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It’s getting hot out there — and by “there” I mean, pretty much everywhere.

NEWS ANCHOR 1: Well, record heat is on the way for much of the nation. By the weekend, 19 states and Washington, D.C. will see record high temperatures.

NEWS ANCHOR 2: India and Pakistan still enduring a heat wave; an unseasonable March was India’s hottest ever.

NEWS ANCHOR 3: Millions are suffering in the Pacific Northwest as a stalled-out heat dome drives temperatures to historic — and dangerous — numbers.

Kristie Ebi is a professor of global health at the University of Washington. Her research focuses on the health risks of climate change. Last summer, she lived through the heat wave in the Pacific Northwest.

Kristie EBI: There’s all kinds of stories around here. An E.M.T. burnt his knees kneeling on a sidewalk trying to help somebody that had fainted. The emergency departments looking for ways to cool people down when they came in with extraordinarily high body temperatures. And one of the emergency departments almost ran out of ice. They weren’t ready for this event. Overall, we were really lucky that everything hung together as well as it did.

Heat waves have become much more common in major U.S. cities over the last few decades, from an average of two per year in the 1960s to six per year in the 2010s. Global temperatures are also rising steadily, at a pace of around 0.17 degrees Fahrenheit per decade since 1901. That might sound small, but in climate science, it’s a lot. 2016 was the warmest year on record, and 2020 was the second warmest.

Climate change is transforming our lives. How will the heat transform our bodies? Our health?

EBI: People have a variety of mechanisms to remove excess heat from the body. But as you sweat more, you can become dehydrated and the very high core body temperature starts affecting your kidneys, your heart, your other organs.

From the Freakonomics Radio Network, this is Freakonomics, M.D.

I’m Bapu Jena. I’m an economist and I’m also a medical doctor. Each episode, I dissect an interesting question at the sweet spot between health and economics.

Today on the show: As temperatures increase, so do some risks to our health. We’ll talk about a few of these concerns:

EBI: Mortality is quite obvious. People are injured and there’s growing research on mental health consequences of these events.

Anita MUKHERJEE: Heat has a unique relationship to violence, and not just general disobedience.

But also — why the worst-case scenario may not be as inevitable as it seems.

Nolan MILLER: I think we should be focusing our attention on how we’re going to live in this hotter world.

EBI: The moment I would say that I saw a big shift was in 2008.

Professor Kris Ebi has been studying the intersection of climate, weather, and health for 25 years. She remembers when things started to change.

EBI: That was the year that the World Health Organization named climate change and health as the issue they focused on that year. There was a meeting at the World Health Organization headquarters, which has an auditorium, but it’s not that large. And about twice as many people showed up as were invited. There were overflow rooms. And it was a moment within the health sector where you started to see a real shift in interest from the decision-makers.

Interest in climate-related health issues might be on the rise, but climate change is, too. The annual State of the Global Climate report released in May of 2022 confirms that the past seven years were the warmest on record, and that temperatures will continue to rise as long as carbon dioxide levels do. And they are: in April of this year, greenhouse gas concentrations hit their highest recorded level ever. There are a lot of reasons why this is really concerning, and one of them is our health.

EBI: Our core body temperature operates effectively within a pretty narrow range and that’s to keep ourselves and our organs protected. As our core body temperature starts to rise, either because we don’t have access to shade, a fan, air conditioning, or other reasons, then you start seeing this core body temperature come up, which starts damaging our organs People have a variety of mechanisms to remove excess heat from the body. Sweating, for example. But as you sweat more, you can become dehydrated and the dehydration, the very high core body temperature starts affecting your kidneys, your heart, your other organs. People die from heart attacks, who would not have died. This is why we call them “excess” deaths. It’s not somebody who was going to die within a day or two. This is a heart attack that wouldn’t have occurred without this event. And people would go on to live for months, years longer, without that particular event.

