E-cigarettes—which contain nicotine in a liquid form and are “vaped,” not smoked—have been marketed as a safe alternative to traditional cigarettes. But e-cigarettes are still so new that scientists are only beginning to piece together their effects on health, especially among the youngest vapers.

This week, a new study has found that vaping might be linked to respiratory problems in teens who pick up an e-cig habit.

Researchers at the Keck School of Medicine at the University of Southern California analyzed responses from more than 2,000 11th and 12th graders in a long-running study on children’s health. They compared the lung health of those who were past or current e-cig users with that of teens who had never vaped, looking at reports of symptoms such as persistent cough, congestion, phlegm, and bronchitis.

The researchers found that when compared with those who never tried e-cigarettes, the risk of respiratory symptoms was almost twice as high among past users, and more than twice as high among current users. And the more teens vaped, the higher their risk.

Although it’s not entirely clear which component of e-cigarettes might pose a respiratory risk, there are a few possibilities. “Metals, glycerol from the e-liquid carrier, and nicotine all have potentially toxic effects on the lung,” said Rob McConnell, M.D., who led the study. The flavorings that are so popular among teens are also potentially hazardous, he added.

More research in this area is needed to confirm these findings. Other factors—secondhand tobacco smoke exposure and socioeconomic differences, for two—might have played into some of the elevated respiratory risk among current e-cig users (though did not explain the problems in past users).

But the new evidence corroborates scattered case reports that have also linked e-cigs to respiratory problems in some users. McConnell plans to continue following these teens to see whether the association between e-cig use and compromised lung health continues into adulthood.

“Teenagers may be unaware of the potential respiratory and other dangers of e-cigarettes, rationalizing that they are a safer alternative than cigarettes,” notes Consumer Reports’ medical director, Orly Avitzur, M.D. “We need to help educate our teens that they are likely risking their health by smoking e-cigs.”

According to the Centers for Disease Control and Prevention, the use of e-cigarettes has increased in recent years, particularly among young people. In 2015, 16 percent of high school students reported using the devices, compared with just 1.5 percent in 2011.