It's 5:30 a.m. Dan Dollevoet pulls on bike shorts and straps on a helmet while his fellow metro Atlanta commuters fasten their seat belts and adjust the radio dial.

Just before pedaling out of his Acworth driveway into the dark on the 33-mile trek to his Norcross office, he turns on his bicycle lights.

If his timing is good, he might catch sight of a rower cutting through the mist on the Chattahoochee River before he merges with the sea of cars on Holcomb Bridge Road.

"I've been driving the interstates for the last ten years and I'm not really familiar with a lot of different areas," said Dollevoet, 38. "It's kind of neat to get out there and see some of the back roads where it's not so commercial."

Dollevoet, a financial analyst, is usually the lone bicyclist on his long commutes.

The Atlanta-based Clean Air Campaign, a public-private partnership that offers incentives for commuters to try alternatives to driving alone, estimates that 25,000 metro Atlanta commuters bike or walk to work — out of more than 2 million.

Ninety percent of metro commuters usually drive alone to work, according to a survey by the Transit Planning Board, a regional group that lobbies for federal funding. Six percent of 1,001 respondents, who could give more than one answer, said they carpool at least once a week; 3 percent said they take MARTA; 2 percent telecommute; 1 percent take express buses; and 1 percent walk. Bicyclists were too few to count in the survey, even counting those like Dollevoet who bike just two or three days a week.

The numbers help explain why this region is one of the nation's smoggiest. Since Georgia Power started cleaning up its coal-fired power plants in the late 1990s, most of metro Atlanta's smog-forming pollution is emitted from vehicles.

"A lot of people are just unaware that their solo daily driving is bad for the environment," Clean Air Campaign spokesman Brian Carr said. "You're trying to undo a mentality of an entire region that's so dependent on cars."

Thursday also marks the start of smog season, when bad-air forecasters warn parents and schools to keep children inside on certain hot afternoons. Between now and Sept. 30, metro Atlantans can expect orange- or red-alert warnings for bad air on about 30 days, if this year is like the past two.

Those are days when man-made pollution mixes with hot temperatures to form high levels of ground-level ozone, an ingredient in the region's toxic smog soup that scars lungs.

Runners, walkers and bicyclists are advised to work out before noon on high smog days. And people with asthma and other respiratory or heart problems are better off avoiding all outdoor activity. A National Research Council panel that advises Congress recently found strong evidence that people are dying from illnesses caused by breathing ozone.

Rebecca Serna, executive director of the Atlanta Bicycle Campaign, said people joining her 570-member biking advocacy group are commuting by bike to be green.

"The growth right now is people worried about the environment, and they want to reduce their carbon footprint," Serna said.

That's one reason Bob Knoll started riding his bike last year 10 miles from his home in Roswell to his job in Alpharetta. The other was to get in shape. He said he's lost 30 pounds and "my stress level is down when I get to work and I'm happier. ... I look forward to the commute."

Two other factors ease Knoll's commute: Most of his ride is along Alpharetta's 6-mile Big Creek Greenway, a multiuse trail, and his office has showers and a locker room.

Dollevoet's motivation was to lose weight and get in shape. He's lost 25 pounds and wants to lose another 25.

"I am not generally a 'Let's save the Earth' kind of guy. We recycle at home, but I'm not biking so I emit less carbon dioxide. I'm commuting because I know it's good for me."

Dollevoet plans to continue his one-way bike commute to or from work several times a week, even when smog levels hit the red zone. He said 95-degree heat is more likely to stop him.