This is one of the most common moisture carrying ingredients other than water used in cosmetics because it supposedly moisturizes better than glycerin.
Used as a fragrance ingredient, wetting agent, surfactant and penetration enhancer; it's supposed to enhance absorption and promote spreading of the product on the skin.
Propylene Glycol is an emollient similar to ethylene glycol and both are used in anti-freeze and other household products. It's classified as a skin irritant and associated with causing dermatitis and hives.
Extensive animal studies have shown reproductive effects at moderate doses and endocrine disruption at high doses, with mutation results on mammalian cells.
If exposed to the eyes, as when it is found in eye creams for example, it can cause mild conjunctivitis.
People with low kidney function for whatever reason are unlikely to be able to process Propylene Glycol efficiently and should avoid exposure.
In a 2007 University of Connecticut study it has also been found to be dangerous for people with liver problems. What were the specific liver conditions were not specified.
Pregnant women and women who are breastfeeding should be especially careful to avoid using products containing this ingredient.
It has already been documented that it can cause varying degrees of neurological issues, including stupor, convulsions and other unspecified "severe mental symptoms". This has been seen when it was taken orally, and so when on the skin the severity of adverse effects might be modified slightly, but the cumulative effect of daily use of skincare products containing this ingredient may prove to be as harmful as when ingested, or close to it