ATLANTA — 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.