Many celebrities have credited e-cigarettes for helping them quit smoking. But actress Katherine Heigl might have brought the most attention to vaping with her appearance on "The Late Show With David Letterman" in 2010.

She and Letterman smoked from her e-cig.

"This oughta get it done," said Letterman. "I know," agreed Heigl. "You have no excuse now to smoke a real cigarette."

It turns out that's not the positive public health message it seemed to be — especially when it comes to teens and vaping.

The exotic-seeming, caloric, sweet-flavored e-cigs have become a high school craze. A 2016 report from the U.S. surgeon general found a 900 percent increase in e-cigarette use by high school students from 2011 to 2015. And the 2016 National Youth Tobacco Survey reported that 1.7 million high school students had used e-cigarettes in the previous 30 days.

Those kids are getting bombarded by many harmful chemicals that are produced during vaping.

A new study published in Pediatrics analyzed urine samples from 103 teens, 67 of whom used only e-cigarettes, 16 of whom smoked regular and e-cigarettes, and 20 of whom smoked neither.

They found that teens who vaped had three times the levels of metabolites of chemicals such as acrylonitrile, acrolein, propylene oxide, acrylamide, and crotonaldehyde in their urine than those who didn't. Several of those are known carcinogens.

So, while you're teaching your kids about the dangers of smoking cigarettes (it's working; smoking rates are dropping), make sure they understand that vaping is no better.