Some of the liquids used in e-cigarettes may in fact have highly toxic effects on human cells tested in a lab, a new study reveals.

Researchers in North Carolina found that some of the ingredients used in so-called “e-liquids” are more toxic than nicotine alone and many of them are more toxic to human cells than the main ingredients in these liquids, which are propylene glycol and vegetable glycerin, according to the findings, published in the journal Plos Biology.

E-cigarettes work by heating a liquid solution composed of propylene glycol, vegetable glycerin, flavorings and typically nicotine, until the liquid vaporizes.