The Phoenix metropolitan area exceeded the federal air-quality standard for ozone levels on 28 separate days this year, compared to none in 2007.
Air-quality officials say the amount of ozone in the skies didn't fluctuate much year over year but what has changed is how much of the pollutant the Environmental Protection Agency says is acceptable.
The EPA lowered the federal health standard in March from its previous level of 0.08 parts per million to 0.075 ppm.
The EPA estimates that as many as 345 counties nationwide will not be able to meet the new standard, although it is not expected to make official non-attainment designations until 2010.
It's likely that Maricopa will be among those counties classified as marginal, a designation reserved for those areas just barely meeting the standard.
If that's the case, officials will be given three years to submit plans for bringing the region into compliance, EPA air analyst Niloufar Glosson said.