I did some little digging in google scholar after seeing your interesting post and found that e-cigarettes have slightly lower toxicity profile than tobacco cigarettes. But as old wisdom goes.. its better not to smoke at all if you want 100% assurance that e-cigarettes are safe. My take on this is no they are NOT safe if you take this for long-term even if it has small level of nicotine and other additives in it. But I have to say that there is no any definitive data or extensive research work to support whether it is good or bad. Till then keep vaping because this is a good way to quite smoking if you are a heavy smoker. Here is a nice review paper on : Will chronic e-cigarette use cause lung disease?