The Los Angeles metropolitan area remains one of the most polluted in the nation, the American Lung Association said on Wednesday.

The Los Angeles-Long Beach area was No. 1 on the list of the 24 metro areas most polluted by ozone, No. 4 on the list of the 24 metro areas most polluted by year-round particle pollution, and No. 7 on the list of the 24 metro areas most polluted by short-term particle pollution.

The Lung Association's latest report, State of the Air 2018, found ozone pollution worsened significantly in 2014-2016 compared to the previous report, while improvements continued in year-round particle pollution and fewer episodes of high particle days, the report said.

But the bottom line, it said, is that "Los Angeles remains the city with the worst ozone pollution."

"This year's report provides continued evidence that the United States must continue to fight climate change and to support and enforce the Clean Air Act to protect the nation from unhealthy air," it said.

The report "adds to the evidence that a changing climate is making it harder to protect human health," it said, adding that with record-setting heat in 2016, high ozone days zoomed, putting millions more people at risk and adding challenges to the work cities are doing across the nation to clean up.

Only Clean Air Act enforcement will "enable the nation to continue to protect all Americans from the dangers of air pollution."

The number of people exposed to unhealthy levels of air pollution increased to more than 133.9 million people during the period covered by the 2017 report (2013-2015).

"The best progress came in the continued reduction of year-round particle pollution, thanks to cleaner power plants and increased use of cleaner vehicles and engines," it said. "Continued progress to cleaner air remains crucial to reduce the risk of premature death, asthma attacks and lung cancer."

More than 7.7 million people, or 2.4 percent, live in 10 counties with unhealthful levels of all three of the greatest pollution threats: ozone and short-term and year-round particle pollution. This is 10.4 million fewer people than in the 2017 report, the report said, but also likely under-counts the number due to missing data.

Two heavily populated counties in California -- San Bernardino County and Los Angeles County -- lacked year-round particle-pollution data, according to the report. Had data been available from those two counties, which traditionally have unhealthful levels of all three measures, the actual number would likely be much higher, likely 19 million people.

With respect to ozone pollution, "Los Angeles remains at the top of this list, as it has for all but one of the 19 reports. Los Angeles also recorded more unhealthy air days in this report, measured in the weighted average, a change from last year when it reached its lowest level ever."

But pollution problems are not confined to Los Angeles.

"The State of the Air 2018 shows that too many people in the United States live where the air is unhealthy for them to breathe," it said. "More than four in 10 people (41.4 percent) in the United States live in counties that have unhealthful levels of either ozone or particle pollution.

"More than 133.9 million Americans live in 215 counties where they breathe unhealthful levels of air pollution in the form of either ozone or short-term or year-round levels of particles."