Americans are more at risk from the deadly effects of heat than ever before, a new study found.

Researchers from the Journal of the American Medical Association say there were 21,518 deaths from 1999-2023 where heat was either the underlying cause or the contributing cause of death, a possible underestimation, they say. In total, heat-related deaths have increased 117% since 1999.

The authors of the study, which was released Monday, used data from the Centers for Disease Control and Prevention to calculate the mortality rates.

While the number of heat-related deaths fluctuated year to year, there was a sharp uptick in the last seven years. According to the data, heat-related deaths increased by 16.8% per year from 2016 to 2023 – and researchers expect the numbers to continue climbing.

“As temperatures continue to rise because of climate change, the recent increasing trend is likely to continue,” the authors wrote.

Last year was not only the hottest year on record but also the most deadly, with a total of 2,325 people dying from heat, the study found. Around 120,000 people were admitted to the emergency room for heat-related illnesses from January to December 2023, according to CDC data. Over 90% of those visits occurred during the peak summer months of May to September. 

Rising temperatures due to climate change have led to record-breaking heat across the country in recent years. After recording the warmest July on record in 175 years, the National Oceanic and Atmospheric Administration said on a call this month that there is a 77% chance that 2024 will be the hottest year yet.

The international community is also grappling with the deadly effects of heat. A recent study published in Nature Medicine estimated that 47,000 people died last year in Europe due to heat. 

Experts say the number of heat-related deaths in the U.S. and globally is likely an undercount. That has to do with the way that heat-related deaths are tracked.

“People may present to the Emergency Department or the hospital with a heart attack, stroke, asthma attack, or kidney failure, and heat is often not recorded in medical records as the underlying cause,” Dr. Céline Gounder, CBS News medical contributor, told CBS News earlier this month.

As the temperatures climb each year, states are scrambling to put life-saving measures into place. The Journal of the American Medical Association authors recommend local governments invest in cooling centers, air conditioning and hydration centers to help people beat the heat. 

Other ways to protect against extreme heat, Dr. Gounder said, include taking advantage of state and federal tax credits for residential renewable energy installation projects, and making sure your home is properly insulated. 

On an individual basis, Dr. Gounder said she recommends cold showers and taking breaks in the shade if you’re outdoors.