File: Dust and pollution in the air near I-10 and Pinal Air Park looking west with the Silverbell Mountains in the background Monday November 10, 2008.

Benjie Sanders/Arizona Daily Star file photo

Pima County's air quality gets an "A" for particle pollution but a "C" for ozone, the American Lung Association says in a new report.

Ozone, also known as smog, is the most widespread pollutant in the U.S.

The annual State of the Air report for 2013 gives Pima County's air the same marks as the year before. However, the report uses numbers that are a couple of years old; the 2013 report covers the years 2009-11. The 2012 report covered 2008-10.

While Pima County officials are pleased at the A grade for particulates, they are concerned about ozone levels. They say the county remains on the cusp of violating the federal ozone standard.

Moreover, the county's odds of violating that standard could rise if the Environmental Protection Agency decides to tighten the rule, said Beth Gorman, a program manager for Pima County's Department of Environmental Quality. The EPA must decide by December whether to propose toughening the long-contested national ozone standard. The EPA had proposed that during President Obama's first term, but Obama shelved the proposal in 2011 under industry protest.

While the air quality here is good on the majority of days, Tucsonans still must be vigilant because unhealthy air days still occur occasionally, primarily for ozone, Gorman said.

"A lot of people are affected. They call here and say, 'I'm worried. I know you put out an air quality advisory, and I'm concerned about my health,' " Gorman said. "A lot of callers want to know if they can go outside" on high ozone days, which usually occur in summer.

Ozone is a tricky pollutant to control since it isn't emitted from a single source, Gorman said. It's formed when nitrogen oxide - from vehicles and power plants - and volatile organic compounds - from gasoline fumes, auto fluids, lighter fluid, etc. - are combined with sunlight and heat.

The new report also says:

• For ozone, Pima County had four "orange" days in 2009-11, up from three in 2008-10 and 2007-09. An orange day, meaning the air is unhealthy for members of sensitive groups to breathe, occurs when clean air standards are exceeded but not formally violated.

• For particulates, the county not only had no orange days during 2009-11, and it's had none since 2006. Particle pollution comes from dirt roads, power plants, vehicles, heavy equipment and industry.

• Nationally, the county slipped to having the 18th-lowest year-round particle pollution levels in 2009-11 from the 15th lowest in 2008-10. Tucson also fell in national rankings for year-round particle pollution among cities, from the fourth lowest in the 2012 report to the eighth lowest in the new report.

• Phoenix's air stayed bad, with Maricopa County keeping its longstanding "F" ratings for ozone and particle pollution. The number of orange ozone days in that county dropped from 34 in 2008-10 to 31 in 2009-11; its orange particle days rose from two to seven.

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Contact reporter Tony Davis at or 806-7746.