Heat Can Take A Deadly Toll On Humans : Short Wave Heat—it's common in summer in much of the world, but it's getting increasingly more lethal as climate change causes more extreme heat. NPR climate correspondent Lauren Sommer talks with Short Wave's Regina G. Barber about how human bodies cope with extended extreme heat and how current information on how hot it feels need updating.

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Heat Can Take A Deadly Toll On Humans

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EMILY KWONG, BYLINE: You're listening to SHORT WAVE from NPR.


Hey, SHORT WAVErs. Regina Barber here with Lauren Sommer, one of NPR's climate correspondents. Hey, Lauren.

LAUREN SOMMER, BYLINE: Hey, Gina. So today we're starting with an experiment that's designed to make you sweat.

BARBER: Well, I am an already sweaty person and, actually, thinking about climate change doesn't help. But, you know, I'm going to do it. I'm ready.

SOMMER: OK. Well, then this is going to be, like, the perfect Venn diagram of that. It's at Penn State University inside this climate-controlled room. And if you're in the study, you go in and you either sit there or walk slowly on a treadmill.

BARBER: OK, that doesn't sound so bad. I could do that.

SOMMER: Yeah, well, here's what professor of physiology Larry Kenney does next.

W LARRY KENNEY: We start to increase the humidity every five minutes, in a stepwise fashion.

BARBER: OK, that sounds awful. As a West Coaster, I am very bad with humidity.

SOMMER: Yeah. I am too. And in that room, it's getting really muggy. And then the test subjects, they've all swallowed this tiny, electronic device that's shaped like a pill, and it records their core temperature.


SOMMER: And what Kenney is looking for is what he calls the critical environmental limit.

KENNEY: The combination of temperature and humidity beyond which either they can't sweat enough or they can't evaporate enough sweat to maintain their body temperature.

BARBER: I feel like I'm actually very familiar with that moment. You know, you feel really sticky and uncomfortable.

SOMMER: Yeah. Yeah, like the sweat is kind of pooling on your skin.

KENNEY: Only sweat that evaporates has any ability to cool the body. Sweat that just drips off the skin is essentially a senseless loss of body fluids.

BARBER: Right. Humid air is full of moisture already. So it's harder for your sweat to evaporate and then it's not doing much to cool you off.

SOMMER: Yeah, exactly. And that's where it gets dangerous. When your sweat doesn't really evaporate at all, that's when people die, you know, even just sitting in the shade, if they don't do something else to cool off. And that's why humidity is kind of sinister.

KENNEY: People need to understand that heat is the most deadly of all weather-related fatalities, much more so than tornadoes, hurricanes.

BARBER: Wow. That's surprising. Although actually, this summer I got really sick from the heat and humidity during my stay in D.C.

SOMMER: Yeah, it sneaks up on you. And that's why when you look at the weather forecast, the high temperature of the day, you know, isn't really telling you the whole story. The National Weather Service has an alert system. It's called the heat index, and it tries to fix that by showing you the full danger of heat. But new research is showing that it underestimates the threat.

BARBER: So today on the show, heat - why it's an invisible killer that only gets worse as the climate gets hotter and what today's heat warning systems could be doing better to protect the public. I'm Regina Barber, and you're listening to SHORT WAVE, the daily science podcast from NPR.


BARBER: OK, Lauren, so we've heard about how humidity can be really dangerous. It's really common in much of the U.S. and the world. But I feel like it's hard to know where that line is. Like, when does a heat wave go from uncomfortable to deadly?

SOMMER: Yeah, yeah. It's hard to know that, and that makes it also harder to communicate to the public. One example of that actually happened in Chicago in 1995. It was July, and a heat wave was coming. And people heard on the weather report that it was going to be over a hundred - so, you know, pretty hot but not unheard of for a Chicago summer. But the air was really humid, and that made it extremely dangerous.


UNIDENTIFIED REPORTER: The heat wave across the Midwest and up and down the East Coast continues to kill people.

SOMMER: That's NPR News in 1995.


UNIDENTIFIED REPORTER: In some cases, people lose a huge amount of fluids through sweating and don't replenish them.

SOMMER: More than a thousand people died in the broader region.


UNIDENTIFIED PERSON: So it's a mess. That's all I can say. It's a mess. It's worse than a plane crash.

SOMMER: Many were people of color and older people who died in their homes trying to ride out the heat.


UNIDENTIFIED PERSON: They bringing some more bodies in I could say, like, every 10 minutes, I guess.

BARBER: That's horrifying, actually.

SOMMER: Yeah. I mean, it was 20 years ago. But, you know, just last year, there was a major heat wave in the Pacific Northwest where around 800 people died.

BARBER: Yeah, I actually experienced that one. So if humidity is really crucial to pay attention to, what's the best way for me and for others to keep an eye on that?

SOMMER: Yeah, you don't have to try to understand the humidity forecast because there's a shortcut, and that's the heat index. It says what the temperature feels like to a human, you know, as opposed to what the thermometer says.

BARBER: OK, great. I already kind of look at the heat index on my phone when I scroll down on the weather report, but I don't think I've totally understood it.

