Speaker 1: This summer has broken heat records in many places around the world. It might not be the hottest summer in recorded history. It might be the second-hottest summer, so congratulations. Not exactly good news. The experience of intense, sometimes deadly heat is a real part of our lives now, and it's here to stay. My colleague, Dhruv Khullar, writes for The New Yorker about medicine and public health. He's also a practicing physician at a New York hospital. And in 2023, Dhruv filed this story about extreme heat and what it's doing to us as we break one temperature record after another.

Speaker 2: Nine, Got it.

Speaker 3: We're going to go up in 20 seconds.

Dhruv Khullar: There are people in the world who set up labs to test how the human body reacts to all sorts of things. They can analyze your endocrine system when you produce the lactates, that'll slow you down.

Speaker 3: Nice work. Nice work.

Dhruv Khullar: There are people who agree to get their bodies tested, like me.

Speaker 3: Let's go, Dhruv. Let's go. Looking strong.

Dhruv Khullar: I'm Dhruv Khullar. I write for The New Yorker, and I'm a doctor. I went to a lab at the University of Connecticut to find out my body's VO2 max, the maximum amount of oxygen my body can absorb and use during exercise.

Speaker 2: We're watching two lines on the graph, and right now they're starting to plateau.

Dhruv Khullar: I'm on a treadmill hooked up to all sorts of sensors, and I'm running hard. Every few minutes, they increase the speed until I hit my absolute max.

Speaker 2: He's hitting his max.

Speaker 3: Four, three, two, one. Nice.

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Dhruv Khullar: The UConn lab is particularly interested in three athletes, soldiers, and laborers. Like construction workers. As a result, they're increasingly interested in how the body reacts to extreme heat. I'm going to be walking basically on this treadmill for the next two hours in 104-degree heat with 40% humidity. They're going to be tracking all sorts of different metrics. UConn's heat lab is an environmental chamber that can take you through from anywhere near freezing to 120 degrees. The longer you stay in there, the worse it gets. It feels basically like I'm in a sauna at this point. Just feel extremely puffy everywhere. You'd have to cut my finger off to get my wedding ring off right now.

Then it got even worse. I have a headache, and I can't even imagine lifting boxes, working outside for hours in this type of weather, although a lot of people-- the UConn lab is working to understand how these conditions impact people who have no choice about spending hours in extreme heat. The US military in basic training or farm workers at harvest time. I headed to another room to clean up. My producer inched towards the lab technicians. They were all huddled around screens flashing my biometric data.

Speaker 4: What was his starting Sean?

Sean: 71.24.

Speaker 4: He lost a little bit more than a kilo. A little bit more than 2.2 pounds in that 90 minutes of walking.

Sean: Of water.

Speaker 4: Yes, of sweat. Isn't that crazy? It's a lot. That's probably why he's not feeling so good.

Dhruv Khullar: I was surprised at how long it took for me to get clear-headed and feel hydrated.

Speaker 4: We just made sweat soup. We took him, placed him in here. We took two gallons of distilled water, and we went through every body part. We started with his hair, his face, back of the neck, his armpits, and we had him wipe everything as we were pouring the water over him. The idea is we wanted to get all of the electrolytes, the sodium, magnesium, the chloride, everything off of his skin and into the water ao we can go ahead and put that into our analyzer and we can get an idea of how much sodium, chloride, potassium, he sweat out during that hour and a half.

Speaker 2: Wow, like reverse Gatorade.

Speaker 4: Reverse Gatorade. Yes, but then the cool thing is, is he knows that value.

Dhruv Khullar: When I sat down with the head of the lab, we got deep into the mechanics of how heat acts on the body.

Douglas Casa: I'm Douglas Casa. I'm the CEO of the Korey Stringer Institute and a professor in the department of kinesiology here at the University of Connecticut.

Dhruv Khullar: Heat is on a lot of people's minds for obvious reasons. These past few years have been the hottest on record. We just lived through the hottest month in history, but you've actually been thinking and talking about heat for many decades now. Maybe you could just tell us how and why you first got interested in it.

Douglas Casa: Yes, absolutely. It's been almost exactly 38 years ago I suffered an exertional heat stroke as a 16-year-old high school athlete. I was running the 10k race, which is 25 laps on the track, and the Empire State Games, which is a summer Olympic-style sports festival in upstate New York. I was representing the Long Island division on the very final lap of that 25 laps. With 200 meters to go, I collapsed. I got back up immediately, ran 200 more meters, collapsed again 50 meters before the finish line, and then I was unconscious for the next few hours.

