What are PEGs and Glycol? The main 3 ingredients used in skincare that are part of the Glycol family are Propylene Glycol, Butylene Glycol and Polyethylene Glycol (also knows as PEG). We believe the ones really to avoid are Propylene Glycol and Polyethylene Glycol. There is much talk about these being the basis of antifreeze (not true – Ethylene Glycol is often used in anti-freeze and never used in skincare). Glycols come in very different molecular sizes, hence the number that often precedes a PEG reference in an ingredient list, ie PEG-100, PEG-9. The smaller the number, the smaller the molecular weight and the easier it is for the PEG to penetrate the skin. This penetration is why you should be careful of the PEGs you apply to your skin, especially if you have undesirable ingredients in that skincare.