This year, 16 of the 34 counties in New York state with air quality monitors received failing grades, compared with 19 out of 33 counties with monitors in 2010. Most counties also have experienced fewer days of unhealthy air.

The Bronx, home to some of the New York region's most important expressways, has some of the state's worst air.That's the good news from State of the Air 2011, the American Lung Association's twelfth report, released this week.

The bad news is that more than nine million New Yorkers -- nearly half the state's population -- live in counties where unhealthy air endangers their health.

"These results show that the Clean Air Act and other clean air laws are working," said Sandra Kessler, Interim President and CEO, American Lung Association in New York. "To ensure all New Yorkers breathe healthy air, it is our job to make sure that Congress doesn't weaken the Clean Air Act and that state government doesn't roll back important clean air regulations."

The report used the most recent EPA data, collected from 2007 through 2009, from official monitors for ozone and particle pollution, the two most widespread types of air pollution. Counties are graded for ozone, year-round particle pollution and short-term particle pollution levels. The report also uses EPA's calculations for year-round particle levels

The Bronx, in New York City, was the only county in the state to receive an F for short-term particle pollution, while Suffolk County on Long Island again had the worst ozone pollution in the state.

Rensselaer and Dutchess shared the dubious distinction of being the only two counties in the state to experience more unhealthy ozone days than during the prior year.

At the other end of the spectrum, Essex and St. Lawrence ranked among the top 25 cleanest counties nationwide for both annual particle pollution, and short-term particle pollution.