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Thousands of Americans die every year from heat, but the United States’ official tally of heat-related deaths does not come close to capturing the full human toll according to recent reporting from NPR. The Centers for Disease Control and Prevention tracks deaths, but it isn’t always easy to attribute a death to heat.
The result is a vast undercount of the number of people whose deaths are heat-related and experts say failing to recognize and address this may cost more people their lives.
NPR partnered with Boston University to find the total number of deaths influenced by heat every year in the United States. The investigation took two years and involved building a statistical model that assessed nearly every county in the country. The full analysis will soon be published in a scientific journal.
From 2004 to 2018, the national number of heat-related deaths compiled by the CDC was low: on average, around 700 a year. The number has crept up in recent years, now averaging around 1,700 but sometimes topping 2,000, as heat risks grow and counting strategies evolve.
But even that number is an underestimate, according to NPR and BU’s analysis. On average between 2018 and 2025, NPR estimates that some 9,000 deaths each year in the country are impacted by heat — more than five times higher than the official average over that same period.
Some years, heat-related deaths can top 10,000.
The West Virginia data from the study indicated a 13-fold undercount. The CDC shows 4.1 deaths per year on average, but the modeling indicated it should be nearly 53.
The heat wave season has extended by a month and a half since the 1960s. In 50 of the country’s biggest cities, the number of heat waves has doubled since the 1980s. Heat domes that settle over the country regularly expose tens of millions of Americans — sometimes well over 100 million at a time — to dangerous heat.
News Director Eric Douglas sat down with Brian Leary, assistant professor and director of tactical performance physiology with the WVU School of Medicine Division of Exercise Physiology to discuss what heat does to the body and what it means in West Virginia.
This interview has been lightly edited for clarity.
Douglas: How does the heat index scale work?
Leary: The heat index, basically it considers two big things. It takes into the actual temperature that we see, and then we take into account humidity. Wind chill focuses on wind because we know as air moves over us, just like a fan on a hot day cools us, on a cold day, that makes us even colder.
In the heat, the worst thing that starts to happen is when it starts to get humid. When we sweat, that’s not just a useless response to us. It’s inconvenient, but it’s wildly important in the sense that the sweating is what actually cools us down and the evaporation of that sweat as it’s coming off the body. And a couple of things impact that ability to evaporate. And the biggest thing that we have is really humidity.
When that humidity goes up, the water has nowhere to evaporate to. On a dry day, say if you’re in Las Vegas and it’s 20% humidity, you can walk around there and it’s 100 degrees outside and you’ll look down and your skin’s very dry. You’re still sweating, it’s just evaporating so quickly you don’t notice it.
Douglas: Let’s talk about heat stress on the body. What does it do to the body as heat indexes start climbing? You’re exercising, you’re working out in the heat, or even somebody who’s older, doesn’t have air conditioning.
Leary: It’s not a competition in the fact that bodily systems are really fighting against each other. But when we start exercising, blood has to go to working muscles. You could be a construction worker out working, you’re still using oxygen, you still need blood to flow around. All of those muscles need blood to carry oxygen to them so they can do what they need to do.
What also happens is we get a lot of blood going to our skin surface. On a hot day, you look down and your face is really red when you come in. The blood vessels dilate. So they open up and you get a lot more blood to the surface.
When that evaporation takes place, that cooling can actually happen at the skin surface and we get cold blood that goes back to the body. But what will happen is we’ve now got a lot of blood going to the muscles, a lot of blood going out to the skin, and particularly in the heat, a lot more going to the skin on a cool day.
There’s two primary organs that also need a lot of blood flow, and that’s the heart and the brain. And they tend to get reduced blood flow on those really, really hot days, particularly in our vulnerable populations. So our elderly individuals, if they’ve already had maybe an underlying risk for cardiovascular disease or cardiovascular disease in general, we’re now putting additional strain on that heart to do extra work, which can set them up for failure from there. And that’s usually when they start to get heat related illnesses.