I had suffered an exertional heat stroke, and without the amazing care of the athletic trainer on site and the EMS EMT providers, and the emergency room physician and nurses, I would have died. I was very fortunate to have amazing care.

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Dhruv Khullar: Did you immediately start thinking that you wanted to make heat and heat stroke, a heat-related illness, part of your career?

Douglas Casa: It's really interesting is that night I talked to the doctor and I talked to the other people who were providing care for me, and started to understand how serious it was that really happened to me. Yes, I had actually, very soon after-- I'd always had an interest in medicine. Up to that point in time, I was thinking maybe medical school or something. When I was pursuing college, I was heading into my senior high school. After that, I knew 100% I was going to pursue sports medicine and research and try to figure out how the body responds to exercise in the heat.

Dhruv Khullar: What is happening in the body when people suffer? I guess heat stroke is the extreme version of this, but there's also a whole spectrum of heat-related illness, heat exhaustion, heat syncope. Why do people feel this way when they're exposed to extreme heat or significant heat for prolonged periods of time?

Douglas Casa: I think the best way to explain it is that when you do intense exercise in the heat, whether it be a soldier, laborer, or an athlete, the blood or the fluid in your body has to now be shared by three main entities. Your skin, so you can keep yourself cool, your heart, so you can maintain your stroke volume and your blood pressure, but also your muscles, so you can maintain that level of exercise and performance that you want. When you don't have the heat, that denominator is two items. You just have the muscle and the heart because you're not sending much blood flow out to the skin, because cooling is not a key factor.

Now the heat brings this massive new part of the equation into it. You have a finite pool of blood, which is the numerator. The denominator is now dividing by three instead of two. Now that becomes even more exacerbated when you get dehydrated because now that finite pool in the numerator is now a smaller number that's being divided by three entities. It's just ultimately this incredible physiological challenge that when you do intense exercise in the heat, when you're dehydrated, you can't meet the demands of those three entities.

It's simply impossible. Something gives, right? What's going to give? The performance will definitely give. You will heat up because you can't cool down as effectively as you want. Then our cardiovascular ability will falter.

Dhruv Khullar: Is that why people who are older or younger, young children, these people suffer heat-related illness?

Douglas Casa: Yes. I think it's important for people to understand there's two different kinds of heat strokes. There's classic heat stroke, and there's exertional heat stroke. Exertional heat strokes are related to what we've discussed so far. It's you're doing some kind of physical exertion and you're driving your body temperature up. I always say that exertional heat stroke is an overwhelming of the thermoregulatory system. You're simply generating more heat in a given period of time than you're able to dissipate or lose. Classic heat stroke is different.

Classic heat stroke, I say, is a failure of the thermoregulatory system. We see it happen to, for instance, infants left in parked cars during hot weather, where they just simply can't thermoregulate anymore, or old people who have comorbidities like heart, lung, other medical problems, who are in the higher stories of apartment buildings and they don't have air conditioning. During heat waves, the heat gets so oppressive, and they don't have the physiological ability to deal with that heat. We have to separate those two things out because there's different populations and there's different strategies you would use to try to prevent that condition from occurring.

Dhruv Khullar: This is an area of medicine where your intuition about what to do matches exactly what you're supposed to, it sounds like. If someone's too hot, throw them in cold water and bring the temperature down as fast as possible. There's not some fancy thing that you should be doing here.

Douglas Casa: It's interesting that you say that. It is that simple, but it seems to be so complicated to get people to do the right thing. A couple of the hurdles have been, for instance, the concept of cool first, transport second. You always want to cool a person on site and then transport them because you don't want to wait for an ambulance to come, have ambulance on site, go back to the hospital, and then wait for the hospital to do cooling. That could be 45 minutes to an hour. You have 30 minutes to get the patient's temperature to under 104 if you want to assure survivability with an exertional heat stroke.

The common start temp for heat stroke is around 108, but it can be anywhere from 105 to 113 is what we've seen and I've seen in my career. We have to get their temp down as fast as humanly possible, and cold water immersion has the best cooling rates. If you have a high school football practice, or you have a finish line of a major medical tent, or you have military bases where people are training, people have cold water immersion tub set up so that we cool them on site. That concept of cool first, transport second. We also had to overcome hurdles with people doing rectal temperature on-site.

For instance, at some high school settings, we have had non-medical people like superintendents of school districts, athletic directors, principals, back at delivering best care for their student athletes because they didn't want a rectal temperature done. You have to do a body temperature to one, assess the condition, but two, to have the objective data to implement cool first, transport second, because the ambulance will arrive, and you want to relinquish care when appropriate. You don't want to have a subjective response. You don't want to say, "Oh, they're looking better," but their temp might still be 108 degrees. You want to have it done objectively.

