Researchers measured levels of toxic substances in urine samples from hookah users before and after they smoked in hookah bars. The average time spent smoking a hookah was just over an hour for most participants.

"There was a 73-fold increase in nicotine levels and a substantial increase in cancer-causing compounds in the urine immediately after smoking," St. Helen said.

In the United States, hookah bars are flourishing. In 2007, maybe 300 hookah bars were in business. Today, searching "hookah bar" on Yelp will yield you nearly 400 hits in Los Angeles, 500 in New York, and even 17 in Louisville, Kentucky.

Despite anti-smoking legislation in more than 35 states, hookah bars continue to thrive because of loopholes and laws that exempt or don't specifically address waterpipes, says Thomas Carr, the director of national policy for the American Lung Association.

Three-fourths of 100 U.S. major cities have laws banning cigarette smoking in bars -- but nearly 90 percent have loopholes that make it easy for hookah lounges to set up shop, according to a study published in the American Journal of Public Health in September 2012.

Some areas in the United States, like San Francisco, have curbed the practice. Roughly 73 municipalities and some states across the country have specifically banned hookah, says Bronson Frick, associate director of Americans for Nonsmokers' Rights.

The FDA also proposed new rules in April that would allow the agency to regulate the manufacturing and marketing of hookah and other tobacco products.