E-cigarettes are not FDA approved. They are not regulated and have not been fully studied for safety and long-term health risks. Although e-cigarettes are likely safer than smoking cigarettes, we know nothing at all about long-term effects of vaping. The manufacturing process is not yet regulated and we are still unsure of the health impact of inhaling the nicotine containing liquid, often flavored, directly into the lungs.
A 2015 study at Harvard tested 51 e-cigarette flavors and found that 39 of the 51 flavors contained diacetyl, a chemical commonly used to add flavorings like butter, caramel, strawberry and butterscotch. These flavorings are linked to bronchiolitis obliterans (a degenerative and irreversible respiratory condition also known 'popcorn lung') as well as other severe respiratory diseases. In bronchiolitis obliterans, small airways become inflamed and scarred, resulting in narrowing of the airways. (1)
We have much to learn about the effect of the vapor on users and others. Many research studies are being done and we should know more in a couple of years.
1. Allen, et al. Flavoring Chemicals in E-Cigarettes: Diacetyl, 2,3-Pentanedione, and Acetoin in a Sample of 51 Products, Including Fruit-, Candy-, and Cocktail-Flavored E-Cigarettes. Environ Health Perspect DOI: 10.1289/ehp.1510185