Conditions like heat stroke are also common, and really dangerous. As temperatures go up, the frequency of heat-related illnesses will go up, too. That’s not all we should expect, though.

EBI: There are a long list of climate-sensitive health outcomes. And I specifically say outcomes because one of the consequences of a changing climate are changing extremes. Being injured in a flooding event that was made more extreme by climate change is not a disease. And that’s why we call them health outcomes. Mortality is quite obvious. People are injured during these kinds of events. So, that is one big category. A second big category are infectious diseases. These are diseases carried by mosquitoes, ticks, in our water, or in our food. Many of the pathogens replicate at a rate that’s temperature sensitive. So, with warmer temperatures, the pathogens grow faster, greater opportunity for us to be exposed and to develop a disease. Under higher carbon dioxide, we’re seeing more pollen from, for example, grasses and ragweed. So, a world where there could be more allergies and asthma. And there’s also growing research on mental health consequences of these events and these mental health consequences can last for years.

Some of that research on mental health is being done by Anita Mukherjee.

MUKHERJEE: I’m an Assistant Professor of Risk and Insurance at the Wisconsin School of Business, University of Wisconsin-Madison.

Anita studies policies that affect vulnerable populations. A few years ago, she and her colleague Nick Sanders got interested in studying what heat does to our behavior. Does it make us more aggressive, quicker to turn violent? You’ve probably heard stories of crime going up in big cities on hot days, but turns out — it’s a tricky thing to measure.

MUKHERJEE: There’s been many different magnitudes documented, but they’re a little bit hard to pin down there can be mitigation or avoidance of heat in society. And so, for example, people might have air conditioning, they might have uneven access to it, they might choose to not go out when it’s hot. People may not report crime when it’s hot. There may be fewer witnesses on the street. Police may change patrolling behavior. There’s literature on all these various confounds.

What Anita is saying is that you can’t just look at crime rates on hot days and conclude that heat leads people to be more aggressive. Behavior changes in a lot of different ways on hot days. For example, what if on hot days, people just spend more time outside, more time interacting with others, more time at outdoor parties? And the number of crimes goes up just because more people are out and about, not because heat made any one person more aggressive than they would otherwise have been. To try and figure out whether heat leads people to behave more aggressively, Anita needed to find a more controlled environment, so, she went looking for one

MUKHERJEE: The prison environment is highly controlled and allows us to estimate the effect of heat on behavior, absent of all those confounds.

Remember, confounds, or confounders, are variables that can bias the effects that you’re trying to measure, making it harder, for example, to truly determine the relationship between, say, heat and violence. Anita and Nick tried to determine if heat causes aggression by focusing on prisons because in those settings, people don’t get to make a lot of choices on how to spend their days. So, we’d worry less about people making any of the sorts of decisions that could also lead to violent behavior.

MUKHERJEE: We started to look at this question using some data that I had by working with the Department of Corrections in the state of Mississippi. And so, that data has really detailed information on inmate behavior, and particularly location across the state. So, we have information, for example, on whether an inmate in a particular county is engaged in an act of violence on a particular day. We combined that rich prisoner information and incident information with information on temperature and rainfall and humidity to understand what the relationship is between heat and these incidents of violence.

The data they used allowed them to look at 36 different prisons across the state, all without air conditioning, over the course of seven years. They were able to control for things that may have impacted the heat level inside a facility, like if the layout of a particular prison made it feel cooler. Then, they tried to figure out how behavior might be related to the weather.

MUKHERJEE: We looked at all kinds of what are called “infractions.” These are the behavioral incidents in prison, which can include new crimes. So, we categorize them as violence, aggressive behavior, disobedience, refusal to work, riotous behavior. And we really only find effects on the violence. Now, it could be a reporting issue maybe on very hot days, guards just spend their energy only reporting the violent incidents and don’t bother recording, like, refusal to work. It suggests that heat has a unique relationship to violence, and not just general disobedience, which we weren’t fully expecting.