Dhruv Khullar: The rectal temperature is much more reliable than oral temperature, thermometer in the armpit. All these other types of temperatures that we're more used to.

Douglas Casa: We've done a lot of research that showed during intense exercise in the heat, aural, A-U-R-A-L, or oral, 0-R-A-L, tympanic, axillary, skin can be grossly inaccurate when people are doing intense exercise in the heat as much as 4, 5, 6, 7, 8 degrees away. There's no specific, consistent relationship that you can do some kind of correction factor. the most feasible field expedient method to get an accurate core body is a rectal temperature, and it can be done within 10 seconds.

Dhruv Khullar: How much of a difference does the humidity make when you're thinking about the danger posed by heat?

Douglas Casa: I would say humidity is huge. I think it's especially important for people who have to exercise in the heat because the main way that a person exercising would cool themselves is through evaporation. That's when that sweat droplet can evaporate into the air. For evaporation to occur, you need a water vapor pressure gradient, meaning it has to be more dry outside from where your sweat droplet is. It's only when the sweat droplet evaporates that we actually cool. That's why almost all the heat stroke deaths that have happened for athletes in America, laborers, and war fighters happens in the southeastern America.

Our recent data showed that the hottest state in America is actually Louisiana, but very close behind is Georgia, Mississippi, Alabama, and Florida. Those states are super oppressive. Classic heat stroke can really affect people anywhere because the heat waves, it's happening in apartments that are so oppressive that there's no ability to climate control. Once it gets hot, it gets so stifling. These people already are dealing with some other medical conditions, so their thermoregulation is really limited.

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Dhruv Khullar: If you can tell us, obviously, people have been talking about climate change for some time, but probably in the last decade or so, it really became part of our political and national discourse. What was it like in the late '80s or '90s, early 2000s, talking about heat-related illness when it wasn't on everyone's mind in the way it is now?

Douglas Casa: I'd say it was a little different. It was relevant within the populations that still had heat issues. If you were dealing with laborers and war fighters, soldiers in training, or athletes, these are three populations that heat stroke has always been one of the three leading causes of death. It's just that climate change has taken this into the everyday world for the everyday American citizen. You don't have to be a laborer working for 12 hours, you don't have to be a soldier in training trying to get through basic training, or a football player during August practices.

Now this is making it affect so many people, even just during daily living, daily workouts, daily things that they're trying to do, but now they have to face the extreme heat.

Dhruv Khullar: I saw a study recently that basically modeled what would happen to Phoenix if there was a heat wave and also a power outage. The number of people, I think it was nearly half the city, might require some level of medical care in that type of setting. Having air conditioning, really, it's a lifesaver. In a lot of places, like the Pacific Northwest, parts of Europe, they don't have a lot of air conditioning, and so I think that's why you often see these nearly mass casualty events in those settings.

Douglas Casa: Let's just look at France in 2003, they had 12,000 deaths in one week in one country. I just saw data that this past year, in 2022, Europe had 61,000 heat stroke deaths, which is just a crazy number for a place that most people would think that have some of the basic resources to protect people. You just mentioned in Phoenix, heat stroke deaths would be in the many thousands if they just had a few days in a row of a power outage, because of all the people who would not be able to cope with that extreme heat.

Every person who's having to live through climate change, they're going to have to make additional efforts to protect themselves. Whether it's when they're going out for a walk, a hike, working in their garden, mowing their lawn, doing a job that might have some heat exposure. The four biggest things you can do to protect yourself is having cold fluids nearby that you drink whenever you want. You don't need a specific break to drink. You're drinking when you need fluids. Second is having cooling strategies in place, so cold wet towels and a cooler nearby that you can replenish those cold wet towels, or even having shade. Anything that would be a cooling strategy that your microenvironment would be cooler.

Third would be implementing work-to-rest ratios based on the environmental conditions. If it's hotter out, more brutal, you have more rest breaks that are longer. Fourth is heat acclimatization, using that first week or two of heat exposure to phase in the intensity and duration of activity so that your body can develop the better physiological responses.

Speaker 1: Douglas Casa is a professor of kinesiology at the University of Connecticut and CEO of the Korey Stringer Institute. He spoke with The New Yorker's Dhruv Khullar. Dhruv is a physician and an associate professor of health policy and economics at Weill Cornell Medical College. At one point last month, 136 million Americans, more than 1⁄3 of the country, were living in what the National Weather Service defines as a major heat risk.

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