California's reign as the U.S. state with the worst air pollution continues as Los Angeles is again the nation's smoggiest metro area, according to a report released Wednesday.

LA isn't alone when it comes to smog-choked cities in California: seven of the nation's top 10 smoggiest cities are in the Golden State, including other sprawling cities such as San Francisco, San Diego and Sacramento.

Los Angeles has had the worst smog for 19 years of the 20-year history of the report.

The bad news came from the American Lung Association's annual "State of the Air" report, which covers the years 2015-2017.

Smog forms on warm, sunny days and is made worse by chemicals that exit vehicle tailpipes and power plant and industrial smokestacks. Warmer temperatures make ozone more likely to form.

The report covers the two main types of air pollution that plague the USA: smog (also known as ground-level ozone) and soot (technically known as "particulate matter").

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The news isn't any good in California when it comes to soot either: Fresno has the nation's highest levels of that form of air pollution, and six of the nation's 10 "sootiest" cities are again in California.

Soot pollution is deadlier and more of a health hazard than smog, causing more premature deaths and lung cancer, the lung association said.

"California's air quality is worsening despite having the strongest environmental regulations in the nation," said John Balmes of the University of California-San Francisco at a news conference Tuesday.

California's soaring population and topography allow air pollution to overcome the state's strict environmental laws, the lung association said. The boom in people brings an increase in cars and trucks on the roads, and many of those people live in valleys and basins, right where pollution tends to settle.

Nearly year-round sunny skies don't help: Those picture-perfect days are a major factor in high levels of ozone pollution.

The state would be much worse off without its strict laws on tailpipe pollution and eliminating coal-fired power plants.

The bad news about air pollution spread nationwide in the report: Smog and soot-choked air is worsening amid heat waves and bigger wildfires as environmental protections are rolled back.

Many cities increased the number of days when soot soared to record-breaking levels, the report says, and "more cities suffered from higher numbers of days when smog reached unhealthy levels."

About 141 million Americans live with unhealthful levels of air pollution, the report says, placing them at risk for premature death, lung cancer, asthma attacks, cardiovascular damage and developmental and reproductive harm.

This is a higher number than either of the reports from 2017 and 2018.

On the other end of the scale, only six metro areas recorded no days when smog or short-term soot pollution reached unhealthy levels, according to the report: Bangor, Maine; Burlington, Vermont; Honolulu; Lincoln, Nebraska; Wilmington, North Carolina; and the Melbourne, Florida, metro area.