Nearly half of Americans are breathing in unhealthy levels of polluted air, according to the latest "State of the Air" report from the American Lung Association.

The report found that 156 million people, which is 25 million more than last year, are living in areas that received an “F” grade for either ozone or particle pollution.

The association said particle pollution can be deadly, and mostly comes from wildfires, wood-burning stoves, coal-fired power plants and diesel engines.

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The report measured "short-term" spikes in particle pollution that occur daily, and the annual average "year-round" levels, which reflect the concentration of particles in each area.

The top cities with the highest level of short-term particle pollution were the Bakersfield-Delano area in California, the Fairbanks-College area in Alaska, the Eugene-Springfield area in Oregon and Visalia, California.

For year-round particle pollution, three regions of California topped the list: Bakersfield-Delano, Visalia and Fresno-Hanford-Corcoran.

Another type of pollution measured in the report was ground-level ozone pollution — also known as smog. The American Lung Association said it's a powerful respiratory irritant whose effects have been likened to a sunburn of the lungs.

The cities most polluted with smog, according to the report, are all in California, including the Los Angeles-Long Beach area, Visalia and Bakersfield-Delano.

The annual report also highlights cities with the cleanest air based on several different measures. But this year, only two cities made the cut for that list — Bangor, Maine and San Juan, Puerto Rico. Last year, there were five cities on this list.