LOS ANGELES (AP) – More than half the nation’s population lives in counties with hazardous smog levels.

The annual American Lung Association study says about 159 million Americans, or 55 percent of the country, are threatened by air that’s heavily polluted with ozone or tiny particles of soot, known as particle matter.

While Maine received good grades for particle pollution, the state fared poorly in most of the nine counties monitored.

Cumberland, Hancock, Knox, Penobscot and York received grades of F; Kennebec got a D, and Oxford a B. There was insufficient data to grade the last two.

Sagadahoc and Piscataquis counties.

“This report is important because it once again confirms that air pollution continues to pose a health threat to Maine people,” said Edward Miller, CEO of the Maine association.

Janice Nolen, the association’s director of national policy, said strides have been made to cut pollution, particularly ozone contamination, but many people remain at risk for smog-related illnesses like asthma, cardiovascular disease and lung inflammation.

“We still have a serious air pollution problem across the country. It’s cleaner … but it’s not clean enough,” Nolen said.

Ozone pollution occurs when hydrocarbons and nitrogen oxides – released when fossil fuels burn or chemicals evaporate – combine with heat and sunlight.

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.

AP-ES-04-29-04 1451EDT