Propylene Glycol
Propylene glycol is rated Clear. Fulton's 1989 rabbit-ear assay scored it 0 out of 5, and the same assay dissolved most of the ingredients it tested in propylene glycol — so the substance that carried nearly every comedogenic number in circulation scored nothing on its own. The number holds, but it rests on one legacy assay, so we hold it at Low confidence.
Common humectant/solvent; non-comedogenic.
What it is
Propylene glycol is 1,2-propanediol, a small, water-soluble diol that works as a humectant, a solvent and — depending on the formulation and what is being carried — a penetration aid. It dissolves things that are hard to dissolve — actives, fragrance, preservatives, botanical extracts — and it holds water, so a formulator reaches for it to keep a product from drying out and to carry ingredients that would otherwise sit undissolved. The Cosmetic Ingredient Review's 2012 assessment records it across a very wide range of cosmetic categories at concentrations up to about 50%. On a label it is Propylene Glycol. One distinction has to be made before anything else on this page, because it is where the error would come from. Propylene glycol is not its esters. Propylene Glycol Monostearate and Propylene Glycol Isostearate are separate ingredients — the diol reacted with a fatty acid — and they carry their own, higher legacy numbers (Morris and Kwan scored propylene glycol isostearate 3–4 out of 5). Those are different materials with different rows. This page is about the plain diol.
Why we rate it Clear
We rate it Clear at Low confidence, and the Low is doing real work — read the limits below before taking the 0 as settled. The number, and its scale. Fulton's 1989 rabbit-ear survey in the Journal of the Society of Cosmetic Chemists prints one row for this exact material: "Propylene glycol — 0," on a 0–5 scale; his earlier 1984 survey, now read, prints it 0 as well. By Fulton's own published key, "a minimal grade of 0 to 1 is not considered significant." So within that assay, propylene glycol registered as nothing. There is a row for the exact material, it is a 0, and it carries no asterisk and no range. The thing worth knowing about that 0. Propylene glycol is not only a substance Fulton scored — it is the substance he dissolved most of the others in. The rabbit-ear method he and his contemporaries used diluted a test ingredient to roughly 10% in propylene glycol before applying it to the ear (what preparation he used when propylene glycol was itself the article, the table does not say — we are not going to assume he diluted it in itself). So the carrier that delivered a great many of the comedogenic numbers now in circulation scored 0 as a test article itself. Fulton's Table II makes the same point from the other side: the dye D&C Red #36 scored 3 in mineral oil, 1 in propylene glycol, and 0 in PEG 400 — the vehicle moves the result, and propylene glycol sits at the inert end of that spread. We are not going to claim Fulton designed the 0 as a control for his own method; he does not say that. We are only reporting what the tables print: the standard vehicle, scored on its own, came back at 0, and the vehicle experiment puts it near the bottom. That is the whole of the comedogenicity evidence for the plain diol, and it is why the rating stops at Low. One legacy rabbit-ear number is not much to stand on, and a 0 for the material that also served as the method's own solvent is reassuring but not an independent, hard-won finding.
What the evidence doesn’t tell you
There is one measurement, it is a rabbit's ear, and the test article is also the assay's own solvent. Fulton's general method diluted a test ingredient to roughly 10% in propylene glycol before applying it to three New Zealand albino rabbits' inner ears, five days a week for two weeks, scoring the follicles for keratosis. What preparation he used when propylene glycol itself was the article — neat, or some other control condition — the table does not record, so the exact row-specific method is not established. It is not a human face, not a finished formula, and not a realistic use concentration. Draelos and DiNardo's 2006 re-evaluation in the Journal of the American Academy of Dermatology tested finished products on human skin and found the isolated rabbit-ear score a poor predictor of what a product does on a person; a 2025 JAAD Reviews survey of the field concluded that no standardised comedogenicity test exists at all. Those objections apply to a reassuring 0 exactly as much as to an alarming 4. We identified no independent comedogenicity measurement of plain propylene glycol. Morris and Kwan (1983), the other rabbit-ear assay on the same 0–5 scale, tested propylene glycol isostearate — an ester — not the diol, so it does not corroborate the number. We also identified no human comedogenicity or comedone-count study of propylene glycol. The 0 is a single legacy figure, quoted onward by the consumer charts; it is not four sources agreeing. And the largest caveat is about the wrong endpoint. Propylene glycol has a substantial dermatology reputation, but it is not a pore reputation. The American Contact Dermatitis Society named it Allergen of the Year for 2018, and the accompanying systematic review in Dermatitis documents it as both a weak sensitiser and an irritant. That is allergic and irritant contact dermatitis — redness, itch, stinging, patch-test reactions — which is a different thing from follicular plugging. Someone can react badly to propylene glycol for reasons that have nothing to do with comedones, and a comedogenic score cannot speak to that risk in either direction. Irritancy is not comedogenicity, and allergy is not comedogenicity. The CIR's safety assessments, which conclude it is safe for use up to 50%, are likewise safety findings, not comedogenicity findings, and we cite them only for what they are.
