What is actually in e-cig juice? Hard to say. There are zero standards, controls, labeling requirements. In at least one test, undesirable stuff was found.
FDA: Electronic cigarettes are a no-no | Booster Shots | Los Angeles Times
What does inhaling the vapor do? In the long run, no-one knows. In the short run, at least immediately after inhaling, vaping appears to have negative effects on lung function.
Electronic Cigarettes Harm The Lungs