There are 70 years of research on PG. As it is the base material used in nebulizers for lung transplant patients we can assume none of the research proved too negative.
Dr E. doesn't seem able to make his mind up. Or perhaps he is doing a balancing act, one day consulting with e-cigarette users, the next making placatory noises to the pharma industry. That's where the money comes from in his line of work, so it doesn't pay to annoy them. People like Prof. Siegel who make very clear positive statements about e-cigarettes do so at considerable risk to their future prospects for employment.
Dr E. doesn't seem able to make his mind up. Or perhaps he is doing a balancing act, one day consulting with e-cigarette users, the next making placatory noises to the pharma industry. That's where the money comes from in his line of work, so it doesn't pay to annoy them. People like Prof. Siegel who make very clear positive statements about e-cigarettes do so at considerable risk to their future prospects for employment.