Tanya Lewis: Hi, and welcome to Your Health, Quickly, a Scientific American podcast series!

Josh Fischman: On this show, we highlight the latest vital health news, discoveries that affect your body and your mind.  

Every episode, we dive into one topic. We discuss diseases, treatments and some controversies. 

Lewis: And we demystify the medical research in ways you can use to stay healthy. 

I’m Tanya Lewis.

Fischman: I’m Josh Fischman.

Lewis: We’re Scientific American’s senior health editors. 

Today we’re talking about the best way to beat the heat this summer. Your body has evolved a natural technique for cooling down rapidly, and it’s remarkably effective. We’ll discuss how to take full advantage of it.

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Fischman: It’s hot out. It’s sweltering. The sun beats down on your head. Breezes are distant memories. Welcome to summer!

Lewis: Hey, it’s not that bad! I prefer warm weather to the cold. I like doing more things outside. It’s easier to convince myself to go for runs and bike rides. And I love those long summer days when it stays light so late out.

Fischman: Okay, I like summer too. But the fact is heat can be dangerous. We’ve been getting more and more blistering summer heat waves. About 1,300 people in the U.S. die because of extreme heat every year. 

Lewis: Yeah, and that’s because high heat makes your body work extra hard to cool down. That can lead to heat exhaustion and heat stroke. Heat can be especially harmful for people with heart and respiratory diseases.

Fischman: Even when it’s just normally hot, say in the high 80s and 90s, it’s pretty easy to get uncomfortable. You sweat, you pant, and you just want to cool down fast. Everybody’s got their favorite tricks for doing that. After you go running, Tanya, what’s your go-to cool-down method?

Lewis: I like to splash water on my face and drink some cold water.

Fischman: Mine is to ditch my shoes and socks as fast as possible, and walk barefoot on a cool floor. And it turns out, according to physiologists who study temperature regulation, both our techniques are actually pretty effective strategies.

Lewis: Wow, the cold floor technique really helps?

Fischman: Yeah, I didn’t know this, but the soles of your feet and the palms of your hands are keys to fast cool-downs. Some athletes have even started using special cooling gloves to recover quickly after a hot workout.

Lewis: Hmm, your palms? That’s not very much surface area. It doesn’t seem like they would cool your whole body down, right?

Fischman: I agree. It’s a bit weird. So I turned to one of the scientists working in this area to explain it.

Craig Heller: I’m Craig Heller. I’m a professor of biology at Stanford. I study human temperature regulation and its role in performance. 

Fischman: Quick heads up: Craig talks about temperatures using the Celsius scale. To get to Fahrenheit, multiply his number by 1.8. Then add 32. 

Lewis: Or you can just remember that when he says 37 degrees, that’s 98.6 Fahrenheit. And 40 Celsius is 104 Fahrenheit.

Heller: Our body temperatures are regulated normally around 37 degrees. By the time we get to 40 degrees, we’re not functioning normally. We live very close to the edge. 

Lewis: This is because we’re mammals—we’re warm-blooded. We’ve evolved to be good at maintaining a warm body temperature. And most mammals have a nice blanket of insulating hair all over their bodies. Even people are covered with millions of hair follicles. The hairs are just a lot thinner and shorter than they are in other animals. 

Fischman: Which means that when it comes to losing heat, we generally suck. 

Our bodies do, however, have a kind of emergency temperature relief valve. Craig has been studying it. 

That valve is a special type of blood vessel. This week it’s my turn to get stuck with the hard science word, so here goes: they’re called arteriovenous anastomoses.

Lewis: Very nice.

Fischman: Why thank you. I practiced. A lot. But let’s call them AVAs from now on. 

Most arteries and veins connect through a bed of very thin capillaries that bring nutrients and oxygen to cells.

AVAs, though, are different. They are direct junctions of arteries and veins, so blood flows through them pretty quickly. 

And the real key to their heat relief function is they are concentrated in just a few places in the body. Here’s Craig again:

Heller: We found that in the palm of the hand, the soles of the feet, and the upper part of the face, which are called non hairy skin, there are special blood vessels, and those blood vessels can shunt the blood from the arteries to the veins directly, bypassing the capillaries.

You know mammals have fur. If you have fur you can't dissipate heat over your overall body surface very efficiently. So mammals have these special blood vessels in their non hairy skin, the pads of their feet, the tongue, the ears in some cases.

Fischman: To see if they could take advantage of AVAs in people, back in the early 2000s Craig and his colleague Dennis Grahn basically McGuyver’d this goofy device. 

They put a Plexiglass cylinder around someone’s hand and sealed it around their arm with part of a wetsuit sleeve. Inside the cylinder, cool water ran over their palm. 

