Kevin Shapiro, a 20-year-old math and physics major at the University of Pennsylvania, first tried a hookah at a campus party. He liked the exotic water pipe so much that he chipped in to buy one for his fraternity house, where he says it makes a useful social lubricant at parties.

Like many other students who are embracing hookahs on campuses nationwide, Mr. Shapiro believes that hookah smoke is less dangerous than cigarette smoke because it “is filtered through water, so you get fewer solid particles.”

“Considering I don’t do it that often, once a month if that, I’m not really concerned with the health effects,” he added.

But in fact, hookahs are far from safe. And now, legislators, college administrators and health advocates are taking action against what many of them call the newest front in the ever-shifting war on tobacco. In California, Connecticut and Oregon, state lawmakers have introduced bills that would ban or limit hookah bars, and similar steps have been taken in cities in California and New York. Boston and Maine have already ended exemptions in their indoor-smoking laws that had allowed hookah bars to thrive.

The ornate glass and metal water pipes are used for smoking an aromatic blend of tobacco, molasses and fruit known as shisha. A 2008 study of 3,770 students at eight universities in North Carolina found more than 40 percent had smoked a hookah at least once, only slightly lower than the percentage who had tried a cigarette at least once.

But researchers say the notion that water filters all the harmful chemicals in tobacco smoke is a myth. So, too, they say, is the idea that because hookah smoking is an occasional activity, users are inhaling much less smoke than cigarette users.

Many young adults are misled by the sweet, aromatic and fruity quality of hookah smoke, which causes them to believe it is less harmful than hot, acrid cigarette smoke. In fact, because a typical hookah session can last up to an hour, with smokers typically taking long, deep breaths, the smoke inhaled can equal 100 cigarettes or more, according to a 2005 study by the World Health Organization.

That study also found that the water in hookahs filters out less than 5 percent of the nicotine. Moreover, hookah smoke contains tar, heavy metals and other cancer-causing chemicals. An additional hazard: the tobacco in hookahs is heated with charcoal, leading to dangerously high levels of carbon monoxide, even for people who spend time in hookah bars without actually smoking, according to a recent University of Florida study. No surprise, then, that several studies have linked hookah use to many of the same diseases associated with cigarette smoking, like lung, oral and bladder cancer, as well as clogged arteries, heart disease and adverse effects during pregnancy. And because hookahs are meant to be smoked communally — hoses attached to the pipe are passed from one smoker to the next — they have been linked with the spread of  tuberculosis, herpes and other infections.

“Teens and young adults are initiating tobacco use through these hookahs with the mistaken perception that the products are somehow safer or less harmful than cigarettes,” said Paul G. Billings, a vice president of the American Lung Association. “Clearly that’s not the case.”