The air over the Salt Lake Valley has become cleaner in recent years, even as Utah’s urban core gets more crowded and covered in more asphalt, with more cars driving more miles.

So it is no surprise that Salt Lake City still has some of the worst air quality in the nation. The capital is listed at No. 7 among large metro areas in a new analysis that compares air-quality metrics for hundreds of U.S. cities.

The analysis, released Jan. 27 by the California-based digital media firm Quote 360, examines 2018 air-quality data processed and posted daily on the U.S. Environmental Protection Agency’s AirNow.gov website.

That year, 76 million tons of emissions were released in the United States, emitting pollutants that contribute to the formation of ozone and fine particulate matter, or PM2.5, according to the EPA.

The agency processes cities’ ambient levels of air pollution into a daily score called “air quality index,” or AQI. Values from 0 to 50 are considered good; from 51 to 100 are moderate; from 101 to 150 are unhealthy for sensitive groups; and above 151 are unhealthy for everyone.

Among large cities, Salt Lake City is essentially tied with Las Vegas and Sacramento when it comes to crappy air. All three cities had an average AQI of 61, although Salt Lake City had fewer days categorized as “moderate or unhealthy” (225) or worse, that is “hazardous,” at just two. That means Utah’s capital city saw 138 days in 2018 where a deep breath could be taken outdoors without worry.

The Wasatch Front experiences high levels of particulate pollution in winter and of ozone in the summer. Those are the seasons when weather is conducive to the formation of those contaminants. Fine particulate matter is associated with increased rates of respiratory and cardiovascular diseases, stroke, cancer and autoimmune disorders, as well as shorter life spans.

A recent comprehensive review of scientific literature conducted by Brigham Young University researchers concluded polluted air shaves between 1.1 and 3.5 years off the average Utahn’s life.

“This loss of life is distributed across most of the population rather than only affecting ‘sensitive groups,’” the research team, led by ecology professor Ben Abbott, stated in an executive summary. “For example, 75% of Utahns lose 1 year of life or more because of air pollution and 23% lose 5 years or more.”

At least 85 percent of the harmful emissions come from fossil fuels, according to the study, which recommends measures that increase vehicles and buildings’ fuel efficiency, eliminating subsidies for fossil fuel production, expanding alternative transportation and pricing carbon emissions.

Overall emissions in the Wasatch Front counties declined 30.5% between 2011 and 2019, from 410 tons per winter day to about 285 tons, according to the Utah Division of Air Quality. Much of the gains were attributed to reductions in vehicle emissions. Despite an ever increasing number of miles driven, these emissions dropped by nearly half, reflecting improvements in automobile engines and the expansion of low-emission, hybrid and electric vehicles.

Utah's air quality might be improving, but many residents are eager to see further reductions in emissions and are hoping for action out of the legislative session that opened Monday.

Topping the air quality wish list are $100 million in appropriations proposed by Gov. Gary Herbert and incentives aimed at renewable energy, energy storage technologies and replacing polluting vehicles.

Some of these investments are part of the air quality “Road Map” released last month by the Kem C. Gardner Institute, a University of Utah economics think tank. The seven-point strategy calls for cutting carbon emissions 80% below 2005 levels by 2050.

According to the Quote 360 analysis, 137 million Americans live in cities where air pollution exceeds federal standards. Dominating the top of the rankings for poor air quality are California urban areas, starting with Riverside, whose median AQI of 97 was by far the nation’s worst.

But the city with the second worst air quality is not in California, but thousands of miles into the Pacific, where volcanic eruptions from Madame Pele have been bathing Hawaii’s Big Island in sulfur dioxide for years. The city of Hilo’s average AQI for 2018 was listed as 87, with 104 days described as “hazardous.” But before changing your vacation plans, it is worth noting that Hilo experienced 127 days of good air, almost as many as Salt Lake City and more than double or triple the number enjoyed by mainland cities with comparable air quality challenges.