptoms of heat exhaustion, heat stroke after only a few hours, even among the healthiest people, » says Mora.