In the association's annual State of the Air report, Mercer County received an "F" grade for ozone pollution.

The Sharon metropolitan area ranked 44th worst in the nation for air quality, the report said. Twenty-five other Pennsylvania counties and 391 counties in the nation also received failing grades for ozone air pollution, a news release said.

Lawrence and Lycoming counties were the only two in the state to receive passing grades this year, the release said.

According to the report, the health of 9.8 million Pennsylvanians is potentially at risk because ozone-smog levels are at dangerous levels, particularly during the summer months. Half the country's population is breathing unhealthy amounts of air pollution, it said.

Of the adults in Mercer County: 6,000 have asthma, 4,300 have chronic bronchitis and 1,600 have emphysema, the report said. The report also said 1,500 children suffer asthma attacks and 23,000 have emphysema. People who work or exercise outdoors are also particularly at risk.

The State of the Air report examines the most recent, quality-assured data about ozone-air quality from the Environmental Protection Agency, the release said. The report grades and ranks counties on how often their air quality exceeds "unhealthful" categories of the EPA's Air Quality Index for ozone air pollution.

Ozone exposure may lead to shortness of breath, chest pain when inhaling deeply, wheezing and coughing, the release said. Long-term exposure can result in reduced breathing ability and increases the risk of respiratory disease later in life.

The areas with the worst pollution were Los Angeles-Riverside-Orange County, Calif.; Bakersfield, Calif.; Fresno, Calif.; Visalia-Tulare-Porterville, Calif.; Houston-Galveston-Brazoria, Texas; Atlanta; Merced, Calif.; Knoxville, Tenn.; Charlotte-Gastonia, N.C.; Rock Hill, S.C.; and Sacramento-Yolo, Calif.