Propylene glycol

No ingredient on this site has a reputation resting on a thinner reed than this one, and the reed is a shared word.

The claim, repeated everywhere, is that propylene glycol is antifreeze. The reality is that antifreeze is traditionally ethylene glycol, a genuinely poisonous alcohol responsible for real poisonings every year, and propylene glycol is the substance the industry switched to when it wanted an antifreeze that would not kill a dog that licked the driveway.

The two share a family name and a rough shape. They do not share a toxicology.

What the regulation says about it

The US definition is unusually candid: propylene glycol “does not occur in nature”, and is manufactured by treating propylene with chlorinated water to form a chlorohydrin, which is then converted to the glycol.

We are quoting that deliberately, because a fair reading of this page has to include the part that sounds least reassuring. It is a synthetic compound made by an industrial route, and the regulation says so without dressing it up. What follows from that is a separate question from what it is made of, and the answer to that question is well established rather than hidden: it has been assessed repeatedly, it is used in medicines and vaccines as a solvent, and the body metabolises it along a pathway that ends in ordinary metabolic products.

The contrast with Glycerin is instructive: glycerin sounds natural and can be animal-derived, and propylene glycol sounds industrial and is entirely synthetic. Neither of those facts is the safety question, and mistaking origin for safety is the error this whole site exists to unpick, set out in the guide on hazard and exposure.

The reason to actually know the name

Contact allergy. Propylene glycol is a recognised skin sensitiser for a minority of people, and it is in an enormous number of products: creams, deodorants, wipes, hair products, and the topical medicines someone with irritated skin is most likely to be using.

That produces a specific and frustrating pattern. Skin flares, the person switches products, the new products contain it too, and nothing improves. If your skin reacts to things without an obvious pattern, this is one of the names on the shortlist worth checking against, which is the subject of the page for skin that reacts to everything.

Children and medicines

Propylene glycol appears in liquid medicines as a solvent, including ones given to children, and this is the one context where quantity has been examined closely rather than casually. Regulators have looked at intake in infants specifically, because a small body and a repeated dose is a different exposure from an adult meeting it in a moisturiser.

The practical version for a parent is narrow: this is a question about a prescribed medicine and its dosing, which belongs with the prescriber, and not a reason to scan a shampoo. The page for parents keeps that proportion, and it is here.