Propylene Glycol
In plain terms
A clear, syrupy liquid that holds moisture and helps ingredients dissolve. It shows up almost everywhere, from lotions to foods to medicines.
What the evidence shows
Propylene glycol is one of the most widely used ingredients in personal care, and the American Contact Dermatitis Society named it Allergen of the Year for 2018. The reason was not that it is a strong sensitizer, since it turns up in only about 2 to 3 percent of patch-test allergy cases, but that it is so common that anyone allergic to it has a hard time avoiding it. It acts as both a weak sensitizer and an irritant, which makes patch tests hard to read. For most people it does nothing at all; for the small group sensitized to it, it is worth knowing by name. It is also approved for food and recognized as safe to eat.
On the label
Listed as propylene glycol, PG, or 1,2-propanediol. It is often confused with ethylene glycol, the poisonous antifreeze ingredient; they are different substances, and propylene glycol is the one regulators treat as safe to eat.