I think in the end Mike Petro may be correct in believing it may ultimately come down to temps used. I don't understand all of it, but enough that it seems a valid theory.
Just a Clarification:
The work that Mike has done is in regards to the Formation of Formaldehyde and Acetaldehydes in e-Cigarette vapor.
And in those regards, Yes... Coil Temperature is the Major Factor.
New studies find carcinogens in vg and pg at high temps, even in tootle puffers