Their research found that on days with unsafe heat index levels — meaning the temperature was 80 degrees Fahrenheit or more — violent incidents were 20 percent more likely than on days where temperatures were more comfortable.

MUKHERJEE: So, the question is: What is the effect of heat potentially outside prison? Because our study is only really estimating the effects inside prison. The effects of heat on violence are likely to be higher in our study than for people not in prison for a couple of different reasons. One is that people in prison may already have a higher likelihood for violence than people outside prison.

So, the question then is how to extrapolate Anita and Nick’s findings outside of prison. On one hand, you might expect smaller effects of heat on violent behavior outside of prison because the general population is less violent. But remember, prison is also a controlled setting. So, the other ways in which heat could affect aggression — by encouraging some people, probably younger people, to go out — would likely play a larger role in the general population.

There’s also the interesting fact about the air conditioning. Outside of prison lots of people presumably would be able to avoid the heat by staying indoors and cranking up the AC, which could, in theory, lower the rate of crime on hot days. If you put all these factors together, though, Anita still thinks that there’s reason to be concerned.

MUKHERJEE: Look, a lot of the world does not have access to air conditioning. There’s intense global warming. We need to think quite seriously about infrastructure updating, and access to mitigation, and other ways to contain violence.

Taking the link between heat and aggression in prisons seriously could be good economics too. Anita and her co-author expanded their findings to the entire U.S. to look at how the cost of medical care from acts of violence in prisons would compare to simply making prisons more comfortable on hot days.

MUKHERJEE: We do combine information on prisoners and where they’re located with the weather data and find that, for example, in a given year that about 110 million prisoner days would be in this very hot kind of vulnerable temperature range. And we find that there would be an additional 4,000 acts of extreme violence each year from that additional heat. So, people can quantify the cost of heat in different ways, but 4,000 incidents of extreme violence is very high. So, certainly, I think if we were to think about the cost of air conditioning versus the cost of violence, one could make the argument that these investments are worthwhile.

After the break, we’ll look at just how worthwhile these kinds of investments might be, inside and mostly outside of prisons.

MILLER: We’ll face a lot of hot days in the future. It leads you to think a little bit differently about where you focus your attention

I’m Bapu Jena, and this is Freakonomics, M.D.

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When the heat dome parked itself over the Pacific Northwest in the summer of 2021, it strained local health infrastructure — the systems that are critical for keeping people alive.

EBI: When you have a significant heat wave, mortality starts within about 24 hours. It doesn’t start immediately. So, the first 24 hours didn’t look too bad. Emergency departments thought, “We’re doing pretty well.” And then we passed 24 hours. There was about a 69-fold increase in hospital related admissions, on top of COVID. And they were struggling to manage the numbers of people who are coming in. And they tried really hard. They really worked to do everything they could do to protect people. When you think about who needs to be at the table with a heat wave early warning system, it’s not just the health department and the meteorological services. You need E.M.T., you need the police, you need the fire department, you need the utilities, the electric utilities, who is responsible for elderly care facilities, who reaches out to the red line districts. And you start going through this really long list and getting those people around the table.

Preparation can make a difference. In 1995, the city of Chicago experienced a multi-day heat wave that resulted in more than 700 deaths. Just four years later, a nearly identical heatwave led to just over 100 deaths — an impressive drop, thanks to better planning on the part of city authorities. That kind of planning is something we’ll need to do more of in the coming decades, according to Dr. Nolan Miller, PhD.

MILLER: I’m not that kind of doctor, as I tell my family. I come from a family of M.D.’s, but I’m a Ph.D.

Nolan is a professor and economist at the University of Illinois at Urbana-Champaign. He may not be “that kind of doctor,” but his work focuses on the intersection of health and the environment. In 2021, he and two colleagues published a paper that looked at the ways society has adapted to changing weather.

