Metro Atlanta and dozens of other urban regions across the country fail to meet the new Clean Air test announced last week by the U.S. Environmental Protection Agency. The regions will have at least five years to meet the new standard for ground-level ozone, a component of smog.

The new standard is 75 parts per billion. That's down about 11 percent from the old limit of 84 parts per billion. EPA scientists determined the current standard was not sufficiently protective of public health. Ozone, which is formed when sunlight mixes with industrial and vehicle emissions, can trigger asthma attacks. It also can cause other respiratory problems, particularly among children and the elderly.

This is the fourth ozone standard since 1971.

Despite cleaner fuels, cleaner car engines and pollution controls on coal-fired power plants, metro Atlanta has not yet met the old standard, which replaced the original ozone standard in 2005. The region's deadline to meet the old standard, which is still in effect, is 2010. Most of the other regions still in violation of the old standard are in California, Texas and the Northeast.

Dr. Michael Chang, a senior research scientist at Georgia Tech who is part of the air quality forecasting team that issues smog alerts, recently discussed the new smog standard.

Chang said metro Atlanta experiences about 30 days of violations in an average year under the old standard, or about one bad-air day out of every five during the smog season, from May 1 to Sept. 30. Under the new standard, he estimates that number will rise to 53 days, or about one every third day.

Q: Has metro Atlanta's air quality improved?

A: It's hard to say whether there's any real trend going on. ... How much is due to weather and how much is due to controls that we've initiated? That's always sort of a fuzzy area. ... Saying that we're making progress is probably a little overambitious.

Q: How will metro Atlanta meet the new ozone standard, especially since given that the region has yet to meet the old one?

A: We've done the cheap and easy things and every part per billion we're going to get is going to be much more expensive than the last part per billion that we achieved. We're running up against some marginal cost curves that makes the next ton much more expensive than the last ton of emissions to reduce. ... The real vexing question is what do we do next or what can we do next.

Q: What will the new standard mean for metro Atlanta?

A: It means that we have to continue to be vigilant and not lose ground where we are. We've always been challenged by growth in our area that has offset gains that we've made technologically or by changing our approach to how we conduct our business. We're challenged just not to backslide when we have 100,000 people move here every year. ... How are we going to provide their mobility, their energy needs? ... How are we going to manage that and still get better?

Q: What has been done so far to improve air quality?

A: Burn bans have been put in place in the northern two-thirds of the state. Some industrial controls have been put in place. ... A national fuel standard is in place. We're benefitting from all that. We've got new engine rules for both heavy-duty diesel as well as small engines.