And then our heat stroke, especially during exercise, we start to get neurological issues because we’re getting less blood going to the brain and the blood that is going to the brain happens to be very, very hot, which is not what our brain wants.
Older adults, they usually suffer what we classically, like if you’ve seen the news from like Spain and Europe with the deaths this summer, they typically get what we call as like a classic passive heat stress. So you’re looking at Europe, places with no AC, and it’s the cumulative effect of a couple of days of heat stress. We know that if you’re exposed to a hot day one day and you go and do the same thing the next day, your core temperature is going to be higher and it feels hotter.
The same thing happens in a house. They don’t get the classical symptoms of a lot of sweating, especially in our older adults. They don’t really know they’re getting too hot. Next thing they know, they’re sitting there passively, their heart’s getting stressed, their brain’s getting stressed because they’re trying to cool themselves off.
They still get the same symptoms as far as neurological issues and heart issues. They just don’t have the big sweating response, mostly because of their age. And they’re not really exercising. They’re just putting a big cumulative load on them.
Douglas: You alluded to it, but part of the problem being with this heat dome that’s sitting on top of us right now is it’s not cooling down at night. Your air conditioner never goes off. Your house never gets cooler when the overnight low is 72 degrees.
Leary: Yeah, you’re not cooling off. And that’s a big one. The overnight lows are not going down. When we sleep, our core temperatures should drop a little bit. That’s just our body shutting down for the night, letting us sleep. When it’s a hot day and it’s hot at night, the core never really drops. So you’re starting your day at an already elevated core temperature and then add in more load on top of that. And then after consecutive days of that, your body’s just slowly overheating itself without you really recognizing what’s going on. And we can only handle very small increases in core temperature. On the cold days, like we’re used to, we’re okay getting cold, we’re just not great at getting too hot.
Douglas: Within what, 5 degrees or so, that’s brain damage, right?
Leary: That’s brain damage.
Douglas: What are your thoughts on the likelihood that we see more heat related illness and death than is tracked typically.
Leary: I think it’s likely that we get more heat-related illnesses than what we think. Primarily in West Virginia, there’s a couple things working against us.
One is, if we look at some of the communities, particularly the rural communities with that, not everyone is going to have whole house HVAC systems. They’re probably going to be using window AC units that might only control one room at a time if they can have that. The utility bills have gone up dramatically. Not only that, the cost of living. So people are making decisions on whether or not they’re using that. And then if we look at the overall health of the state, and we look at things like the percentage of individuals with diabetes, cardiovascular disease, obesity, all of those are contributing risk factors to a heat-related illness.
You’ve already got a vulnerable population that doesn’t have potentially the resources to cool themselves off, or maybe the elderly communities and some of these rural communities, even the ability to get to a place that’s cooler for the day. And they have the underlying risk factors.
An individual that has a heart attack at home, how is that getting attributed to whether or not that was a heat-related heart attack versus a heart attack because they already had underlying cardiovascular disease. And that’s a very hard thing to differentiate out.
Douglas: Is there anything we haven’t talked about?
Leary: Medications have a big interaction with all of this as well. It’s not just the individuals with heart disease; they might be on a blood pressure medication. Part of that might be a diuretic. A diuretic is going to cause you to have a fluid loss, which means you have less blood flow going around to release some of that pressure.
But less blood flow means we’ve got less blood flow going to the skin to cool ourselves off and we have less fluids that we can sweat off to cool ourselves down. There’s a lot of factors when we start looking at the underlying things and the burden locally here that are potentially contributing to a lot of numbers that we’re just not capturing.
I think we do a really good job with those modifications of saying signs and symptoms of what happens if you start taking a medication, but not how that’s going to interact with your day-to-day life and your environment as you’re walking around. I think we just assume that you’re taking it and you’re fine, not you’re taking it, but it could also cause an added risk on the back end.
You’re not automatically healthy now that you’re taking the medication. The medication can also cause additional risk that if you’re not aware of, can cause some issues on that back end.