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Which is worse? Smoky bars or busy streets?

Which is more harmful to your health — a smoky bar or a city street filled with diesel truck fumes? Well, you might want to skip your next happy hour.

Smoky bars and casinos have up to 50 times more cancer-causing particles in the air than highways and city streets clogged with diesel trucks at rush hour, according to a study that also shows indoor air pollution virtually disappears once smoking is banned.

Conducted by the researcher who first showed secondhand smoke causes thousands of U.S. lung cancer deaths each year, the study found casino and bar workers are exposed to particulate pollution at far greater levels than the government allows outdoors.

“This paper will help localities pass smoking bans,” predicted the author, James Repace, a biophysicist who works as a secondhand-smoke consultant after spending 30 years as a federal researcher. “It shows how beneficial smoking bans are for hospitality workers and patrons.”

Ventilation systems can't keep up

Repace tested air in a casino, a pool hall and six taverns in Delaware in November 2002 and in January 2003, two months after the state imposed a strict indoor smoking ban.

“They are the most dangerous” substances in secondhand smoke, said Repace, a visiting assistant clinical professor at Tufts University School of Medicine in Boston.

Repace said his research also showed that ventilation systems — sometimes touted by tavern, restaurant and casino groups as an alternative to smoking bans — cannot exchange air fast enough to keep up with the smoke.

The study, published in the September issue of the Journal of Occupational and Environmental Medicine, was partly funded by the nation’s largest philanthropic organization devoted to health care, the Robert Wood Johnson Foundation of Plainsboro, N.J.

The eight indoor places had an average PPAH level of 134 nanograms, or billionths of a gram, per cubic meter — five times the level in the air outside. By comparison, the average rush-hour levels of PPAHs on Interstate 95 in Wilmington and in Boston’s Roxbury neighborhood, heavily polluted by diesel and truck emissions, were 7 and 18 nanograms, respectively.

After the smoking ban took effect, levels of both cancer-causing substances dropped 90 percent or more in all of the indoor places tested, with the air quality nearly indistinguishable from outside air.

“It demonstrates really clearly that a smoking ban results in a massive improvement in air quality,” said Dr. Jonathan Foulds, director of the tobacco dependence program at University of Medicine and Dentistry of New Jersey’s School of Public Health. “Here in New Jersey, and in many other states that don’t have an indoor smoking ban, this should be used to put pressure on the legislators.”

Timothy Buckley, associate professor of environmental health science at Johns Hopkins Bloomberg School of Public Health in Baltimore, said other research has shown dramatic air quality improvement after smoking was banned in workplaces, but this appears to be the first study in bars or casinos.

“The magnitude of that effect is striking,” Buckley said.

As of July 1, a total of 727 U.S. municipalities had some smoking restrictions, with 312 banning smoking even in bars and restaurants, according to the nonprofit American Nonsmokers’ Rights Foundation.

Delaware, New York and Massachusetts prohibit smoking in all workplaces, restaurants and bars. California and Connecticut have similar bans, but with exemptions for workplaces with five or fewer employees.