MILLER: So, when we think about adaptation, we really think about two different kinds of adaptation. So, one is adaptation that we observe in the present to past conditions. So, places that have historically been hot have adapted to living in a hot environment. In places that have historically, historically been cold have adapted to living in a cold environment. And then we expect to see, moving forward, that as the climate continues to change through the rest of the century, that places will continue to change and adapt to the new conditions that they face in the future.

So, where are those kinds of adaptations happening? And how successfully are they reducing mortality, and improving other health outcomes? Nolan and his co-authors tried to answer these questions using data from Medicare.

MILLER: 97 percent of people over the age of 65 in the United States are on Medicare, which means we observe people who live you know, north to south, east to west, everywhere in the country. And that allows us to do analysis at a very fine level. The Medicare data contains virtually every interaction these people have with the healthcare system. So, we are using that data and then we’re using temperature and weather readings to look at the impact of short-term variation in temperature. So, what happens if you get a very hot day? What is the impact of that on things like mortality or hospital spending or hospitalizations?

Nolan and his colleagues were looking at the intersection of three factors: health, temperature, and location.

MILLER: Both hot and cold days are are harmful. They increase mortality. But then what we’re able to do because of the granularity of the Medicare data is to separately, break this down and estimate separate relationships by historical weather. And so, we can say, well, how does this relationship differ depending on whether you’re in a historically cold area or historically warm area.

So, in terms of health effects: is there a difference between a heat wave in a place that’s always had heat waves, and a heat wave in a place that’s having them for the first time?

MILLER: What we find is evidence of adaptation in the sense that places in the north of the country that have historically been cold, they tend to deal very well with cold temperatures and much less well with hot temperatures because they don’t tend to see these very hot days. Whereas places in the south of the country that have a history of dealing with hot temperatures, it’s the opposite. They don’t do very well with a cold day. If you get a cold day in Dallas, you know, it shuts the entire city down, but they’re very good at dealing with heat. And so, places have adapted to their historical temperatures. They have low mortality on the kinds of days they typically face and they have high mortality on days that are extreme for their region.

The climate is changing everywhere. Cooler places are getting warmer, and hotter places are too. That raises the question: As temperatures continue to rise, who will suffer the most — people who live in places that are already hot, like Florida, Texas, and Arizona, or people who live in places that are adapted to the cold?

MILLER: The conventional wisdom says that the effects of warming will be, felt most acutely in the hotter regions of the country in the south. And we show that when you take historical adaptation into account, actually the largest effects will be sort of in the middle of the country that historically are good at dealing with cold days and have not seen a lot of hot days. Instead of seeing the largest effects happen in the Southern part of the county, you actually see that the Southern part of the country that has historically been warm will tend to do better because they’ll get more hot days, but they do good on hot days. They do well on hot days. So, we don’t expect to see large mortality effects in the south. The places that actually will have the largest effects are the places that are kind of in the middle of the country. So, like South Dakota, Nebraska, Kansas, Iowa, Illinois, Missouri. These are places that will be seeing fewer very cold days in the future, but they’re adapted to dealing with cold. They’ll have more hot days in the future, and they’re not particularly good at dealing with hot days.

Adaptation might be as simple as getting air conditioners into more homes — or as complex as building the kind of inter-agency coordination that Kris Ebi mentioned earlier. But what Nolan’s findings suggest is that the worst-case predictions about increasing heat and our health can be mitigated — if we adapt, which some places have. And that’s important because the heat is here, and doesn’t show any signs of slowing down.

MILLER: We should be focusing our attention on how we’re going to live in this hotter world. It seems very unlikely that we’re going to summon the will to stop the progression of climate change that would involve basically stopping emitting carbon, stopping burning things. People are pretty good at dealing with heat. And so, we just need to make sure that we get those technologies out there and give people ways of coping with the changing climate, because I don’t think we’re going to stop it at this point as much as we might like to.

If that sounds like giving up, Nolan insists it’s the opposite.

