This article is from way before ecigs were around. I'm not sure if posting a link is allowed and my apologies if they aren't.
Im not bashing ecigs, they helped me quit smoking. I was just oblivious to what PG was and never really researched it. It's pretty toxic stuff though, but so are cigarettes and the polluted air we are constantly exposed to.
I don't know of any studies to prove anything indicating the effects but I just wonder I how it couldn't be harmful. Thanks for your insight...
https://www.atsdr.cdc.gov/phs/phs.asp?id=1120&tid=240.