The annual American Lung Association study says about 159 million Americans, or 55 percent of the country, reside in 441 counties threatened by air that's heavily polluted with ozone or tiny particles of soot, known as particle matter.
California has a majority of the top 10 worst-polluted counties. The Los Angeles metropolitan area placed at the top for most ozone pollution for the fifth consecutive year.
The Houston, Texas, and Knoxville, Tenn., areas ranked fifth and ninth, respectively.
According to the report, San Bernardino ranked number one among counties nationwide in ozone pollution. Riverside County had the nation's worst annual and short-term particle matter pollution.
Overall, 34 of California's 58 counties failed one or more of three clean air tests in the study.
John Kirkwood, the association's chief executive officer, tells the CBS News Early Show that the reportagainst smog, just not enough of it.
"There have been improvements in smog and there have been improvements over the last 20 years in particulate levels," he said. "But we still haven't reached the level in the country that's healthy for people."
Janice Nolen, the association's director of national policy, said many people remain at risk for smog-related illnesses like asthma, cardiovascular disease and lung inflammation.
Ozone pollution occurs when hydrocarbons and nitrogen oxides — released when fossil fuels burn or chemicals evaporate — combine with heat and sunlight. The problem is separate from the depletion of the ozone layer, which occurs high in the atmosphere. The ozone layer protects the Earth from the sun's harmful ultraviolet radiation, but ozone at ground level can be harmful.
Particle pollutants can come from fireplaces, autos and agriculture. They can lodge deep in the lungs, contributing to heart problems and sometimes leading to death over longer periods of exposure.
Industry representatives counter that cleaner burning engines are helping reduce smog.
The Environmental Protection Agency said its own analysis, to be released next month, shows ozone levels were down significantly across the country, with many areas seeing their lowest concentrations since 1980.
"You wouldn't realize we have made such incredible progress in reducing pollution from this report," said Joel Schwartz, a visiting fellow at the American Enterprise Institute.
The EPA recently told officials in 31 states they must develop new pollution controls because the air in some of their counties, home to more than 150 million people, does not meet air quality standards.
The EPA said that all or part of, mostly in the eastern third of the country and in California, do not meet the federal health standard for smog-causing ozone. Officials have three years to develop plans to come into compliance.