Why is it Nasty?
PEG stands for Polyethylene glycol. PEGs increase the absorption of chemicals through the skin, especially those PEGs with a lower molecular weight. This is of concern as many products contain a mixture of chemicals, not all of which are completely benign. Moreover, the manufacturing process of PEG leaves an impurity in the ingredient called 1,4-dioxane which is a known carcinogen. PEG can facilitate the absorption of 1,4-dioxane into the skin causing potentially negative health side effects.
How do I find it?
Typically, you will see PEG listed with a number such as PEG20 or PEG100; these numbers represent molecular weight.