Heat is the deadliest form of extreme weather. Why are fatalities so hard to track?

As blazing heat settled over the state last month, Idaho’s health department received news that two residents had died due to heat-related causes. They made an unusual decision: rapidly inform the public.

“For years we’ve been announcing the first rabid bat in the spring, or the first flu death in the fall,” said state epidemiologist Christine Hahn. “But we hadn’t been doing anything about heat-related deaths.”

Last year was the hottest in human history, and there are signs that 2024 could be even more scorching. As summer begins in the United States amid record-breaking temperatures and unrelenting humidity, some local officials and health experts are sounding the alarm about the dangers of extreme heat, whose effects can be deadly but hard to trace.

Extreme temperatures have been linked to cardiovascular deaths, chronic kidney disease mortality and respiratory failure. Heat can put undue stress on organs. The heart pumps faster to get blood flow to the skin; kidneys work harder to preserve the body’s water. Those with preexisting conditions are more at risk in hot weather.

Identifying and tracking heat deaths is challenging, especially in the United States, where comprehensive national data can be hard to obtain. Experts say the number of heat deaths in the United States is probably much higher than recorded - a situation that can affect policy responses and leave people more vulnerable and less informed about the dangers of heat.

“Extreme heat causes the greatest mortality of all extreme weather,” said a 2023 World Meteorological Organization report. Heat-related deaths have steadily increased over the last few years from 1,563 in 2021, to 1,702 in 2022, and, provisionally, 2,297 deaths in 2023, according to the Centers for Disease Control and Prevention.

The CDC typically does not release its official tally of annual heat deaths until well after summer. Maricopa County, Ariz., which has made tracking heat deaths a priority, has recorded 6 this year; 111 are under investigation and include cases where the county medical examiner suspects a heat-related death.

The CDC says it tracks heat fatalities through death certificates. If an emergency room patient diagnosed with heat stroke or a heat-related illness dies, local authorities would investigate the case, run toxicology tests, and mark that as a heat death. That data is sent to state, tribal and territorial databases, which then send it to the CDC.

Some cases can be less clear-cut, such as someone who goes into cardiac arrest during a heat wave.

“Imagine you’re in an emergency department. Somebody comes in with a heart attack. They have underlying cardiovascular issues,” said Kristie L. Ebi, a professor of global health at the University of Washington. “How do you know, for this person versus the person next to them, which one was exacerbated by the heat and which one wasn’t?”

A more precise way of tracking heat deaths, experts say, is to measure what are known as “excess deaths.” Analysts look at the typical amount of people who die on a given day and then see how many more people died when there were higher temperatures during the same time of year.

An excess-mortality study published in Nature last year estimated more than 60,000 heat-related deaths occurred in Europe during the summer of 2022 - the continent’s hottest on record until 2023.

The data for these estimations can be hard to attain, said Gregory Wellenius, director of the Boston University Center for Climate and Health. It requires having several years worth of information on how many people died each day or week in as many parts of the country as possible.

“Data from the CDC on deaths are publicly available for much of the U.S., but are provided in enough detail only going back a few years and only for counties with larger populations,” Wellenius said. “The lack of access to more complete data makes estimating the number of people dying from heat challenging.”

Wellenius co-authored a 2020 excess-death study that estimated an average of 5,608 deaths a year in the United States were attributable to heat between 1997 and 2006. He said the data was publicly available only through 2006 and he is in the process of applying to obtain more recent figures.

“CDC is working on an approach to compute excess deaths nationally but we are in the process of obtaining necessary data inputs for implementing this methodology,” the office said in statement to the Washington Post.

It’s unlikely that more accurate information will became available quickly, said Ebi. But knowing the figures can be important to help governments plan for and communicate with the public during high temperatures.

“Nobody needs to die in a heat wave,” Ebi said.

Sometimes people don’t even know they are vulnerable, said Rupa Basu, an epidemiologist at the California Environmental Protection Agency. Medication including beta blockers (which can slow heart rates), blood pressure medication (which can be dehydrating) and anti-depressants (which can prevent sweat glands from functioning properly) can make people high-risk in the heat, she said. Fit, young hikers can succumb to high temperatures on the trails.

Hahn, Idaho’s state epidemiologist, has been in the role since 1997. Boise, the state’s capital, experienced record-breaking summer temperatures in 2021 and 2022; 15 heat-related deaths were recorded in the state. Last year, a resident died of heatstroke while on a challenging hike with little shade in the state’s south.

It was the first time Hahn decided to quickly put out an announcement about a heat death. It felt so preventable, she said.

“The compelling reason that we should put the information out is because people can do something about it,” Hahn said. She doesn’t know how often she plans to release these reports. “We will put out more information if we feel like there’s something new to share with the public.”

That said, “The numbers are probably a tip of the iceberg,” Hahn said. “There are probably some deaths that don’t get officially recorded that way.”