TORONTO - A new fad is threatening to undercut the hard-fought gains of laws that have placed strict limits on smoking in public, experts say.

The smoking of hookahs -- waterpipes evocative of Cairo's Kasbah or a Saharan oasis -- is surging in popularity among young adults, research suggests.

And in a number of places, laws aimed at keeping bars and restaurants cigarette-free don't ban the aromatic smoke swirling from these exotic waterpipes, which are sometimes known as shisha, narghile or hubble bubble.

"From a public health standpoint, we really do need to nip this in the bud before we've got a hookah lounge on every corner," says Pippa Beck, a policy analyst with the Non-Smokers Rights Association.

Beck notes in Ottawa, where she is based, more than a dozen restaurants or bars offer hookahs.

"It really is a problem, and certainly for young people who otherwise wouldn't smoke cigarettes. They seem to think this isn't a big deal."

A new study on hookah use by young adults in Quebec suggests Beck's assessment is on track. The study was published Monday in the journal Pediatrics.

The work, by a team at the University of Montreal that has been studying smoking in Quebec teens for years, shows that about 23 per cent of 871 study participants reported smoking a waterpipe at least once in the previous year.

While most reported smoking hookahs only on rare occasions, about four per cent of the participants said they smoked hookahs at least once a month.

Though anti-smoking laws are making cigarette smokers virtual pariahs, paradoxically hookah smokers can and do indulge in bars and restaurants.

In some cities, clean air laws cover only the smoking of tobacco -- which sometimes provides an out for hookah bars that claim they offer tobacco-free product. In other places, exemptions are given to this type of establishment.

"The thing that bothered me about it, actually, is its visibility," says Jennifer O'Loughlin, the senior author of the study.

"In a country that is banning smoking cigarettes in so many public places and smoking is becoming so contrary to the social norms, are we going to let waterpipe replace smoking in terms of visibility?"

The craze isn't just sweeping Canada. Hookah use is on the rise throughout the United States and elsewhere, says Dr. Wasim Maziak, an epidemiologist who studies tobacco addiction at the University of Memphis in Tennessee.

"It's still under the radar currently. But I think there's a kind of major awakening," Maziak says, pointing to the fact that the U.S. National Institutes of Health have started funding research into hookah use.

"This is spreading so fast.... People now understand it's really a major public health threat."

Hookah smoking has been practised for centuries, but was on the wane in the last century, Maziak says. "It used to be old men in the Middle East smoking this raw tobacco."

But in the 1990s, the trend reversed. Maziak puts the change down to the fact that while in days gone by shisha smokers had to prepare their own blends, commercially prepared product became available, in a range of flavours such as apple and even Earl Grey tea.

What goes into it often isn't clear. Beck says some claim to contain no tobacco, but it can be almost impossible to verify that without doing laboratory analyses. Product labelling is either minimal, indecipherable or non-existent.

"Some is labelled, some isn't. It's hard to tell what's in it. The labelling might be in Arabic," she says.

Beck says she knows of some public health units in Ontario that have analyzed some shisha products to see if they actually contains tobacco. Some do, others don't seems to be the conclusion.

But even if there is no tobacco, the practice of drawing smoke into the lungs is not a healthy one, Beck says. O'Loughlin concurs.

"Waterpipe smoke contains harmful constituents -- nicotine, carbon monoxide, carcinogens. And it may contain greater amounts of tar and heavy metals, including cobalt, chromium and lead," she says.

"Waterpipe use has in fact been linked to lung cancer, heart disease, infectious disease and pregnancy related complications. So that's a list that would certainly raise my anxiety."

Another thing that worries people watching the trend is the fact that while most North American hookah smokers don't indulge daily, they get a high dose of smoke when they do. A session generally lasts over an hour and people partaking often take many hits off the pipe.

A World Health Organization report on waterpipe smoking in Egypt suggests the smoke exposure in a hookah session there could be equivalent to that of 100 to 200 cigarettes. Maziak says his team has analyzed blood samples taken from waterpipe smokers, finding "very high levels" of nicotine.

Experts worry that people using hookahs don't fully understand the risks, buying into myths that the water in the pipes filter out dangerous components in the smoke.

O'Loughlin says this trend is one to which regulators need to start paying attention.

"Here's a product that is not regulated in Canada or the United States that people are turning to," she says. "And I think at a minimum it warrants monitoring surveillance to make sure that its prevalence doesn't increase over time."