Charleston County received a failing grade for ozone air pollution in a report released Wednesday by the American Lung Association, but a state official said the air quality here meets national standards.

In last year's association report, the county received a passing grade of "C" for its ozone levels. Those most at risk from air pollution are people with asthma, chronic bronchitis, emphysema, cardiovascular disease and diabetes. There are more than 100,000 people in the county who have at least one of those conditions, according to the report.

"This report is alarming and shows that we really have a lot of work to be done," said Nancy Vinson of the Coastal Conservation League.

The association determination of the county's ozone levels was based on its interpretation of three years of daily air monitoring data done by the state. During that time, the county had 12 "orange days" for high ozone levels, meaning the air was unhealthy for sensitive groups.

"For some people on those days it is unsafe but that's not everybody, although everybody needs to breathe clean air," said June Deen, director of advocacy for the association in Georgia and South Carolina.

Berkeley County received a "C" for ozone pollution. Dorchester County was not included in the report, which is based on data from 2005 to 2007. Spartanburg and Richland counties had the worst ozone pollution. Other counties receiving failing grades for ozone were Abbeville, Aiken, Cherokee, Chester, Darlington, Pickens, Richland, Spartanburg, Union and York.

Myra Reece, chief of the state Department of Health and Environmental Control's Bureau of Air Quality, said that air quality in the Charleston area usually meets national standards, although there are some occasional fluctuations that could affect sensitive groups. "It's two different ways of gauging air quality," Reece said.

Receiving an "F" from the association for ozone does not mean that the county is flunking air quality standards every day, she said. In 10 years, the county had 69 days of ozone levels that were unhealthy for sensitive groups and one day when an alert was issued for all residents.

Ozone, the primary ingredient of smog air pollution, attacks lung tissue by reacting chemically with it. The ozone level found in the upper atmosphere is beneficial because it shields us from much of the sun's ultraviolet radiation. However, ozone air pollution at ground level, where people can breathe it is harmful. The raw ingredients for ozone are produced primarily when fossil fuels, such as gasoline, oil or coal, are burned or when some chemicals evaporate. Scientists have established a relationship between increased ozone levels and higher deaths, according to the association report.

The reports states that 61 percent of Americans live in counties that have unhealthy levels of either ozone or particle pollution. Sources of hazardous air range from school bus emissions to dirty power plants.

"Of particular importance to people living in South Carolina are ocean-going vessels, which will soon become a leading source of air pollution in the United States," the report states. The association supports an effort by the U.S. Environmental Protection Agency to significantly reduce pollution from ships.