If you vape e-cigarettes (or know someone who does), you’ll want to pass on the flavored varieties. Research is finding that the chemicals used to create these flavors damage lung cells…putting vapers at risk for serious respiratory diseases.

Up to 90% of e-cigarettes are flavored, and the variety of flavors keeps growing. Increasingly, the health impact of the chemicals used to create these flavors is coming under scrutiny. For instance, in 2015, Harvard researchers found the chemical diacetyl in 39 e-cigarette flavors they tested. Diacetyl, a synthetic flavor that mimics butter, has been linked to the irreversible lung disease called “popcorn lung,” so-named because it was first found among workers who inhaled the chemical while working in factories that produced butter-flavored microwave popcorn.

More recently, Harvard University’s T.H. Chan School of Public Health looked at another chemical flavoring in e-cigarettes that is used as a safer alternative to diacetyl—2,3-pentanedione. The researchers cultivated cells from healthy human cilia, the fingerlike projections that line the lungs and bronchial passages and help clear them of mucus, debris and unfriendly microbes. They then exposed the cells to diacetyl or to 2,3-pentanedione and analyzed the results.

Results: Both chemicals significantly affected the cilia cells in ways that demonstrated decreased production and function of cilia. Both chemicals also interfered with a gene that breaks down and removes toxins from lungs and interfered with a gene that has been linked to lung cancer. And impaired ciliary function is closely linked to COPD and asthma—suggesting that 2,3-pentanedione is no safer than diacetyl.

Note: These are only two of approximately 25 chemical flavorings used in e-cigarettes considered high-priority respiratory health concerns. And there are thousands of varieties of flavored e-cigarettes.

Of course, the best way to avoid the health risk of flavored e-cigarettes is to not use e-cigarettes at all. But if you do vape—either to help quit regular cigarettes or because you think it’s a “healthier” alternative—stick to kinds that don’t list flavoring on the label.