After a person exercised, those AVAs pulled in hot blood from the core of the body. The blood gave off its heat to the cooler water, which was at about 56 degrees. Then, cooled down, the blood would circulate back to the body’s core and lower the heat there. People returned to regular body temperatures in just a few minutes.

Heller: We couldn’t believe it.

Fischman: This stuff gets published in places like the Journal of Applied Physiology. And since these guys are at Stanford, a university with a bunch of elite sports teams, it starts getting attention in the gym. Because athletes work out hard, get overheated and exhausted, and normally have to quit for the day, or several hours. But Craig and Grahn built a few more versions of this cooling mitten and handed them out. 

Athletes would put them on between workout sets, cool down in about three minutes and jump up and do another set. Craig tells a story of one guy who did 618 pullups in about twenty minutes. Some women athletes did 900 pushups in that short time period. 

Lewis: Wow, that’s about 899 more pushups than I can do. And he’s selling the gloves now, right? 

Fischman: Yeah, they’re called CoolMitts. Heller says some pro football players on the San Francisco 49ers also adopted the gloves. 

Tanya: I wouldn’t mind a pair of those on the New York subway in summer, just saying. But we should be clear that we’re not endorsing the product.

Fischman: No, we’re really not. It’s probably a fine device. But it hasn’t been exhaustively tested in a variety of people. And it costs about $1,500 bucks. But product aside, there is some cool science behind it. Literally.

Lewis: Ha-ha. So when it gets really hot, and I feel signs of heat stress—heavy sweating, clammy skin, muscle cramps, dizziness—what’s a good way to cool down if I’m not putting on one of those gloves? Should I dunk my body in an ice bath?

Fischman: Heller says that could work. The problem is it’s not very convenient. I don’t have a giant ice bath handy. Do you?

Lewis: No, but I did use to stand in an ice bath after high school cross-country practices. But seriously, could I just stick my feet in a bucket of ice water?

Fischman: Not so much. You’ve got AVAs in your feet, but remember the idea is to get more blood flowing through them. Icy water is a shock, and it makes blood vessels constrict. So you’re actually get less blood through your AVAs, not more. The water in Heller’s gloves, in the mid-50s, is cool but it’s not too cold.

Lewis: Ok, that makes sense. What about running my hands or forehead under a cold tap?

Fischman: That’s your postrun remedy today, right? Heller says that’s smart. The water is cool but not freezing, and you’re getting it onto the AVAs in your palms and your face.

Lewis: What about air conditioning? Does that help at all?

Fischman: AC is good. It’s not the fastest cool down, but it definitely helps. 

You can also drink cold water. That will bring your core temperature down pretty quickly. Be careful not to guzzle a huge amount, though. Too much water dilutes the fluids that carry signals amongst your cells, and that can lead to heart trouble and seizures, among other things. 

Lewis: Yeah, we talked about that on the last Your Health, Quickly episode. What about a towel soaked in cold water, draped over my neck?

Fischman: That’s actually a terrible idea, according to our body heat expert.

Lewis: Wait, really?

Fischman: The reason is the brain has a thermostat that it uses to trigger the body’s natural cooling mechanisms, like sweating or passing blood through those AVAs. That brain region is located near the back of the neck. It uses neck skin temperature, and blood temps in major vessels there, to measure how hot you are. 

So your cold towel is going to fool the brain’s thermostat into thinking that your body has cooled down. It’s going to shut down all your other natural cooling methods. And you’ll stay uncomfortably and sometimes dangerously hot.

Lewis: Wow, good to know! I guess I will stick with splashing cool water on my face and hands.

Fischman: That is the mammal-approved heat fix. And it should certainly help you chill out. 

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Fischman: Your Health, Quickly is produced by Tulika Bose, Jeff DelViscio and Kelso Harper. It’s edited by Elah Feder and Alexa Lim. Our music is composed by Dominic Smith.

Lewis: Our show is a part of Scientific American’s podcast, Science, Quickly. You can subscribe wherever you get your podcasts. If you like the show, give us a rating or review!

And if you have ideas for topics we should cover, send us an email at Yourhealthquickly@sciam.com. That’s your health quickly at S-C-I-A-M dot com.

Fischman: For a daily dose of science, sign up for our new Today in Science newsletter. Our colleague Andrea Gawrylewski delivers some of the most interesting and awe-inspiring science news and opinion to your inbox each afternoon. We think you’ll enjoy it. Check it out at sciam.com/newsletters.

Lewis: For Your Health, Quickly, I’m Tanya Lewis.

Fischman: I’m Josh Fischman.

Lewis: We’ll be back in two weeks. Thanks for listening!