Where you’ll see it
Propylene glycol is one of the more common ingredients in cosmetics, in creams, lotions, gels, cleansers, serums, deodorants and hair products, where it holds water and dissolves what water cannot. It also appears in medications and food. The CIR records its cosmetic use at concentrations up to about 50%. On a label it reads Propylene Glycol. It has a second life this page cares about more than most: it is the laboratory vehicle the legacy comedogenicity assays ran on. When a chart tells you an oil or ester "scored 4 in Fulton," the 4 was measured on an ingredient dissolved in this. That is not a reason to worry about propylene glycol — it is the reason its own score is a 0.
Sources
Each source says what it actually is. Several widely-cited “sources” for comedogenic ratings are republishing the same 1989 assay, and counting them as independent agreement is how a thin evidence base gets made to look thick.
- Fulton JE. "Comedogenicity and irritancy of commonly used ingredients in skin care products." J Soc Cosmet Chem 40:321–333 (1989) The primary source of nearly every comedogenic score in circulation, read off the journal scan rather than via a list quoting it. Method: the test ingredient diluted to roughly 10% in propylene glycol, applied to the inner ear of New Zealand albino rabbits, three rabbits per assay, five days a week for two weeks, follicular keratosis scored 0–5. Table I prints one row for this exact material — "Propylene glycol — 0" — and Fulton's key defines 0–1 as "not considered significant." His Table II records the same dye scoring 3 in mineral oil, 1 in propylene glycol and 0 in PEG 400. A single rabbit-ear assay, and one in which propylene glycol is the method's own vehicle; cited as the sole comedogenicity figure for the plain diol, not as independent corroboration.
- Morris WE, Kwan SC. "Use of the rabbit ear model in evaluating the comedogenic potential of cosmetic ingredients." J Soc Cosmet Chem 34:215–225 (1983) An independent rabbit-ear assay on the same 0–5 scale, six years before Fulton 1989. Cited here for what it does NOT contain: it has no row for plain propylene glycol. It scored propylene glycol isostearate — a fatty-acid ester, a different material — at 3–4. It therefore does not corroborate the diol's 0, and naming it as though it did would be the material-substitution error this site exists to catch.
- Fiume MM, Bergfeld WF, Belsito DV, et al. "Safety Assessment of Propylene Glycol, Tripropylene Glycol, and PPGs as Used in Cosmetics." Int J Toxicol 31(5 Suppl):245S–260S (2012) An expert-panel safety review, and the source of the composition and use-concentration figures on this page. It re-affirms the 1994 conclusion that propylene glycol is safe for use in cosmetics at concentrations up to 50%. A SAFETY assessment, not a comedogenicity assay: it says nothing about pores, and is cited only for what it is and how the ingredient is used.
- McGowan MA, Scheman A, Jacob SE. "Propylene Glycol in Contact Dermatitis: A Systematic Review." Dermatitis 29(1):6–12 (2018); with the accompanying editorial Jacob SE, Scheman A, McGowan MA. "Propylene Glycol." Dermatitis 29(1):3–5 (2018) The American Contact Dermatitis Society's Allergen of the Year 2018 designation and its supporting systematic review. Cited to keep an endpoint straight, not to support the rating: these document propylene glycol as a weak sensitiser and an irritant — allergic and irritant contact dermatitis — which is a different clinical endpoint from comedogenicity. They are the reason propylene glycol has a reputation, and the reason that reputation is not about pores.
- acne.org comedogenic ingredient list A consumer chart that lists propylene glycol at 0. Cited as one of the places a reader will have met the number, not as evidence for it: it is downstream of the same legacy rabbit-ear assays, not an independent measurement.
Others in the same family
Humectants behave similarly enough that the evidence for one is often wrongly read across to the others. These are its structural relatives, not a guess. What the evidence says about humectants as a family ›
Last reviewed 2026-07-13 · How we decide