Heat is the deadliest weather extreme. Why are deaths difficult to record?

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

Extreme temperatures are linked to cardiovascular deaths, chronic kidney disease, and respiratory failure. Heat can put excessive strain on organs. The heart pumps faster to supply blood to the skin; the kidneys work harder to retain water in the body. People with underlying health conditions are at greater risk in hot weather.

Identifying and tracking heat deaths is challenging, especially in the United States, where comprehensive national data is hard to obtain. Experts say the number of heat deaths in the United States is likely much higher than reported—a situation that can influence policy responses and make people more vulnerable and less informed about the dangers of heat.

“Extreme heat causes the highest mortality rate of all weather extremes,” said a 2023 report by the World Meteorological Organization. The number of heat-related deaths has steadily increased in recent years, from 1,563 in 2021 to 1,702 in 2022 and tentatively to 2,297 deaths in 2023, according to the Centers for Disease Control and Prevention.

The CDC’s official count of national heat deaths for 2024 probably won’t be available until next year. Maricopa County, Arizona, where tracking heat deaths is a priority, recorded six deaths this year; 111 are currently under investigation, including cases where the county medical examiner suspects heat-related deaths.

The CDC says it tracks heat deaths using death certificates. When a patient dies in the emergency room who was diagnosed with heat stroke or a heat-related illness, local authorities investigate the case, run toxicology tests and label it a heat death. That data is sent to state, tribal and territorial databases, which then forward it to the CDC.

In some cases the situation is less clear-cut, such as someone who suffers cardiac arrest during a heatwave.

“Imagine you’re in the emergency room. Someone comes in with a heart attack. They have cardiovascular problems,” says Kristie L. Ebi, a professor of global health at the University of Washington. “How do you know which person next to them has gotten worse with heat and which hasn’t?”

A more accurate way to track heat deaths, experts say, is to measure what’s called “excess mortality.” Analysts look at the typical number of people who die on a given day and then determine how many more died when temperatures were higher. at the same time of year.

According to a study on excess mortality published in Nature last year, there were an estimated more than 60,000 heat-related deaths in Europe in the summer of 2022 – the hottest summer on record on the continent until 2023.

The data for these estimates are difficult to obtain, says Gregory Wellenius, director of the Boston University Center for Climate and Health. You need several years of information on how many people died daily or weekly in as many parts of the country as possible.

“CDC data on deaths is publicly available for much of the U.S., but it is detailed enough only for recent years and only for counties with larger populations,” Wellenius said. “The lack of access to more comprehensive data makes it difficult to estimate the number of people dying from heat.”

Wellenius is co-author of a 2020 study on excess mortality that estimated that an average of 5,608 deaths per year in the U.S. 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 requesting more recent numbers.

“The CDC is working on an approach to calculate excess mortality at the national level, but we are still in the process of obtaining the necessary data inputs to implement this method,” the office said in a statement to the Washington Post.

It is unlikely that more accurate information will be available soon, Ebi said, but knowing the numbers could be important to help governments plan and communicate with the population during high temperatures.

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

Sometimes people don’t even know they’re at risk, says Rupa Basu, an epidemiologist with the California Department of Environmental Protection. Medications such as beta-blockers (which can slow the heart rate), blood pressure medications (which can cause dehydration) and antidepressants (which can impair sweat gland function) can put people at risk in the heat, she says. Fit, young hikers can succumb to the high temperatures on the trails.

Hahn, an Idaho state epidemiologist, has served in that role since 1997. Boise, the state capital, experienced record-breaking summer temperatures in 2021 and 2022, with 15 heat-related deaths recorded in the state. Last year, a resident died of heat stroke during a challenging hike with little shade in the southern part of the state.

It was the first time Hahn decided to quickly report a heat death. It felt so avoidable, she said.

“The compelling reason we should publish this information is so people can do something about it,” Hahn said. She doesn’t know how often she plans to publish these reports. “We will release more information when we feel there is something new we want to share with the public..”

However, the numbers are “probably just the tip of the iceberg,” Hahn said. “There are probably some deaths that are not officially recorded in this way.”