Wikipedia or other reference sites can give you the full lowdown on PG. You will find it in many foods, medicines, and toiletries, so you are already consuming it on a daily basis. “Smoke” at concerts and such is created by vaporizing PG. It is “generally recognized as safe” to inhale by the FDA. However, small portion of the population is sensetive to PG.
You may be puzzled that PG is also anti-freeze. Most anti-freeze products were made of ethylene glycol many years ago. PG is now favored because it is dramatically less toxic than EG. However, if you’re still opposed to inhaling anti-freeze, then by that same logic you shouldn’t ingest salt either.
PG is responsible for transporting the flavouring mainly but also contributes to throat hit. VG produces more visible vapor than PG but mutes flavour and throat hit. PG is thinner than VG; the ratio of PG and VG will affect the wicking properties of your e-liquid,
Generally, standard pre-manufactured atomizers work well with 70/30. Vapers who use more advanced gear such as RBAs and RDAs for building their own atomizers often prefer higher amounts of VG.