Butylene Glycol
Butylene glycol is rated Clear. The only primary score identified in this audit is Fulton's 1989 rabbit-ear score of 1 on a 0–5 scale, a grade his own published key files under "not significant," and no independent legacy replication was identified.
Common humectant/solvent; non-comedogenic.
What it is
Butylene glycol is a small synthetic diol — a short molecule with two alcohol groups — that formulators reach for mainly as a humectant and a solvent. As a humectant it holds water in the product and helps keep it at the skin's surface; as a solvent it dissolves other ingredients and thins the feel of a formula, which is why it turns up in so many water-based textures. The 1985 Cosmetic Ingredient Review safety assessment lists its cosmetic functions as humectant and solvent and describes it as a water-miscible liquid of low oral and cutaneous toxicity. That report's own use survey put concentrations of the period in the low single digits up to around ten percent — historical survey data rather than a current measurement, and separate from supplier guidance, which we have not sourced. One point of identity is worth stating plainly, because it matters lower down. The cosmetic ingredient labelled Butylene Glycol is 1,3-butanediol. Other butanediol isomers exist, and the older literature does not always record which one it tested. On a label it reads simply Butylene Glycol.
Why we rate it Clear
We rate butylene glycol Clear, at Low confidence, and the confidence is low for a specific reason: the rating rests on a single rabbit-ear assay and nothing beside it. That assay is James Fulton's 1989 survey in the Journal of the Society of Cosmetic Chemists, the source of nearly every comedogenic number in circulation. We read its Table I off the journal scan. Butylene glycol is there, and it scored 1 on Fulton's 0–5 scale. The table prints the name without an isomer, so attributing that 1 to the modern INCI ingredient, 1,3-butanediol, is a reasonable reading rather than something the paper states. His method was the standard one for the period: the ingredient diluted to roughly 10% in propylene glycol, applied to the inner ear of three New Zealand albino rabbits five days a week for two weeks, the follicles then scored for keratosis. The number matters less than it looks, and Fulton says so himself. His published key reads: a grade of 0 to 1 "is not considered significant"; 2 to 3 is "borderline"; 4 to 5 is "considered positive." A 1 sits inside the band its own author defined as insignificant. So the single measurement we identified for this ingredient is a low score that Fulton would not have called a finding — and that is the whole of the positive evidence pointing toward Clear. There is no second reading to set beside it. Morris and Kwan's 1983 assay — the other study that scored ingredients on the same 0–5 scale, in a different laboratory, and the one that contradicts Fulton elsewhere — has no butylene glycol row at all. Fulton, Pay and Fulton's separate 1984 rabbit-ear paper, on its own 0–5 scale whose key reads 1–2 as not significant and 3 or above as positive, prints no butylene glycol row either, and Nguyen, Dang and Maibach's 2007 rabbit-ear study, on a 0–4 scale, does not name it among its fourteen test materials. We found it in none of the other legacy documents we hold. So "Clear" here does not rest on two labs agreeing. It rests on one low number from one rabbit-ear study, and the honest version of the rating says exactly that.
What the evidence doesn’t tell you
Everything above rests on a rabbit's ear. Three animals, the ingredient at about 10% in propylene glycol, applied to the inner ear — not a human face, not a finished formula, and not the vehicle or the exposure a cosmetic gives. The rabbit-ear model has been contested for decades: Draelos and DiNardo's 2006 re-evaluation in the Journal of the American Academy of Dermatology found isolated rabbit-ear ingredient scores a poor guide to what finished products do on human skin, and a 2025 JAAD Reviews survey of the field concluded that no standardised comedogenicity test exists and that "non-comedogenic" is an unregulated label. Those objections apply to a reassuring 1 exactly as much as to an alarming 4. We identified no human comedogenicity study of butylene glycol — no trial that counted comedones on human skin after it was used. The Clear rating is therefore an inference from one low animal score and the absence of any signal against it, not a positive human result. Two things routinely folded into this question are not about pores, and we are keeping them separate. First, allergy. There are published case reports of allergic contact dermatitis to butylene glycol, including a 2014 report of a reaction to 1,3-butylene glycol (PubMed 25182293) and a 2020 report on butylene glycol in cosmetics (PubMed 32342510). Allergic contact dermatitis is an immune reaction — redness, itch, an eczema-like patch — and it is a different endpoint from comedone formation; it neither raises nor lowers a comedogenic score. It is uncommon, but it is real, and it is simply not what "comedogenic" measures. Second, the confident claims you will read in both directions — that butylene glycol "may clog pores," or that it is "low-comedogenic and ideal for acne-prone skin" — trace to no primary comedogenicity source we could find, either way. We are not repeating them. One smaller caveat of identity: Fulton's table prints "Butylene glycol" without naming the isomer. The cosmetic ingredient is 1,3-butanediol, and we are taking that to be what he tested — the reasonable reading, but not something the paper states.
Where you’ll see it
Butylene glycol turns up across water-based skincare: toners and essences, lightweight serums and gels, many lotion and "water cream" bases, sheet-mask fluids and plenty of cleansers, where it does the quiet work of holding water and carrying other ingredients into solution. Because it belongs to the solvent-and-humectant base rather than to the actives, it often sits high on an ingredient list — but list position reflects how much water phase a formula needs, not a dose that tells you anything about pores, and we are not going to read one off the other. On a label it is Butylene Glycol.
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 source of the only comedogenicity score we identified for this ingredient, and the origin of nearly every comedogenic score in circulation. Read off the journal scan rather than through a list quoting it. Method: the ingredient diluted to roughly 10% in propylene glycol, applied to the inner ear of three New Zealand albino rabbits five days a week for two weeks, follicles scored for keratosis under a micrometer. Table I scores "Butylene glycol" 1 on a 0–5 scale, and Fulton's own key files 0–1 as "not considered significant." It is a rabbit-ear assay, not a human study, and Fulton called the whole survey "not at all definitive but simply designed to stimulate research."
- Fulton JE Jr, Pay SR, Fulton JE 3rd. "Comedogenicity of current therapeutic products, cosmetics, and ingredients in the rabbit ear." J Am Acad Dermatol 10(1):96–105 (1984) An earlier, separate rabbit-ear study from the same lead author, on its own 0–5 scale whose key is not the 1989 key: 1–2 is read as not significant and 3 or above as positive, so its numbers cannot be carried across to 1989 numbers without each paper's own key. Cited for a verified absence: it prints no butylene glycol row, so it adds no second reading in either direction.
- Nguyen SH, Dang TP, Maibach HI. "Comedogenicity in rabbit: some cosmetic ingredients/vehicles." Cutan Ocul Toxicol 26(4):287–292 (2007) A later rabbit ear-canal study on a 0–4 scale — a third scale, easy to misread as Fulton's. Its abstract names all fourteen test materials (esters, fatty alcohols, hydrocarbons, one butter and one surfactant). Cited for a verified absence: butylene glycol is not one of them.
- Cosmetic Ingredient Review. "Final Report on the Safety Assessment of Butylene Glycol, Hexylene Glycol, Ethoxydiglycol, and Dipropylene Glycol." J Am Coll Toxicol 4(5):223–248 (1985) An expert-panel safety assessment, cited only for what it can support: what butylene glycol is and does in a formula — a humectant and solvent, a water-miscible liquid of low oral and cutaneous toxicity. It is a safety review, not a comedogenicity study, and it says nothing about follicular plugging. Safety is a different question from comedogenicity, and we are not using one to answer the other.
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 2026-08-12 · How we decide