I was using mercury as an example to counter your claim that heavy metals don't vaporize at a relatively low temp. Like the temps produced by a vaporizer. Plus, we don't know what heavy metals make up the 5.0PPMs allowed in USP Glycerin. We do know, or should know from a simple google search, that at least some heavy metals are carcinogens. But I'm sure you checked before you dismissed my claim - because you said you did.
http://www.researchgate.net/publica...s_on_Human_Health/file/79e415082be0ce7530.pdf
I would expect levels of mercury in seafood flavored e-liquids, particularly tuna.
Canned Tuna May Contain Excessive Mercury - Scientific American
So yeah, FDA approved = safe. Not.