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Danger signs to look out for: What happens to the body in extreme heat

ADRIAN FLORIDO, HOST:

The extreme summer heat that's impacting millions of people will likely continue through August. And of all extreme weather conditions, heat is the most deadly. In an average year, it kills more people in the U.S. than hurricanes, tornadoes and floods combined. Joining us now to explain what happens to the body when temperatures rise is NPR's Maria Godoy. Hi, Maria.

MARIA GODOY, BYLINE: Hi, Adrian.

FLORIDO: Can you tell us at what point does heat turn lethal?

GODOY: Yeah, when it overwhelms your body's ability to cool down - so if the surrounding temperature is around your body temperature, which is about 98.6 degrees Fahrenheit. For most of us, your body sweats to cool itself. But if it's really, really hot and really, really humid, that sweat is not going to evaporate as easily. And that makes it harder to cool down. And if you can't get your internal core temperature down and it rises to around 104 degrees or so, that's where you can get into trouble.

FLORIDO: What kind of trouble?

GODOY: Well, for one thing, heatstroke. So when exposed to heat, your body pumps more blood to the skin to try to cool off, which means less blood and less oxygen is going to the organs and the gut. I spoke with Ollie Jay. He's a professor of heat and health at the University of Sydney, where it gets really hot. And he says when it's deprived of oxygen for too long, the lining of your gut can start to break down.

OLLIE JAY: Nasty things like endotoxins that usually reside and stay inside the gut start leaking out of the gut, crossing the endothelial barrier and entering the circulation. And that sets off a cascade of effects that can ultimately result in death.

GODOY: And that's just one way heat can kill.

FLORIDO: I mean, that alone sounds awful. But what are some of the other risks of extreme heat?

GODOY: Well, cardiovascular collapse is another big risk. Because your body is sending more blood to the skin to try to cool off, your heart has to pump faster to keep your blood pressure up.

JAY: We might have a heart rate of 60 beats per minute. All of a sudden, we might be asking our heart to contract a hundred times per minute or 110 times per minute. So now you're asking a heart to do a lot more work. And if there's an underlying infirmity because of cardiovascular disease, this increases the risk of a catastrophic cardiovascular event.

GODOY: And then, of course, there's dehydration. When you get dehydrated, it puts even more pressure on your heart, and it really strains your kidneys.

FLORIDO: So, Maria, is there a specific temperature that can lead to these kinds of really dangerous outcomes?

GODOY: No. You know, there's not one set number. It depends on how humid it is. Are you in direct sunlight or shade? Is there a breeze? Camilo Mora is a professor at the University of Hawaii at Manoa. He points to the story of a California family who died while out on a hike on a very hot day.

CAMILO MORA: It was the husband, the wife, the kid and the dog. And they went out for a walk, and they got caught up in this heat wave during this walk. And the four of them were found dead, including the dog.

GODOY: And...

FLORIDO: Gosh.

GODOY: The temperature that day got to around 109 degrees. Mora says those kinds of conditions could kill within a few hours, even if you are young and healthy. He says studies have found that young, fit soldiers can start showing symptoms of heat exhaustion within just a couple of hours. And things can turn potentially lethal even faster if you're more vulnerable.

MORA: If you are an old person or a very young person, we're talking only a few hours at most of exposure, and you are going to be already on critical conditions.

FLORIDO: So, Maria, what should you do if you start to feel like you're being overwhelmed by heat?

GODOY: Yeah. It's critical to recognize the signs and symptoms of heat exhaustion early. So if you feel lethargic, you have a headache, drink plenty of water, and try to cool down ASAP. Wet your clothes. Immerse your feet in cold water if you can. Get out of the heat. If you vomit or show lack of coordination, that's more serious. And if you don't cool down, you could progress to heatstroke. And, of course, pay attention to those heat health advisories.

FLORIDO: I've been speaking with NPR's Maria Godoy. Thanks very much, Maria.

GODOY: My pleasure.

(SOUNDBITE OF NIPSEY HUSSLE SONG, "OCEAN VIEWS") Transcript provided by NPR, Copyright NPR.

NPR transcripts are created on a rush deadline by an NPR contractor. This text may not be in its final form and may be updated or revised in the future. Accuracy and availability may vary. The authoritative record of NPR’s programming is the audio record.

Maria Godoy is a senior science and health editor and correspondent with NPR News. Her reporting can be heard across NPR's news shows and podcasts. She is also one of the hosts of NPR's Life Kit.