WEDNESDAY, April 29, 2015 (HealthDay News) -- More than four in 10 Americans, or about 138.5 million people, are breathing unhealthy air, a new report finds.

Despite the fact that many cities have shown improvements in air quality, other metropolitan areas have recorded more days of air that contained high levels of both ozone and particle pollution. And a handful of cities saw their highest number of unhealthy days ever, according to the American Lung Association's annual report on air quality.

"Everyone has the right to breathe healthy air. We must meet our air pollution challenges head-on to protect the health of millions of Americans living with asthma and chronic obstructive pulmonary disease [COPD]," said Harold Wimmer, president and CEO of the lung association.

One expert in respiratory disease agreed.

"We know that urban populations with poor air quality have higher incidence of atherosclerosis and COPD," said Dr. Len Horovitz, a pulmonary specialist at Lenox Hill Hospital in New York City. "There is even the question of whether lung cancer may be linked to air quality. We need to clean up our environment."

The best news from the new report came in statistics that showed a continued lowering of year-round particle pollution in the eastern United States, due mostly to cleaner diesel fuels and power plants.

Particle pollution involves tiny particles that are small enough to become trapped in the lungs and enter the bloodstream. Recent research has also suggested this type of pollution can raise the risk of lung cancer, the lung association said.

But the researchers also found signs that climate change is making it harder to keep the air clean in the western half of the country, where droughts and heat waves can trigger episodes of high particle pollution.

Six areas were dubbed the "cleanest cities" -- Bismarck, N.D.; Cape Coral-Fort Myers-Naples, Fla.; Elmira-Corning, N.Y.; Fargo-Wahpeton, N.D.-Minn.; Rapid City-Spearfish, S.D., and Salinas, Calif.

However, six other areas in California earned the dubious distinction of being the worst for short-term particle pollution. They included Fresno-Madera, Bakersfield, Visalia-Porterville-Hanford, Modesto-Merced, Los Angeles-Long Beach, and San Jose-San Francisco-Oakland.

Still, there's reason for hope. A number of cities showed marked improvements in ozone levels in this year's report. Of the 25 metro areas most polluted by ozone, four California cities -- Bakersfield, Los Angeles, Sacramento and Visalia -- as well as the Washington-Baltimore area, were the top six cities in reaching their lowest number of unhealthy days since the lung association began tracking air pollution levels 16 years ago.

But Wimmer said there's much more work to be done.

"One challenge is that many more people are actually at risk than even our estimates show," he said. "We use the current ozone standard as the basis of much of our assessment, but that standard is weak, and out of date, and does not reflect what we know harms children and people with lung disease."

Dr. Philip Landrigan is dean for global health at the Icahn School of Medicine at Mount Sinai in New York City. He called air pollution "a serious threat to the health of Americans of all ages."

Children, especially, "are highly vulnerable to air pollution," said Landrigan, who is also chair of the department of preventive medicine at Mount Sinai Health System. "Their small lungs and narrow airways are easily damaged by ozone, fine particulates and other components of air pollution. Asthma and pneumonia are the consequences."