A layer of pollution can be seen hovering over Los Angeles, California on October 17, 2017.

Los Angeles retained its dubious distinction as the U.S. city with the dirtiest air, but Phoenix, Houston and New York also made the top 10 list, the American Lung Association said Wednesday.

According to the 2019 "State of the Air" report, four in 10 Americans live with unhealthy air. It said increased heat and a more active wildfire season in California, Oregon, Washington and Montana also contributed to a particle pollution problem.

The report, which covers the period 2015-2017, breaks down pollution using three different measurements: days of unhealthy ozone, or smog; days of unhealthy particle pollution, or soot; and annual particle pollution.

The report identified six other California cities in the top 10 it also graded an "F" for smog-fouled air, including Visalia, Bakersfield, Fresno, Sacramento, San Diego and the San Jose-San Francisco-Oakland area. The ranking is based on ozone pollution, an invisible pollutant the association said can damage lungs and even shorten life.

"This is our 20th annual report, and Los Angeles has ranked number one for ozone pollution in all but one of those reports," said William Barrett, director of clean air advocacy for the American Lung Association in California. "Bakersfield was top of the list one year back in 2005 or 2006."

The report said ozone pollution worsened in much of the nation during 2017. It said increased heat likely drove the increase in ozone and added that the three years covered in this report rank as the three warmest years ever recorded.

"Nearly 141.1 million Americans live in 244 counties where they breathe unhealthful levels of air pollution in the form of either ozone or short-term or year-round levels of particles," the 167-page report said.

The West and the Southwest continued to dominate the most-ozone-polluted list. Texas has three cities that received an "F" for smog: Houston, Dallas-Fort Worth and El Paso. Colorado has two: Denver and Fort Collins. Arizona, Nevada and Utah also had one apiece.

Other cities making the top 25 list had a higher average of unhealthy days in 2015-2017, including New York and Chicago. Houston, Phoenix, Las Vegas, Denver, Philadelphia, Washington, D.C.-Baltimore and Salt Lake also made the list.

Short-term particle pollution was made worse, Barrett said, by several devastating wildfires in 2017, including the wine country blazes in Northern California. Also, pollution was created by the massive Thomas Fire in Southern California's Ventura and Santa Barbara counties.

"Ventura went from having an 'A' last year to having an 'F' this year for particle pollution days, and that's because of the Thomas Fire," said Barrett. He said the Thomas Fire also had a pollution impact across other nearby counties, including Los Angeles.

The report said California, Washington and Oregon saw a jump in the number of days when particle pollution levels spiked due to wildfires. Specifically, the Los Angeles region had two days when it said levels shot up to "hazardous" and there was at least one day in portions of Oregon and Northern California during wildfires.

"Wildfires are not the only source of high particle pollution days," said the report. "Other sources including wood stove use (especially in Fairbanks), older diesel vehicles and equipment, and industrial sources (as in Pittsburgh) contribute to a lot of particle pollution."

Weather pattern changes also can create situations where there are spikes in particle pollution, according to the report. It said Pittsburgh is the only city in the 25 most polluted that is east of the Mississippi River.

Finally, the report also said 14 of the 25 cities on the year-round particle pollution list have shown improvement. They include Cleveland, Detroit, Houston, Philadelphia, Chicago and Indianapolis.