LAWRENCEVILLE - Although metro Atlanta already recorded two code-orange smog days in March, the official season to change fueling and exercise times begins Tuesday.

Michael Halicki, a spokesman for the Clean Air Campaign, said there's no way to predict how many smog-alert days the region will see in 2007. But warm, dry temperatures increase the possibility of ground-ozone and particle pollution levels that make it harder to breathe.

The worst year, he said, was 1999, when the levels exceeded federal standards 69 times. Last year, that number was 30 days and levels were in the teens for the three years before that.

"We're starting to go into drought conditions, and when there's rain, there's a relatively mild smog season," Halicki said. "It's cooling things down, but praying for rain isn't exactly the best strategy."

Days are rated as "good," "moderate," "unhealthy for sensitive groups," "unhealthy," "very unhealthy" and "hazardous" on a color scale that ranges from green to purple. Orange is unhealthy for sensitive groups.

Halicki said the name smog season is a misnomer, taken by Atlantans to mean any kind of air pollution. In fact, smog is a combination of smoke and fog and the pollutants in air here are twofold.

One is ground-level ozone, an unstable compound that is created when nitrogen oxides combine with hydrocarbons.

Nitrogen oxides are created by things that burn - like lightning, vehicles and power plants - and hydrocarbons are present in things with strong odors, like gasoline and oil-based paints.

Because they are less likely to bind together in cool weather, people are urged to buy gasoline in the evenings from May until September, Halicki said. Exercising outside is better in the morning.

Halicki said the pollutants are present year round, but adding summer heat is like putting them in an oven.

"When the sun goes down, the ozone levels quickly dissipate," he said. "It breaks into its component parts. Ozone won't stay in ozone form."

The other measure, particle pollutants, are about one-30th the diameter of a human hair. They get caught in people's lungs and are responsible for systemic health problems, but ground-level ozone causes day-to-day illness in people with asthma and causes breathing problems in others, he said.

On smog days, the number of children with asthma who visit the emergency room is up 37 percent, Halicki said.

Changing transportation behavior - carpooling, taking public transportation or reducing car trips - is one element of the push to reduce the number of smog days, Halicki said. Cleaner fuels and technology are also important.

"We haven't really improved significantly, but we haven't really gotten worse," he said. "But smaller concentrations have a greater impact than what we previously thought. We need to do better than the status quo."

To sign up for smog alerts or for more information on smog season, visit www.cleanaircampaign.com.