MILLER: We need electricity. If you said, “Well, let’s live in a world without electricity, there would be a lot of people who died from that.” We’re trying to keep all the balls in the air at the same time and that involves burning stuff. It involves producing carbon. And we want to work on technological progress to reduce that as much as possible. But we also, I think should start focusing our attention on dealing with the consequences, because even now, a significant part of these consequences are baked in, literally.

Nolan’s findings illustrate a larger principle in economics that I think is important to understand. When we try to measure the harms or benefits of a treatment or policy, we can’t assume that people’s behavior will always stay the same. Let me give you an example: if someone’s at risk for heart disease because of high cholesterol, they may start taking a statin, or stop eating certain foods, to prevent this risk from increasing. The worst predictions about their health, based on their high cholesterol, may not happen, because they adapt, either by changing their behavior or taking advantage of a new technology — like a drug — that allows them to adapt. Accounting for people’s ability to adjust helps economists more accurately predict the health effects from heat, or from any threat to our wellbeing.

In its monthly climate trends report released at the end of May, the National Oceanic and Atmospheric Administration predicted above-normal temperatures for the large majority of the United States this coming summer. That means more heat-related illnesses, more cardiac events, and, if Anita Mukherjee’s research holds up, more violent behavior. But, as I just explained, Nolan Miller’s work finds that those predictions are not inevitable. So, what should we be doing? Here’s Kris Ebi again.

EBI: The temperature’s going to change no matter how we choose to approach it. And it is a choice. We can look at all the ways that we can take advantage of those temperatures while reducing our greenhouse gas emissions, protecting people during periods of high temperature, and find ways to capitalize on that, to develop more resilient societies.

That’s it for today’s show. I’d like to thank Anita Mukherjee, Nolan Miller, and Kris Ebi. And thanks to you, as always, for listening. Make sure to leave us a review wherever you get your podcasts, and let us know what you thought about today’s episode! Send us an email at

Coming up next week:

 SHMERLING: My grandmother — she would say that she could predict the weather, that if it was going to be colder or damper, her arthritis would be more severe.

We’re going to talk about an age-old question:

 RODMAN: This association, right — it’s many hundreds of years old. It’s probably thousands of years old. It’s based on this much older, cultural understanding of medicine that has died out in the West for 130, 140 years.

And how we know what we think we know

MOREWEDGE: A lot of the kinds of emotional responses that we have, both positive and negative, are heightened by uncertainty.

Freakonomics, M.D. is part of the Freakonomics Radio Network, which also includes Freakonomics Radio, No Stupid Questions, People I (Mostly) Admire and Off Leash. All our shows are produced by Stitcher and Renbud Radio. You can find us on Twitter and Instagram at @drbapupod. This episode was produced by Julie Kanfer and Mary Diduch, and mixed by Eleanor Osborne. We had help this week from Alina Kulman. Our staff also includes Neal Carruth, Gabriel Roth, Greg Rippin, Rebecca Lee Douglas, Zack Lapinski, Morgan Levey, Ryan Kelley, Jasmin Klinger, Emma Tyrrell, Lyric Bowditch, Jacob Clemente, and Stephen Dubner. Original music composed by Luis Guerra. If you like this show, or any other show in the Freakonomics Radio Network, please recommend it to your family and friends. That’s the best way to support the podcasts you love. As always, thanks for listening.

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MILLER: I think the middle of the country is going to be a nice place to live? We’ll get rid of the cold days a few more hot days. I think a lot of people will appreciate that. I, actually, am a person who likes the cold and so, maybe not for me, but that might be the answer for a lot of people.

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  • Kristie Ebi, professor of global health at the University of Washington.
  • Nolan Miller, professor of finance at the Gies College of Business at the University of Illinois Urbana-Champaign.
  • Anita Mukherjee, professor of risk and insurance at the Wisconsin School of Business at the University of Wisconsin–Madison.