The U.S. battle against smoking has moved outdoors. City parks, public beaches, college campuses and other outdoor venues are putting up signs telling smokers they can't light up. Outdoor smoking bans have nearly doubled in the last five years to nearly 2,600.
But some experts say there's not good medical evidence that cigarette smoke outdoors can harm the health of children and others around.
Hundreds of studies link indoor secondhand smoke to health problems like heart disease. That research has supported city laws and workplace rules that now impose smoking bans in nearly half of the nation's bars, restaurants and workplaces.
Studies have clearly established that even a brief exposure indoors to cigarette smoke can cause blood to become sticky and more prone to clotting. 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 specify the health effects of outdoor smoke.
"If you can smell it, it's obviously there," said James Repace, a 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 Georgia. 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.
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.
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.
"People don't have to be dropping dead for you to regulate something," Repace said.
Not so fast, said Simon Chapman, an Australian researcher. He once won a prestigious American Cancer Society award for his anti-smoking efforts. But in recent years he's become a vocal opponent to wide-ranging outdoor smoking bans.
He worries such bans are motivated by desires to make smoking seem like an unusual, socially unacceptable behavior.
"Next you might say 'Let's not just stop there. Let's not have people smoke anywhere they might be seen'" he said.