I've asked this before in other threads, but how do people expect to be shown that e-cigs are safe to use for 20+ years without them being used for 20+ years? How can researchers possibly prove it one way or the other? Really, all researchers can do is test extreme exposure on lab animals, which still doesn't always show if something is safe or not for humans. Then all they can do is test the ingredients for known carcinogens or known toxicity and estimate that they would be ok if those levels are low.
The fact of the matter is, there is no way to know the long-term effects of e-cigarettes until they have been used by humans long-term.
Consider how sacchrin was considered dangerous for human consumption AFTER it had been on the market for years and now it has been taken back off the danger list and is considered safe again. Or how Chantix was approved and it's dangers only came out after it had real-life use. I know people want reassurance, but I'm convinced that there is really just no way to know the long-term effects of vaping with any lab tests. Unforunately that makes us, as first generation vapers, the guinnea pigs. But eventually someone is going to have to be.