Atlanta drivers are all too familiar with the Georgia Department of Transportation electronic signs warning, "Ozone Alert Day." The warning flashed on 35 days last year. On 22 of them, ozone hit unhealthy levels.

The problem prompted Gov. Zell Miller to sign an executive order requiring all state agencies, departments and universities in the 13-county area to submit a detailed ozone action plan by March 31. The order called for plans that would reduce single-occupancy vehicle trips by 20 percent.

"Our long-term goal is to do that all summer every summer, whether there is an ozone alert or not," Miller said. "Our aim is that every state employee carpools or takes mass transit or telecommutes at least one day a week as a common practice."

Plans to start such things as telecommuting, alternative work schedules and incentives to use transit are being finalized.

"There are 13 counties in the metro area that don't meet the EPA ground-level ozone standards," said Pam Earl of the Georgia Environmental Protection Division.

The EPD has a team of forecasters who meet every day to forecast the next day's ozone levels. If they believe the ozone will reach unhealthful levels, they designate the next day an "Ozone Action Day."

The warnings had an impact. "On some Ozone Action Days, we saw a reduction in peak-hour traffic," said Jeane Pierce, who coordinates the EPD Voluntary Ozone Action Program.

Ozone is a colorless gas. It forms when nitrogen oxides and volatile organic compounds combine in sunlight. The ozone "season" is May 1 through September 30.

"Generally, our ozone concentrations are highest during June, July and August," said Jerry Walker, a plant pathologist with the University of Georgia College of Agricultural and Environmental Sciences. "This year we had some in May."

May 13 was Atlanta's first ozone action day this year.

Pollutants from cars, trucks and other sources merge with hot, stagnant air to create high ozone levels. Ground-level ozone is a serious lung irritant.

High ozone levels can cause breathing difficulties in the elderly, children, athletes, people who work outside and those with respiratory problems. It makes outdoor activities unhealthy.

"I think we should be concerned," Walker said. "This is the worst U.S. pollutant we have. Atlanta contributes to this through the number of automobiles we have."

Walker worries, too, about plants in high ozone levels. For the past 24 years, he has followed ozone levels closely. He has found that some plants can't take it.

"For example, tulip trees (yellow poplars) and grapes have a hard time," he said. "We found some damage to wild black cherry trees. And if those high ozone levels came earlier in the summer -- say in June or July -- we could have seen more damage to trees and to flowers such as petunias."

Walker is studying how ozone affects plants in the new Envirotron at the Georgia Experiment Station in Griffin, Ga. The state-of-the-art facility has special growth chambers to help researchers study environmental effects on plants.