Heat kills thousands in the U.S. every year. Why are the deaths so hard to track?

After a string of scorching days in June 2023, the body of an 88-year-old man was discovered in his home in Maricopa County, Arizona. His air-conditioner, set to 70, was blowing hot air. The temperature inside was nearly 110 degrees. Maybe he had heart problems. Maybe a different organ broke down. Maybe he was taking medications that did not work as they should. Did extreme heat cause or contribute to his death?

After further investigation, the answer was yes, officials said. But scenarios like this can play out in any heat wave across the country, and in many cases, the answer cannot be determined immediately — if at all. That dearth of information is the reason that heat-related deaths in the United States remain a frustrating mystery.

Researchers estimate that heat kills more people than any other extreme weather event, and the number of heat-related deaths reported by the U.S. Centers for Disease Control and Prevention has steadily risen in recent years.

In 2023, the agency reported that heat played a role in approximately 2,300 deaths, though this number may be revised as more records are processed. But some researchers say the actual number is far higher. One study that examined data from the late 1990s to the early 2000s concluded that the average number of fatalities annually was roughly 10,000.

The CDC relies on death certificates reported by local authorities for its tally, but the way these certificates are completed varies from place to place. Many local officials do not have the time, funding or staffing needed to investigate heat-related questions. And officials do not apply a consistent set of criteria to determine whether heat contributed to a death — or even consider heat as a potential factor when filling out death certificates.

“It’s a complete mess,” said Gregory Hess, the chief medical examiner in Pima County, Arizona, which borders Maricopa County. “There’s no uniformity on how to do this in the U.S.”

Researchers and activists say the lack of effective tracking is leading to needless deaths. If cities monitored and responded to heat deaths as if they were a deadly viral outbreak or similar public-health crises, an aggressive and targeted response could help keep more people from dying, activists argue. The issue is more urgent than ever, experts say, as climate change increases the frequency, duration and intensity of heat waves.

“Essentially, all heat-related deaths are preventable,” said Kristie L. Ebi, a professor at the University of Washington who studies the health risks of climate change.

A few localities that have increased efforts to track these deaths say they’re already seeing results.

When officials in Maricopa County see reports of deaths they suspect are related to heat, Jeff Johnston, the chief medical examiner, directs his team members to fan out across the county — one of the hottest in the country — and investigate.

They head to houses, bus stops, trailer parks and other places where the deaths were recorded. They dig into the circumstances surrounding each death, such as whether the person had any preexisting medical conditions or access to a functioning air-conditioner.

Johnston said his office’s findings have been used to block utilities from shutting off customers’ electricity on the hottest days and to pass a law that requires landlords to allow people living in mobile homes to install air-conditioners.

“All of these interventions help reduce mortality,” Johnston said.

Last year, Maricopa County recorded 645 heat-related deaths after a record stretch of blistering temperatures. This year, the county has already confirmed 114 heat-related deaths amid temperatures in Phoenix that forecasters say could make this summer the hottest on record. Nearby Pima County, which includes Tucson, has recorded nearly 70 heat-related deaths, including 36 in July alone.

For the physicians, coroners, medical examiners and other professionals who are filling out death certificates, determining fatalities caused directly by heat is relatively straightforward. But reliably identifying deaths where heat contributed is more difficult. High temperatures can increase the risk of deaths from heart attacks and other forms of cardiovascular disease, and prolonged exposure to heat can put stress on the body in other ways.

“It gets a little speculative,” said James Gill, the chief medical examiner of Connecticut and a former president of the National Association of Medical Examiners.

Gill said medical examiners need to be careful with their judgments so they do not appear arbitrary. He gave an example of finding bodies in apartments days after a heat wave: “Do you just call all of them heat-related deaths?” he said. “You need some criteria.”

But developing the protocols and training necessary to identify heat-related deaths takes time and investment that many offices do not have, said Hess of Pima County. His office did not start tracking deaths where heat played a contributing role until 2023. That year, the county recorded 176 heat-related deaths.

“We had to convince county administration, who holds the purse strings, that we would like to be able to do this,” he said. “Somebody’s got to cough up the money.”

Stacey Champion, a community advocate in Phoenix, said the CDC already has a form that could help. The agency released a supplemental form for heat in 2017 that death investigators could use, including questions about whether the person who died had access to cooling measures, their history of heat illness and the weather. It is unclear how many places use this form.

Champion said this form could be a “game changer” in helping investigators more consistently capture and report information about heat-related deaths.

The CDC wrote in a statement that while it recommends that death investigators use this form, it does not track the use of the form.

In other parts of the country, the number of heat-related deaths is overshadowed as a priority by more common causes of death, such as traffic accidents and opioid overdoses, said John Balbus, director of the Office of Climate Change and Health Equity in the U.S. Department of Health and Human Services.

“Public health departments at all levels are generally understaffed and underfunded and have to contend with a lot of competing priorities,” Balbus said.

In Oregon, for example, heat was not widely considered to be a significant deadly threat until 2021, when a heat wave in the Pacific Northwest killed hundreds of people across the region. During that heat wave, the state began reporting on a daily basis the number of possible heat-related deaths, said Kyle Kennedy, a public information officer for the Oregon State Police.

Miami-Dade County in Florida has taken several measures to prevent heat-related illnesses and deaths, such as purchasing and installing 1,700 air-conditioning systems for public housing residents. But Jane Gilbert, the county’s chief heat officer, said the county is not able to closely track deaths where heat is a contributing factor. She said Miami-Dade County is considering adopting a similar approach to Maricopa County’s, but she is juggling other needs and would require additional support, expertise and funding.

“This needs to be addressed at a larger scale than at a local government level,” Gilbert said.

The time it takes to investigate deaths is another complicating factor. In addition to the dozens of heat-related deaths that Maricopa County has cited this year, there are 465 possible cases still pending, some of which could take months to resolve.

The wait for toxicology or other test results and additional investigations add to that turnaround time. The volume of cases and staffing levels in a medical examiner office’s are a factor, too.

Given these challenges, some researchers support using an alternative measure called “excess deaths.” This approach calculates the difference between the expected number of deaths based on historical conditions and the observed number of deaths in a specific period. The study that estimated about 10,000 cases annually in the nation used this method.

The CDC said in a statement that it was working on its own approach to compute excess deaths from heat on a national level but needed more data for that analysis.

Greg Wellenius, a professor in environmental health at Boston University who co-authored the 2020 study, said that excess death counts can offer a clearer picture of the scope of heat-related deaths on a national level, but they are less helpful for guiding local officials. Death certificates, which can capture the circumstances surrounding each death, can help officials implement targeted measures, he said.

Even if excess deaths become a more widely adopted measure, health officials will continue to fill out those certificates. And if those are done right, they can save lives, Johnston of Maricopa County said.

“We’re making real-time adjustments to try to prevent the next death,” he said. “Today, tomorrow, next week and each summer.”

This article originally appeared in The New York Times.

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