ATLANTA (AP) — First it was bars, restaurants and office buildings. Now the front lines of the "No Smoking" battle have moved outdoors.

City parks, public beaches, college campuses and other outdoor venues across the country are putting up signs telling smokers they can't light up. Outdoor smoking bans have nearly doubled in the last five years, with the tally now at nearly 2,600 and more are in the works.

But some experts question the main rationale for the bans, saying there's not good medical evidence that cigarette smoke outdoors can harm the health of children and other passers-by.

Whether it is a long-term health issue for a lot of people "is still up in the air," said Neil Klepeis, a Stanford University researcher whose work is cited by advocates of outdoor bans.

Ronald Bayer, a Columbia University professor, put it in even starker terms.

"The evidence of a risk to people in open-air settings is flimsy," he said.

There are hundreds of studies linking indoor secondhand smoke to health problems like heart disease. That research has bolstered city laws and workplace rules that now impose smoking bans in nearly half of the nation's bars, restaurants and workplaces.

In contrast, there's been little study of the potential dangers of whiffing secondhand smoke while in the open air. But that hasn't stopped outdoor bans from taking off in the last five years. The rules can apply to playgrounds, zoos, beaches and ball fields, as well as outdoor dining patios, bus stops and building doorways.

"Secondhand smoke is harmful. It's particularly harmful to children," said Councilwoman Mary Cheh of the District of Columbia, one of more than 90 U.S. municipalities or counties considering an outdoor smoking law.

But is it really dangerous outdoors?

Federal health officials say yes. Studies have clearly established that even a brief exposure indoors to cigarette smoke can cause blood to become sticky and more prone to clotting. How long that lasts after just one dose isn't clear, officials say. The best-known studies so far have measured only up to about a day afterward.

Repeated exposures are more dangerous, and can worsen your cholesterol, increase the odds of plaque building in arteries, and raise the risk of chest pain, weakness, or heart attack.

Health officials say there's no reason to think that can't happen from breathing in smoke outdoors.

"There's no risk-free level of secondhand smoke," said Brian King, an expert on secondhand smoke with the Centers for Disease Control and Prevention.

However, it's hard to pin down the health effects of outdoor smoke. There have been some studies — fewer than a dozen — that tried to measure how much secondhand smoke can be found outdoors. Some have found levels that rival what people may breathe indoors, depending on which way the wind is blowing or whether there's an overhang or sheltered area that can trap smoke. One study detected significant fumes as far as 44 feet away from a smoker.

"If you can smell it, it's obviously there," said James Repace, a Maryland-based scientist-consultant who's done some of the outdoor studies.

Two small studies tested about two dozen nonsmokers at a smoky outdoor dining area in Athens, Ga. The saliva tests detected significant jumps in cotinine, a substance produced when the body metabolizes nicotine.

That doesn't mean it's causing chronic illness, though. Repace thinks only two kinds of people may face a serious health risk outdoors — those with severe asthma and staff at outdoor cafes where smoking is allowed.

Indeed, health advocates in some places have focused on sites like sidewalk cafes, feeling they can't make the case for beaches or open-air parks.

"We only get involved where there's a real health risk," said Flory Doucas, co-director of Canada's Quebec Coalition for Tobacco Control.

Still, cigarette smoke in a place like a park can be a nuisance to some, just like loud music or dog waste.

That's how Roger Montiel of Atlanta views it.

"I don't really like the smell and I don't like it blowing in my face. If I'm enjoying a day at the park, I'd rather it not be part of that experience," he said while walking through a downtown park recently.

That's good enough reason for outdoor bans, Repace said. "People don't have to be dropping dead for you to regulate something," he said.

Well, not so fast, said Simon Chapman, an Australian researcher. He once won a prestigious American Cancer Society award for his anti-smoking efforts and formerly was editor of a medical journal focused on smoking's dangers. But in recent years he's become a vocal opponent to wide-ranging outdoor smoking bans.

He and Bayer worry such bans are really motivated by desires to make smoking seem like an unusual, socially unacceptable behavior. Ban proponents see that as a worthy goal; Chapman thinks it's a bad precedent.

"Next you might say 'Let's not just stop there. Let's not have people smoke anywhere they might be seen'" he said. "And then is it legitimate to say that any behavior that people don't like should be disapproved of because people might see it."

In Atlanta, a city council member decided to act after an encounter with a smoker in a park. A ban on smoking in the city's parks went into effect last summer. Technically, a violation could result in a fine of as much as $1,000. But — as in other cities — Atlanta officials have not arrested anyone.

"Enforcement generally has been someone says 'put that out,' and they put it out," said George Dusenbury, Atlanta's parks and recreation commissioner.

At Woodruff Park, a 6-acre downtown hangout, nearly a dozen smokers could be spotted in the park on a recent, sunny Friday morning. The regulars said they knew about the rules, but found ways to get around them.

"Smoke rises. I don't see a reason why it should bother other people out here," said Tommy Jackson, 55, lighting up with a friend at the edge of a paved footpath through the park.

Park worker Rufus Copeland said he's seen only a small drop in smoking since the green and white "Smoking Prohibited" signs went up last year. He steers smokers to the sidewalk rimming the park. But people still smoke. "It's hard," he said.

Brianna Mills, a 26-year-old nursing student from suburban Marietta, sat down for a quiet moment in the park with her Newports, unaware of the ban.

"It's supposed to be a free country," said Mills, who developed her habit 10 years ago. "It's like: 'Where can you smoke?'"