Lung damage in e-cigarette users might be caused by toxic chemicals in the vape liquid, according to new research. More than 800 vapers have fallen ill in recent months and at least 17 people nationwide have died from vaping-related illnesses. The cause is still unclear—but researchers from the Mayo Clinic have found the lung damage they’ve seen looks similar to a chemical burn. “It seems to be some kind of direct chemical injury, similar to what one might see with exposures to toxic chemical fumes, poisonous gases and toxic agents,” Dr. Brandon Larsen told CNN. However, researchers still don’t know exactly which chemicals could be causing the injuries. “Based on what we have seen in our study, we suspect that most cases involve chemical contaminants, toxic byproducts, or other noxious agents within vape liquids,” said Larsen.