http://www.dermaxime.com/propylene_glycol.htm
Propylene glycol (PG, Propan-1,2-diol, Propylenglycolum)
This much talked about product, together with all other glycols and glycerol, is a humectant or humidifying agent, solvent and delivery ingredient used in cosmetics.
Since it can penetrate the outermost layer of the skin and carry the active ingredients into deeper layers of the epidermis it is an extremely common ingredient in cosmetics and is found in a variety of products.
In the cosmetic industry propylene glycol is used in very small amounts to keep products from melting or freezing in extreme temperatures and assists the active ingredients in a product to penetrate the skin.
Propylene glycol further enhances the performance of Sodium PCA in absorbing and retaining moisture in the skin. It is a synthetic compound and some individuals may find that it irritates the skin if used in high concentrations.
Only minute amounts of propylene glycol are added to cosmetic preparations, which is nowhere near the estimated acceptable daily intake of 25 mg per kg as established in the 17th Report of the Joint FAO/WHO Expert Committee on Food Additives, 1974.
If a too high concentration is used, it can be irritating to the skin, but the way in which we use it, it poses no problem of possible skin irritation.