An NPR investigation finds that heat kills thousands of Americans every year. But official records radically undercount the toll. In this series, we are bringing you stories that reveal why this happens and what can be done.
Heat is killing thousands more Americans every year than official data suggests, and the U.S. health system is struggling to document its often silent role.
Debbie Willson is acutely aware of how a death can fall through the cracks.
Debbie and her son, Adam Willson, lived in a sun-scorched neighborhood in Tucson, Arizona. They were both born with a genetic disorder called DiGeorge syndrome, which caused them heart problems. Adam also had cerebral palsy and some developmental delays, so Debbie cared for him full time, even after he became an adult.
Despite those challenges, Adam was thriving as he reached his late 20s, Debbie says. He was obsessed with movies, especially horror movies.
"Freddy, Jason, Michael, Chucky, all those horrors," she says, laughing: He loved them. "I tolerated those."
He worked at a local movie theater and showed her a horror script he'd written in his spare time. He was looking into film programs to apply to.
DiGeorge syndrome also made both Adam and Debbie highly sensitive to heat. Too much heat exposure could trigger seizures and strain their already stressed hearts. On really hot days, even stepping outside would be too much for the two of them, Debbie says, so they'd retreat back into their house and hope the swamp cooler could keep them comfortable.
But in southern Arizona, heat is inescapable. That's especially true as climate change drives temperatures higher.
Between the 1950s and 1970s, Tucson could expect less than one day over 110 degrees each year. By this decade, that average had shot up to nine days per year and has gone as high as 18 in a single year.
The city's annual number of extremely hot days — days whose highs are hotter than 95% of 1991-2020 temperatures — has grown by nearly an entire month since 1970.
June 2024 was no different: hot. Nearly every day that month topped 100 degrees, with 14 days at 105 or higher.
Partway through the month, the swamp cooler broke down. But money was, as always, tight, and Debbie couldn't afford to fix it. The house heated up and stayed hot. Debbie was anxious, especially when Adam started having trouble with his heart. She immediately booked an appointment with his doctor.
The morning of that appointment, Debbie went into Adam's room to wake him up. But his body was cold and limp.
Adam had died overnight. He was 27 years old.
How heat deaths go uncounted
Adam Willson's death is likely one of the thousands of heat-related deaths that go uncounted in the U.S. every year.
Federal records show that on average, about 1,700 people died from heat-related causes each year in the U.S. between 2018 and 2025. But a new investigation from NPR and Boston University finds that the number is likely about 9,000, or five times higher — and even that number may underestimate heat's deadly impact.
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There are a number of reasons those deaths don't show up in state or national death counts, says Quinn Adams, an epidemiologist at Boston University and the lead researcher on the NPR-BU investigation.
Heat is rarely the sole cause of death, often exacerbating existing health conditions. Because heat doesn't leave behind an obvious mark, its role in a person's death is often left off the death certificate, even when it's a key contributing factor.
"We know that during extreme heat events, a lot of people who die, die because they have preexisting conditions," Adams says.
"Heat is working to make these other conditions worse and make people die faster than they otherwise would have."
"You have to ask the question"
Debbie is sure heat played a role in Adam's death. She says this was also the belief of the emergency response team that came to her house that terrible morning. Adam had been sweating in the overheated house for days, stressing his heart.
But his death certificate says only "Unknown natural causes."
That's not unusual, says Brian Drummond, an emergency medicine physician in Tucson.
It can be challenging and time-consuming to figure out whether heat contributed to a death. Like in Adam's case, there are often multiple contributing factors. When there's no single obvious culprit, doctors may mark the cause as unknown.
In other cases, doctors may record only the most obvious cause of death — for example, a heart attack.
The question is: What caused that heart attack? Medical experts know that exposure to heat can stress the heart so badly that it can trigger a fatal event.
But doctors like Drummond, along with medical examiners and coroners certifying deaths, need conclusive evidence to make that link, he says. That evidence is often invisible, unless it is explicitly sought out.
"If I never ask a patient, 'Were you exposed to extreme heat?' or 'Do you have the ability to cool the place you live?', I would never diagnose them with heat-related illness, unless their temperature came in at 105," says Drummond.
Even for someone like Drummond, who is well-versed in the ways heat can influence medical problems and is knowledgeable enough to look for them, it can be tricky to ascertain exactly whether it played a role in a specific case.
Either way, he says, to more accurately track heat deaths, first "you have to ask the question."
"Without it, you'll never get to the real answer."
Investigating heat deaths
Pima County, Arizona, where Adam lived, has in recent years prioritized asking that question and identifying whether heat played a role in deaths.
It has trained death investigators in the medical examiner's office to ferret out the faint clues of heat's influence on a death.
Did a call to an air conditioner technician go unanswered? Was the deceased taking a medication that suppressed their ability to sense heat? Perhaps they had a health condition like diabetes or heart disease that made them vulnerable to dehydration or heart attack if their body became overtaxed.
These strategies have improved Pima County's ability to catch heat-related deaths. It's one place where official records largely matched the findings of the NPR-BU analysis.
Yet stories like Adam's suggest both may still undercount the problem.
In Pima County, and throughout the country, many deaths never go to the medical examiner's office. In Arizona, for instance, deaths are investigated automatically if they are suspicious or violent, if they involve diseases of concern to public health or if the person who died didn't have a doctor.
Adam's death, like many others, didn't fall into any of those categories.
Debbie could have called the Pima County medical examiner to request an autopsy, but she didn't see the point, she says: Adam was dead either way. Instead, she called the primary care doctor who had seen Adam for years. He certified Adam's death.
That straightforward process is standard practice in many parts of the country, especially when someone has a known medical condition and, like Adam, is under the care of a physician.
Changing such practices to more thoroughly investigate the role of heat is far from straightforward, says Drummond, the Tucson physician.
"People are interested in this, but it's hard," he says. "It takes time and money and commitment."
Without changes to such practices — for medical professionals, coroners, medical examiners and all others who collect death information — the true toll of heat will go drastically undercounted, says Adams, the Boston University epidemiologist.
That has consequences. When we don't know the scale of the problem, Adams says, we can't address it appropriately.
Debbie Willson wonders whether her son would still be alive if there had been services available to help people like her cope with the heat.
For Debbie, Adam's loss is still painfully fresh. Her son is gone: his brightness, his sharp humor, the shape he brought to the whole family — gone.
The horror script he'd written will sit on the shelf forever.
"He had his whole life ahead of him," Debbie says. "He had so much planned for himself."
Edited by Rachel Waldholz and Sadie Babits
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