An NPR investigation found that people in the U.S. are dying from heat much more often than official counts show. We explore the reasons why — and how lives can be saved.
Thousands of Americans die every year from a chronically underreported cause: heat.
As summer heat intensifies because of human-driven climate change, the United States' official tally of heat-related deaths does not come close to capturing the full human toll. The data feeding into nationwide death counts is collected inconsistently and rarely thoroughly.
The result is a vast undercount of the number of people who die because of heat. And experts say failing to recognize and address this may cost more people their lives.
So NPR set out to create a more comprehensive national count.
We 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.
An accurate assessment of the true scope — as well as understanding who is affected, where and why — is critical for developing lifesaving solutions, says Quinn Adams, a research scientist at Boston University's Center for Climate and Health and the lead partner in the joint NPR-BU investigation.
Beyond the statistical assessment, NPR set out to find people whose deaths slipped through the cracks — those who might not have died if heat hadn't pushed them beyond their breaking point — to understand exactly how heat could be missed as a factor. We will be bringing you their stories in the coming days.
But first, here are three key takeaways from our research — and why it matters.
How big is the undercount?
The scale of the undercount is significant.
The primary keeper of cause-of-death records in the U.S. is the Centers for Disease Control and Prevention (CDC). It keeps an annual tally of heat-related deaths in the U.S., which is compiled from state reports based on whether heat is noted on individual death records.
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, 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, we estimate that some 9,000 deaths each year in the country are impacted by heat — a factor of five higher than the official average over that same period.
Some years, heat-related deaths can top 10,000.
The true figure could be even higher, says Adams, because of some limitations in the data and the conservative nature of our analysis.
Official records may vastly undercount heat-related deaths
The Centers for Disease Control and Prevention (CDC) compiles death information from around the United States. Between 2018 and 2025, it reports an average of 1,700 deaths related to heat each year. NPR’s modeled estimate shows the number may be more than five times that, or around 9,000 people on average each year.
The dangers from heat are growing every year because of human-caused climate change. 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.
"The frequency and the duration of these heat events has been expanding very rapidly," says Adams. "That's what we can expect moving forward into the next few decades: having more heat waves, and having them last longer, and having them be more intense such that people don't get the escape from the heat that they need to physiologically reset."
Maximum temperatures across the U.S. have risen
So far, 2026 has recorded the hottest 12-month average of daily high temperatures across the contiguous United States for the past 60 years. The last five years have all been among the highest over that span. The number of heat waves per year has also doubled since the 1980s, and the heat wave season has lengthened by a full month since that time.
The reasons for the undercount are complex. But a crucial issue, says doctor and researcher Sameed Khatana of the University of Pennsylvania's Perelman School of Medicine, is that heat's role in a death is often masked by other more obvious causes of death, like a heart attack.
"Heat is what people often call the silent killer, compared to things like hurricanes and typhoons," says Khatana.
Heat, he says, can often be the trigger that pushes someone's health condition — like cardiovascular disease — from manageable to fatal. "The heart starts beating faster and harder," he explains, to shunt blood to the skin to cool the body. And eventually, if someone's heart is already weak, it may not be able "to keep up with the demand that was required of it," he says. That can lead to something like a heart attack.
But often, Khatana says, because heat leaves no obvious mark behind for doctors or coroners to examine, the cause of death gets marked down only as a heart attack. Heat isn't noted on the death certificate, and therefore it is not included in the tallies reported to the municipality, the state and eventually the federal recordkeepers.
Such omissions are understandable, Khatana says. But repeated thousands of times, in all parts of the country, they create a blind spot that obscures the scale of heat's danger.
National risks — even in unexpected places
The NPR and BU investigation found that heat is killing people in nearly every corner of the country.
Our analysis finds that Florida and Texas have the most heat-related deaths overall. That is in part due to their large populations. But those states remain among the most impacted even when adjusted for their size.
But the losses are not limited to states historically considered hot.
