Even light hookah smoking can cause changes in airway cells, a new study suggests.

“With hookah, smoking a bowl is the equivalent of smoking a pack of cigarettes,” study leader Dr. Ronald Crystal, chairman of the Department of Genetic Medicine at Weill Cornell Medicine in New York, told HealthDay.

“When you talk to the hookah smokers, however, there’s a general belief that it is safer than cigarette smoking. We looked at the airways, lower respiratory tract, and in the blood vessels and found biologic abnormalities in all of those who smoked hookah,” he said.

The study included 21 young adults who had used hookah for fewer than five years, and smoked about three bowls a week. They were compared with 19 peers who were not hookah smokers. The researchers examined cell samples from participants’ airways. They found hookah smokers had changes in the cells lining the lungs. They also had elevated carbon monoxide levels in their blood. One hookah session seemed to expose users to seven to 11 times more carbon monoxide, compared with one cigarette.

One hookah session also gave smokers two to four times the amount of nicotine, 100 times more tar, and 17 times the amount of formaldehyde, compared with one cigarette. In addition, hookah users reported coughing more and bringing up more mucus, the researchers report in the American Journal of Respiratory and Critical Care Medicine.

“This is a small study, but our study results justify initiating large epidemiologic studies to further assess the harmful effects of waterpipe smoking,” Crystal said in a news release. “It is uncontrolled – there are no regulations pertaining to its use – and the data raises red flags that even limited use may cause lung damage.”

Hookah use roughly doubled among middle and high school students in the United States from 2013 to 2014, according to the Centers for Disease Control and Prevention.