Of all extreme weather conditions, heat is the most deadly. It kills more people in the U.S. 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 soaring 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 heatstroke

When the surrounding temperatures approach your internal body temperature – which is about 98.6 degrees Fahrenheit for most of us – your body starts to cool off through evaporative cooling, better known as sweating. But when it's very humid out, that sweat won't evaporate as well and cool you down.

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

"So, nasty things like endotoxins that usually reside and stay inside the gut start leaking out of the gut, entering the circulation. And that sets off a cascade of effects that ultimately result in death," Jay says.

For example, those toxins can activate white blood cells, says Camilo Mora, a climate scientist and professor at the University of Hawaii at Manoa who has researched how heat can turn fatal. "They say, Oh my God, we're getting attacked 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 irreversible," Jay adds.

Cardiovascular collapse

The second way people die in high heat also has to do with your body pumping more blood to the 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 per minute, 110 times per minute. So now you're asking the heart to do a lot more work," Jay says.

Those spikes in the heart rate can be triggers for a heart attack, he 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 as much as a liter and half per hour, Jay says. And if you don't replenish those fluids, you get dehydrated and your blood volume shrinks, which makes it harder to maintain blood pressure. That can strain your heart and your kidneys.

"People with kidney disorders can be at greater risk of a negative health outcome during extreme heat exposure," Jay says.

Mora notes another danger to the kidneys that people who work physically demanding jobs in high heat outdoors face. Rhabdomyolysis causes muscle tissue to break down, releasing proteins into the blood that can clog kidneys. This usually occurs in the acute phase of heatstroke. Jay says there's also some evidence that habitually working outdoors in high heat without proper hydration can increase the risk of chronic kidney disease.

What you can do to stay safe

Watch for the first signs of mild heat exhaustion:

• headaches
• dizziness
• lethargy
• feeling unwell in general

If that happens, Jay says, get out of the heat and into the shade or indoors ASAP. Drink plenty of water and wet your clothes and skin. Immersing 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 vomiting or seem to lose coordination – signs of neurological disturbance.

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

How hot is too hot?

Experts say there's no absolute temperature at which extreme heat can turn dangerous.

"It depends on the individual," says Lewis Halsey, a professor of environmental physiology at the University of Roehampton in the U.K. "It depends on how acclimated they are to heat. It depends how long they're exposed to the heat for. It depends on how they're experiencing this heat."

If sweating is our superpower to keep cool, then "the kryptonite to that superpower is humidity," Halsey says.

So a person might start feeling overwhelmed much sooner in higher humidity at lower temperatures than if they're in dry heat, he says. Direct sunlight will heat us up faster than when we're in the shade. A nice breeze could help sweat evaporate and cool us off.

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

He points to the story of a young family who died after becoming dangerously overheated while hiking on a 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 those kinds of conditions could kill within a few hours — even if you are young and healthy.

"The military has done a lot of research into heat exposure and they find the first symptoms of heat exhaustion, heatstroke after only a few hours, even among the healthiest of people," Mora says.