Suffolk County has the worst smog pollution in New York and for the 14th consecutive year received an "F" grade in an American Lung Assocation report evaluating air quality.
Nassau County does not monitor for ozone, the main component in smog, and was not rated. Air quality in neighboring Queens County, which measures the invisible gas, fell to an "F" this year from a "D" last year.
"Suffolk County has remained near the bottom of the list of counties with bad ozone grades for the life of this report," said Michael Seilback, vice president of public policy and communications for the American Lung Association of the Northeast.
Ozone is created by emissions from cars and other sources in a reaction with sunlight. Topography, weather and other factors can affect levels.
During the study from 2009 to 2011, Suffolk reported 28 days when ozone levels were unhealthy for sensitive groups such as people with asthma. Three days were unhealthy for all populations.
"It's not just a matter of the air pollution produced locally," Seilback said. "It also has to do with air pollution coming to Long Island from other places."
Suffolk's unhealthy days have decreased dramatically since a spike recorded between 1997 and 1999, said Janice Nolen, lead author of the study and assistant vice president for national policy and advocacy at American Lung Association.
Despite the bad ozone report, Suffolk and Nassau join six other New York counties that experienced lower levels of particle pollution, the tiny bits of acids, metals, chemicals, soil and dust pushed into the air by car exhaust, coal-powered plants, wood burning and other activities. Both counties earned an "A."
Air quality across the country has improved since the association began its yearly reports, but an estimated 131.8 million people -- the equivalent of four in every 10 residents -- are living in counties with unhealthy conditions, said Jeff Seyler, president and chief executive of the American Lung Association of the Northeast.
Inhaling ozone and particulates can cause wheezing, shortness of breath, an increase in risk for developing respiratory diseases, diminished lung capacity and death.
The group says cleaning up power-plant, carbon and vehicle emissions would improve air quality.