While Houston's air quality has improved in some respects, it still gets an "F" for levels of ozone or smog — the sixth worst in the nation.

Those findings were released Thursday in the 15th annual national air quality report by the American Lung Association. The report compiled air pollution data for the most recent years available (2010, 2011 and 2012) and found that while Harris County has improved in the areas of year-round and short-term particle pollution, it still lags in the critical area of ozone pollution, moving up from seventh place last year.

“The air in Houston is certainly cleaner than when the American Lung Association started the State of the Air Report 15 years ago," Jennifer Cofer, interim executive chief officer for the American Lung Association, Plains-Gulf Region, said in a statement. “The continued reduction of particle pollution-soot, smoke, dust-is due to cleaner diesel fleets and cleaner power plants to a large degree as well as some private/public partnerships on projects in Houston such as paving the roads near the Port of Houston. However, Houston still fails to meet the national standards for ozone (smog)."

The most recent findings show air quality declined in 2010-2012 across much of the nation, even though many areas reduced particle pollution, a pollutant recently found to cause lung cancer.

Especially alarming is that levels of ozone, a powerful respiratory irritant and the most widespread air pollutant, are higher than revealed in the previous year’s report. In fact, nearly five out of 10 people live where the air they breathe earned an "F."

The top cities graded "F" for ozone pollution are:

No. 1: Los Angeles-Long Beach, Calif.

No. 2: Visalia-Porterville-Hanford, Calif.

No. 3: Bakersfield, Calif.

No. 4: Fresno-Madera, Calif.

No. 5: Sacramento-Roseville, Calif.

No. 6: Houston-The Woodlands, Texas

No. 7: Modesto-Merced, Calif.

No. 8: Washington, D.C.-Baltimore-Arlington

No. 9: Dallas-Fort Worth

No. 10: Las Vegas-Henderson, Nev.