Also known by the acronym PPG, polypropylene glycol is a polymer, or large molecule, of propylene glycol. Like propylene glycol, it is used as a humectant and a delivery agent in cosmetics. Despite reports that it can be a skin sensitizer, researchers have found that it does not present a health risk for people when used in cosmetics.
References for this information:
Critical Reviews in Toxicology, April 2013, issue 4, pages 363-390