SALT LAKE CITY — Prolific wildfire seasons, hot, stagnant summers and population growth are all contributing factors that helped land Salt Lake City on a list of 54 U.S. cities struggling with ozone pollution.

A new reportby Climate Central analyzed U.S. Environmental Protection Agency data and found unhealthy ground level ozone pollution is a challenge for millions of residents across the country, even as the United States has experienced a 16% decrease in ozone levels since 2000.

The invisible pollutant formed by a chemical reaction is particularly dangerous for vulnerable populations such as the young, elderly or those with respiratory problems like asthma, leading to increased visits to doctors and emergency rooms.

The report notes Salt Lake City had signficantly more unhealthy ozone days in 2017 and 2018 than the observed average from 2000 to 2014.

"Salt Lake City had 31 unhealthy ozone days compared to an average of 22" during that 14-year period, the report said.

"A number of factors may be contributing, including its increasing population and its bowl-like topography, which can act as a trap for pollutants," it said.

In the past four years, Salt Lake City was among 40 U.S. cities that had a recurring ozone problem, experiencing at least 20 unhealthy ozone days.

Palm Springs, California, led the list, however, with a whopping 450 unhealthy ozone days during that four-year period, followed by Los Angeles at 394 and Bakersfield at 372 days.

Bryce Bird, director of the Utah Division of Air Quality, isn't surprised the Salt Lake metropolitan area made the list, given how the summer seasons played out in 2017 and 2018.

"Last summer was a high ozone summer for us, as was 2017," he said. "Even though emissions are down, we had wildfire smoke impacts and very little monsoonal activity."

Bird said prolonged periods of high pressure kept the afternoon thunderstorms from coming in, which help to scrub the air of the harmful pollutant.

In 2016, the area experienced a cool summer, leading to fewer high ozone days, and this year — so far — is shaping up to be better than the two previous years, he said.

"This summer so far has turned out to be a good one with more rain than in past summers. Unless we have a really hot August, we will be closer to 2016 than 2018," Bird said.

When the EPA lowered the ozone pollution threshold from 75 parts per billion to 70 parts per billion in 2015, it helped to trigger a new slate of areas across the country out of compliance with the federal standard.

The EPA subsequently designated the Wasatch Front as a "nonattainment" area for ozone and classified it in the marginal category. The same classification was leveled at Duchesne and Uintah counties below an elevation of 6,250 feet.

Bird said that led to a three-year evaluation period that began in 2018 in which state regulators are collecting data.

If the standard is not met based on the three-year average, the Wasatch Front risks getting bumped up to the moderate category, he said.

"Our values were high enough last year that we will likely not be able to show attainment with the standard," he said, adding that Utah will then have to develop a plan demonstrating how it will lower ozone pollution levels.

Ozone is a pollutant that plagues much of the state with background "drift" from Western wildfires, even if Utah is not burning.

The state is also one of eight across the country where ozone pollution is monitored year-round, according to the Climate Central report.

The Uinta Basin experiences troublesome ozone pollution, but in the winter months when stagnant air helps to trap emissions in the bowl-like area.

A multimillion-dollar study is probing the atmospheric conditions in the basin, which is Utah's most active oil- and gas-producing region. A number of strategies are in place to tamp down industry emissions, including the use of special equipment to detect leaks.

Climate Central describes itself as an independent organization of scientists and journalists researching and reporting the facts about changing climate and its impact on the public.