In 2018, the Denver-Aurora-Lakewood metropolitan area experienced approximately 131 days of degraded air quality, according to a new report.

The Environment Colorado Research & Policy Center and and Colorado PIRG Education Fund used air quality data from the U.S. Environmental Protection Agency to determine how frequently ground-level ozone and fine particulate matter were elevated nationwide.

“In 2018, 108 million Americans lived in areas that experienced more than 100 days of degraded air quality,” the report concluded. Most of the affected areas were urban. However, 12 rural counties also saw ground-level ozone and fine particulate matter in excess of the EPA’s “little to no risk” threshold.

Although the EPA’s air quality index – which is a scale from zero to 500 – considers a measurement of 101 to be the start of unhealthy air quality for sensitive groups, the report included “moderate” air quality measurements of 51 or higher in its calculation.

“The data show that America’s existing air quality standards aren’t doing enough to protect our health,” said co-author Elizabeth Ridlington of Frontier Group, a liberal-leaning research firm. “As the climate warms, higher temperatures and more severe wildfires increase air pollution and the threat to human health.”

The Colorado Springs metro area had 119 days with air quality measurements of 51 or higher. The Riverside and San Bernardino metro area in Southern California, home to more than 4.6 million people, had the highest rate of degraded air days, totaling 227.

Ground-level ozone, a component of smog, is associated with respiratory problems and premature death. The transportation sector accounts for approximately 60% of the emissions that form ground-level ozone. Wildfire smoke across many western states also contributed to poor air quality.

Among the report’s recommendations were a call to increase use of zero-emission vehicles, maintain strong fuel-economy standards, and expand urban tree cover as a means of storing carbon and filtering pollution from the air.