As dangerous heat bears down on the central and eastern United States this week, a new study shows heat-related deaths across the country are on the rise.

While 2023 was the hottest year on record and led to at least 2,325 heat-related deaths in the U.S., more than 21,518 people have died from heat since 1999, according to a study published Monday in JAMA, the journal of the American Medical Association.

“It’s very likely that we’re going to continue to face these kind of extreme heat issues ,” said Jeffrey Howard, an associate professor in public health at the University of Texas at San Antonio and the lead author of the study. “It’s not something that’s going to go away.”

Heat kills more people in the United States than any other type of extreme weather, according to researchers. The study noted a 117 percent increase in heat-related deaths over the past 24 years, with a significant upswing since 2016.

The Southwest has seen a disproportionate amount of those deaths. About 48 percent of heat-related deaths took place in Arizona, California, Nevada, or Texas, Dr. Howard said. That detail was not included in the study, he noted.

The latest heat wave, expected over the next few days, will close out a period of relatively cool weather with a blast of unseasonably hot temperatures in the Upper Midwest and Mid-Atlantic.

Extreme heat plus humidity means people could experience temperatures that feel like 115 degrees Fahrenheit, or about 46 Celsius, in some places. As of Tuesday, more than 19 million people were under an excessive heat warning.

The JAMA study analyzed publicly available data from the Centers for Disease Control, the federal agency that tracks heat-related deaths, and adjusted for changes in age and population size over time. While previous U.S. studies had analyzed earlier data, up until 2018, Dr. Howard said his work on more recent years revealed an increasing trend.

But heat-related deaths are hard to track. The C.D.C. relies on death certificates from local authorities but there is no consistent criteria to determine the contribution of heat to a death. The tally of deaths from extreme heat could actually be higher, with an average annual number of 10,000 deaths across the United States from 1997 to 2006, according to a 2020 study.

“There’s still a lot of issues in terms of peeling back what’s going on underneath these numbers,” Dr. Howard said. For example, researchers still need to determine how much of the trend is from increasing temperatures and how much is from better data collection.

Warming can be deadly around the globe. Heat contributed to 47,000 deaths in Europe last year, according to a recent study. That number could have been even higher if air-conditioning, better public information and other strategies had not been implemented across the continent, researchers found.

The JAMA study published on Monday suggested that the authorities in high-risk areas expand access to hydration and cooling centers and invest in more air-conditioning.