Hookah smoking is a popular pastime for college students, but it comes with some of the same risks as cigarette smoking.

Smoking hookah may be bad for your health, especially if you do it more than once a week.

Research published today in Cancer Epidemiology, Biomarkers & Prevention shows that young adults who smoke at hookah bars have elevated levels of nicotine and other dangerous compounds in their urine. More time spent puffing on the water pipe results in higher levels of cancer-causing compounds.

A hookah is a type of pipe used to smoke tobacco. It is particularly common in the Middle East, and has been around for centuries. When the smoke is inhaled from the tobacco bowl, it is cooled in a water chamber before being inhaled.

As smoking hookah grows in popularity among American college students, researchers like lead study author Gideon St. Helen of the Center for Tobacco Control Research and Education at University of California, San Francisco (UCSF) want people to know that the habit may not be as harmless as it seems. He studied 55 healthy hookah smokers ages 18 to 48 who smoked hookah at bars in the San Francisco area.

As many as 40 percent of U.S. college students smoke hookah, according to St. Helen’s study. He said that he even saw a family with children smoking a hookah together at a Persian restaurant in the state of Georgia.

The level of nicotine in urine after an hour of sharing a hookah containing a bowl of tobacco is about the same as after smoking a cigarette, St. Helen told Healthline. But cancer-causing agents such as benzene also spike, in some cases to even higher levels than after smoking a cigarette.

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Eight people in the study were flagged as “suspected cigarette smokers” due to high nicotine levels in their urine, but they remained in the study. Forty percent of study participants also admitted to having smoked marijuana in the past month.

UCSF researcher Dr. Neal Benowitz co-authored research last year that highlighted the danger of benzene in hookah smoke. Volatile organic compounds (VOCs) and polycyclic aromatic hydrocarbons (PAHs) are the byproducts of the tobacco mixing with the charcoal briquette in a hookah pipe.

St. Helen told Healthline that his new research builds on these findings because it measures hookah intake in a more casual, applicable setting—the hookah bar.

Benowitz said last year that a once-a-week hookah smoker would not be likely to become addicted to nicotine. But in the new study led by St. Helen, the researchers noted, “Although the addictiveness of water pipe tobacco smoking is not established, nicotine levels reported here are likely to cause physiologic changes in nicotine acetylcholine receptors in the brain that would sustain nicotine addiction.”

Jed Rose, director of the Duke Center for Smoking Cessation, told Healthline, “It is not surprising that inhaling hookah smoke, which is obtained from burning tobacco, would lead to increased levels of the same chemicals associated with cigarette smoking. And cigarettes smoking does present a risk for both disease and addiction.”

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What gets people hooked on hookah? While St. Helen’s research suggests it could be nicotine, others argue that it’s the combination of the pleasant smell of the tobacco and the chit chat of the bar.

Hookah tobacco usually comes in flavored varieties like apple and grape. Like e-cigarettes, those who oppose hookah smoking often claim it is a vice intended to woo children.

Mike King owns and operates Hickey Brothers Cigar Store and Hookah Lounge in Rock Island, Ill. He told Healthline that on Thursday nights his business is teeming with students from local colleges. For $15, they can smoke one bowl of tobacco in a hookah for an hour. “It’s like a Starbucks,” King said. “These are non-smokers, occasional cigarette smokers, from 18-year-olds to 60-year-olds. They’re not doing it every day.”

He said it’s no secret why his business is successful. “Here’s why I win at this game. There are 18- to 20-year-olds who have no place to go other than the movies or the bowling alley. They can’t get into their bar because they’re too young and they can’t get into a teen club because they’re too old.”

Instead of being “stuck in the dorm while their older friends are at bars,” they come to the hookah bar and hang out, usually once a week, King said.

“Once they turn 21 I usually never see them again,” he said. “ Do you want to talk about what’s addictive? When I owned a bar, the people I saw the first day were the people I saw the last day. They never went away.”

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