Propylene GlycolUpdated a month ago

A type of alcohol commonly used in skin care and cosmetics. It is a controversial ingredient associated with skin irritation. Numerous patch tests on humans have shown that it has a very low risk of causing a skin reaction even at concentrations as high as 20%. Since some people may still be sensitive to it, we decided not to use it in our products.

Sources: 
https://online.personalcarecouncil.org/ctfa-static/online/lists/cir-pdfs/PR560.PDF
https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.0105-1873.2005.00693.x