Most Americans — and Utahns — face more days of extreme heat each summer than they did a few decades ago, according to a study that highlights the health effects of that trend.

The study released Tuesday by the environmental advocacy group Natural Resources Defense Council (NRDC) finds that two-thirds of the U.S. population, or about 210 million people, live in counties that have encountered more extreme heat days each year over the past decade than they did from 1961 to 1990.

An even higher percentage of Utahns, 86 percent, live in counties experiencing more days of extreme heat than in the past, according to the report. The NRDC cited Utah as one of 20 states, plus Washington, D.C., with populations especially hard hit by extended and sweltering heat waves. Heat stress, the report said, currently causes almost 250 emergency room visits per year in the state.

Extreme heat days, according to the study, are those days between June 1 to Aug. 31 with peak temperatures hotter than 90 percent of the rest of the days that summer. The NRDC analysis found 11 of Utah’s 29 counties have experienced more than five additional extreme heat days on average each summer over the past 10 years — including the population centers of Washington, Utah and Cache counties.

The NRDC report broadly aligns with findings of federal agencies, including the National Oceanic and Atmospheric Administration and the Environmental Protection Agency. The EPA has noted that unusually hot summer days have occurred more often in the past few decades, while hot summer nights are going up at an even faster rate. Meanwhile, unusually cold winter temperatures have become less common since the 1980s.

The trend is taking a toll on health, with heat killing more Americans than any other type of extreme weather, experts said in a conference call tied to the study’s release. They urged action by health departments and municipalities to combat those health effects — while stressing the importance of cutting emissions contributing to climate change.

“If we keep pouring climate pollutants such as carbon dioxide into the atmosphere, we will face a level of global warming in which our best efforts will be inadequate to prevent adverse impacts of heat on our health,” said Linda Rudolph, director for the Center for Climate Change and Health at the California-based nonprofit Public Health Institute.

Rudolph said low-income families living in so-called “urban heat islands” — areas with lots of concrete and few cooling green spaces — are most vulnerable as the number of extreme heat days increase. They may be unable to afford air conditioning and have aged family members unable to reach a community cooling station.

“The elderly are least able to compensate for high ambient temperature and are at greatest risk from extreme heat,” said Samantha Ahdoot, an assistant professor of pediatrics at Virginia Commonwealth University.

But infants are also at risk, as well as young athletes practicing outside in the midsummer heat.

Ahdoot recalled treating a 15-year-old football player who complained of leg pain that had spread to his back during a hot day of football practice. It turned out the youth had suffered “severe heat illness,” Ahdoot said, with muscle breakdown and kidney injuries. He was treated for three days with IV fluids at the hospital.

And it was not an isolated case, she said. One 2011 study in the American Journal of Preventive Medicine found the number of exercise-related heat injuries in the U.S. doubled between 1997 and 2006.

“It’s important we recognize the vulnerabilities of these groups so that we can protect them,” Ahdoot said.

Experts urged cities to consider adding more green spaces, as well as cooler road and roof surfaces to combat urban heat islands. And while repeatedly stressing the importance of reducing climate emissions, they also called for early warning systems for high-heat days and adding more cooling centers for vulnerable groups.

The report noted through August, 2017 has been the third-hottest year on record in the continental U.S. And 2016 was the second-warmest on average for the U.S., according to NOAA data, just behind 2012.

In Utah, average temperatures have climbed almost 2 degrees since the early 1990s. And if carbon pollution levels continue to rise at their current pace globally, average temperatures in the state could be 13 degrees higher than the historical averages by the end of the century, the NRDC said.