It depends. There is nothing in ecigs that isn't in real cigs. Nicotine is probably the most dangerous thing in ecigs but you are using none. PG, VG, water, alcohol if any - should all be safe. The dicey part is the flavorings. Most flavorings should be totally safe to vape. But there are some, mainly flavors with butter or custard flavor ingredients that contain diacetyl. Diacetyl is safe to eat but anecdotally not safe to inhale - no studies have been done but diacetyl is in popcorn fake butter flavoring and some guys in a popcorn factory got lung cancer or something and it got blamed on diacetyl. So even though there is only a tiny amount of diacetyl in some flavors, if you want to totally avoid diacetyl you should buy juices from a company that says its flavorings are diacetyl free. I mostly just avoid custardy flavors unless the company says they are diacetyl free.