SOMMER: Right. Yeah. And, like, you know, one example of it might be, like, it's 92 degrees out, but if the humidity is at 70%, it feels like 112 degrees. You know, it's kind of meant to be like this cheat sheet for what the real impact of humidity is on the human body. But here's the thing, it only shows what the heat feels like for someone standing in the shade.

BARBER: OK, that seems like a problem because most people aren't in the shade when they go outside. And there are people who may have to be in the sun, like outdoor workers.

SOMMER: Yeah, exactly. And the sun can add 15 degrees to the temperature. So at that point, you know, you could be talking about heat that's lethal.


SOMMER: But the heat index - only being in the shade leaves out those groups in the sense that they don't have a tool to help them figure out what the danger is. And then the heat index is also designed for a particular person. So it's for a person in the shade who is 5-foot-7 and a hundred and forty-seven pounds and healthy.

BARBER: Wow. OK. So that doesn't show how the heat would feel to somebody more vulnerable to it, like if you have a health condition.

SOMMER: Yeah, exactly. So older people are more vulnerable, you know, pregnant people, people with chronic health issues - they're all more susceptible to extreme heat. And the index doesn't really tell them what they need to know. And the last thing is, new research has come out that shows the heat index is miscalculating how hot the body feels at high temperatures and humidity. So it's underestimating the danger.

BARBER: So why is that?

SOMMER: Yeah. So it's done with modeling, which calculates how a body manages heat. And when it was created in 1979, the modeling couldn't really handle high temperatures and humidity. So what the National Weather Service did is it extrapolated and filled in the rest of those missing temperatures by using the lower temperatures as a template.

BARBER: Oh, no.

SOMMER: Yeah. So David Romps, who's a professor of earth and planetary science at UC Berkeley, he and his colleagues fixed the model to more accurately include humidity and higher temperatures. And when they ran it, they found that the heat index was off by as much as 28 degrees at high temps. So...


SOMMER: ...If you look at that Chicago heat wave I mentioned, it's one example he says.

DAVID ROMPS: The National Weather Service reported the heat index at the time as peaking at hundred and twenty-four degrees Fahrenheit, which is really hot. But if you go back and you look at the values that went into that calculation, the heat index actually hit a hundred and forty-one degrees Fahrenheit.

BARBER: Whoa, that's kind of mind blowing. So the heat felt so much worse and had a bigger health risk than the heat index had let on.

SOMMER: That's what his research is suggesting. You know, it works well at lower temperatures, but at higher temperatures, it's not showing you the full risk.

ROMPS: Using the correct heat index would allow us to identify those handful of times where the heat is so severe that it is pushing our bodies close to the breaking point.

BARBER: So what does the National Weather Service have to say about that?

SOMMER: So I spoke to Kimberly McMahon, who is a public weather services program manager at the National Weather Service, and she says they're reviewing the study.

KIMBERLY MCMAHON: Generally, heat is seen as a nuisance, and people still want to go about their day. So we are working with the CDC, EPA, as well as many other of our federal partners, to continue to try to find better and more widespread ways of alerting the general public, our emergency managers and our decision makers that heat is dangerous, and these are the things that people need to watch out for.

SOMMER: She says they're also piloting a new heat warning system in the West, which is called heat risk. And it has a tiered warning system, so, like, these different warning levels with information specifically for vulnerable groups.

BARBER: Oh, wow. OK.

SOMMER: California has also just passed a bill that requires the state to set up a ranking system for heat waves so the public can get a clearer warning.

BARBER: OK. So how do they plan to translate that into, like, public action? Like, there's a heat warning system now, but how will people know about it and will they do anything after they're alerted?

SOMMER: Yeah. Right, 'cause even if those warnings are absolutely perfect - right? - they still have to persuade people to act, to do something. You know, and that can mean don't go outdoors during the hottest part of the day or get to a cooling shelter if you don't have cooling at home, for example. And, like, if you're pregnant, don't book your doctor's appointment in the middle of the day. It's really hard to change those behaviors. And I spoke to Kristie Ebi, a professor at the Center for Health and the Global Environment at the University of Washington about that.

KRISTIE EBI: Not that many people take the actions because I don't see myself at risk. And this is where we certainly need better understanding of how to communicate to people who are in higher-risk groups but don't see themselves at higher risk, that, in fact, they do need to take action.

SOMMER: But she says the research is showing you really need to tap into social networks to reach those vulnerable people. So those networks and the people around them are reinforcing the message. And those can be, you know, neighborhood groups, senior centers, church groups or, you know, those who work with the unhoused. And that's where a lot of cities are trying to improve right now. Because, you know, heat waves are only getting worse. Climate change is making them longer and more intense. So people really need to know when something unprecedented is on the way.


BARBER: Lauren, thanks for sharing this reporting and also making me pay more attention to the heat index.

SOMMER: I'm glad and, yeah, the - happy to be here.


BARBER: This episode was produced by Rebecca Ramirez, and it was edited and fact-checked by our senior supervising editor Gisele Grayson. The audio engineer for this episode was Robert Rodriguez. Beth Donovan is our senior director of programming, and Anya Grundmann is our senior vice president of programming. I'm Regina Barber. Thanks for listening to SHORT WAVE, the daily science podcast from NPR. Stay cool out there.

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