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Sitting within half a metre of a smoker on an outdoor patio can expose you to as much secondhand smoke as sitting in a smoky bar, according to a new study.

"The argument is made that smoke dissipates outside so there is no need to regulate," Neil Klepeis, an assistant professor of civil and environmental engineering at Stanford University and co-author of the study, said in an interview.

"We found that it doesn't disappear so rapidly that you can't breathe it in."

The study, which is bound to add fuel to the continuing debate about how strictly smoking should be regulated in public places, is billed as one of the first to examine exposure to tobacco smoke outdoors.

The research, published in the Journal of the Air & Waste Management Association, was conducted by positioning electronic monitors at 10 sites around Stanford, Calif., including sidewalk cafés, pub patios, patios located both in front of and behind restaurants, a park bench and a sidewalk outside the airport.

The monitors measured particles found in smoke, specifically one called particulate matter 2.5. The microscopic particles - 30 times smaller than a human hair - can penetrate deep inside the lungs and cause cancer. In addition to cigarettes, PM 2.5 is produced by burning wood, diesel engines and other forms of combustion.

Dr. Klepeis said that clean air has fewer than 20 micrograms per cubic metre of such particles, but researchers measured plumes of outdoor smoke that had more than 1,000 micrograms.

By comparison, the PM 2.5 produced by motor vehicles was negligible, he said.

Michael McFadden, author of the book Dissecting Antismokers' Brains, dismissed the research as propagandist, saying non-smokers rarely sit in close proximity to smokers and breathe in their smoke.

In a posting on the website of Smoker's Club Inc., he wrote that anti-smoking groups had "come up with some kind of scientific sounding research to justify these outdoor smoking bans."

Wayne Ott, a professor of civil and environmental engineering at Stanford and co-author of the study, said, however, that it is not uncommon for smokers and non-smokers to sit close by, particularly in outdoor settings where smoking is not often regulated.

He said in a release, for example, that "if you're at a sidewalk café, and you sit within 18 inches [45 centimetres] of a person who smokes two cigarettes over the course of an hour, your exposure to secondhand smoke could be the same as if you sat one hour inside a tavern with smokers."

The researchers noted, however, that unlike indoor tobacco smoke, which can linger for hours, outdoor smoke dissipates rapidly when a cigarette is extinguished.

A typical cigarette lasts about 10 minutes. According to the study, a non-smoker can be exposed to pollutant concentrations of 500 micrograms of PM 2.5 during that 10 minutes.

"If you're exposed multiple times to multiple cigarettes over several hours in an outdoor pub, it would be possible to get a daily average of 35 micrograms or more," Dr. Klepeis said.

According to the U.S. Environmental Protection Agency, exposure to 35 micrograms or more of PM 2.5 over a 24-hour period poses a health hazard. That level of exposure can lead to serious health problems, including asthma attacks, chronic bronchitis, irregular heartbeat, non-fatal heart attacks and even premature death in people with heart or lung disease.

The majority of provinces and territories - British Columbia being the notable exception - have comprehensive smoke-free legislation, but they cover only indoor smoking and outdoor smoking within a couple of metres of public buildings. Some municipalities, however, restrict smoking in outdoor settings such as restaurant and bar patios.