Cyclopentasiloxane
Cyclopentasiloxane is a volatile silicone rated Clear, and the 0 that follows it around is not its own. We identified no comedogenicity assay of D5 itself. The 0 was scored against "cyclomethicone", the retired umbrella name for a family of cyclic silicones that D5 belongs to but was never singled out from — neighbour context for this material, not a measurement of it.
No assay of D5 we can find; the 0 belongs to cyclomethicone, an umbrella term.
What it is
A volatile cyclic silicone. Chemically it is decamethylcyclopentasiloxane — five silicon-oxygen units joined in a ring, with methyl groups on the silicon, molecular weight about 371. It is not an oil and not a fat, and it carries no fatty acids. Formulators use it for feel: it is thin, it spreads a dry silky slip rather than a greasy one, and then it substantially evaporates from the skin rather than remaining on it, which is why it sits behind a lot of hair serums, face primers, foundations, sunscreens and antiperspirants sold on a lightweight, non-greasy finish. One point of naming has to come first, because it is the whole of the material-identity question on this page. Cyclopentasiloxane is often shortened to D5. Until the mid-2000s it was usually sold, and labelled, under the name cyclomethicone — which was an INCI umbrella term for a mixture of cyclic dimethylsiloxanes, chiefly D4, D5 and D6, before the individual ring sizes were each given their own name. D5 is one of the ring sizes that umbrella covered. What share of any particular commercial cyclomethicone it accounted for is not something we hold a source for, and we are not going to supply one from memory. The category and the single molecule are not the same thing, and the distinction decides what the one number we hold actually measured.
Why we rate it Clear
We rate it Clear at Low confidence, and the confidence is Low because we identified no measurement of this molecule at all. The one relevant reading in the corpus is a rabbit's ear, scored against the category the molecule belongs to. Cyclopentasiloxane was not itself tested in the 1989 assay that produced most of the comedogenic numbers in circulation. The category it belongs to was. Fulton diluted ingredients to roughly 10% in propylene glycol, applied them to the inner ear of New Zealand albino rabbits, three rabbits per assay, five days a week for two weeks, and measured the width of the follicular keratosis. His table has a silicones block, and it holds three rows: simethicone 1, dimethicone 1, and cyclomethicone 0, on his 0-to-5 scale. There is no row printed "cyclopentasiloxane" anywhere in the paper. The 0 is scored against "cyclomethicone" — the umbrella name, at that date, for a mixture of cyclic dimethylsiloxanes, chiefly D4, D5 and D6. Fulton records no supplier and no composition for the sample, so which ring sizes were in it, and in what proportion, cannot be recovered. Two things about that 0 are worth stating plainly. It sits at the bottom of Fulton's own scale — his key reads "a minimal grade of 0 to 1 is not considered significant" — so even taken at face value it is a non-finding rather than a clean bill of health earned against a hard test. And it is the whole of the comedogenicity record we could trace anywhere near this material: one legacy rabbit assay, one investigator, one blanket category name, scoring at the cleanest end of the scale. So the reading that survives is modest. Cyclopentasiloxane sits inside a category a single rabbit-ear study scored 0, once, under a name that no longer means only D5, and nothing we identified measures D5 itself. That is a reasonable basis for Clear. It is not a basis for Moderate or High, and we do not reach for either — a category score is not the molecule measured on its own, and one legacy assay is not a body of evidence agreeing with itself.
What the evidence doesn’t tell you
We identified no comedogenicity study of cyclopentasiloxane tested on its own — not in a rabbit, not on a human, and not looking inside a follicle. Every number attached to it, including the 0, descends from Fulton's single "cyclomethicone" row, and that row scored a blend under an umbrella name, not D5 by itself. Whether the 0 would hold for pure cyclopentasiloxane, or whether the reading was moved by whatever else was in the 1989 sample, is not knowable from the paper, because the paper does not say what the sample was. The claim the reader most often meets — that silicones form a film that suffocates the skin and traps sebum and bacteria in the pore — is, as far as we can find, untested for cyclopentasiloxane. That it substantially evaporates is well documented, and it is the reason the trapping story is a hard one to tell about a volatile. But "it does not stay on the skin" is a different measurement from "it does not plug a follicle", and we have not found the study that tested the second. Unproven is not the same as disproven, and that cuts in the reassuring direction here as much as it cuts against the scary one. The rabbit-ear method behind the 0 is contested in the same way it is everywhere else on this site. Draelos and DiNardo (2006) found that ingredient scores from the rabbit assay do not reliably transfer to human skin, and a 2025 review in JAAD Reviews found that no standardised comedogenicity test exists at all and that the label "non-comedogenic" is unregulated. Those objections weaken any confident number here, in either direction. Finally, a restriction that gets offered as evidence and is not. Cyclopentasiloxane is D5, and D5 is subject to an EU restriction on its use in cosmetics. We have not read the consolidated text of that regulation, so we print none of its scope, thresholds or dates here, and nobody should take a summary of it from this page. What bears on this page is the part that does not depend on the text at all: a restriction on a cosmetic ingredient is not a comedogenicity finding, and anyone citing it as proof that cyclopentasiloxane clogs pores is changing the subject.
Where you’ll see it
On a label it appears as Cyclopentasiloxane, usually near the top of the ingredient list. Hair serums and anti-frizz products, face primers and "blurring" products, foundations, sunscreens, antiperspirants and deodorants, and lightweight moisturisers sold on a dry, non-greasy finish. Two notes for the reader trying to match it on a label. Older products and some suppliers still use the name cyclomethicone, which is an umbrella covering D4, D5 and D6 rather than another word for D5, so a label carrying it does not tell you which ring sizes are in the product, or in what proportion. And D5 is subject to an EU restriction in cosmetics, the consolidated text of which we have not read — a regulatory matter, and not a finding about pores.
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 only comedogenicity data we hold that bears on this material, and the source of most of the scores this site distrusts elsewhere. Method: 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 measured with a micrometer. The relevant row sits in the silicones block of Table I and reads "Cyclomethicone 0" — a category name, not "cyclopentasiloxane", and Fulton gives no supplier or composition for what the sample was. His key puts 0 in the band "not considered significant". Cited as a legacy category score, never as a measurement of D5 on its own.
- Johnson W, Bergfeld WF, Belsito DV, et al. (Cosmetic Ingredient Review Expert Panel). "Safety Assessment of Cyclomethicone, Cyclotetrasiloxane, Cyclopentasiloxane, Cyclohexasiloxane, and Cycloheptasiloxane." Int J Toxicol 30(6 Suppl):149S-227S (2011) An expert-panel safety review, cited here for two things we could read from its scope rather than from figures inside its paywalled full text: that "cyclomethicone" is the umbrella term the named ring sizes (D4 through D7) were split out of, so cyclopentasiloxane and cyclomethicone are not simply synonyms; and that D5 is a volatile silicone that evaporates from skin. It assesses systemic and irritation safety, and it does not assess comedogenicity. A safety conclusion is not a pore finding, which is why no number from it appears on this page.
- Commission Regulation (EU) 2024/1328, on the restriction of the cyclic siloxanes D4, D5 and D6 in cosmetic products Cited to draw a line, not to support a claim. This is the restriction routinely offered as proof that "silicones are bad". Cyclopentasiloxane is D5 and is one of the substances it covers. We have not read its consolidated text, so this page prints no scope, threshold or timetable from it, and no reader should take those details from us. The line we are drawing does not need them. A restriction on a cosmetic ingredient is not a finding about skin, follicles or acne.
- "Comedogenicity in cosmeceuticals: a review of clinical relevance, regulatory gaps, and future directions." JAAD Reviews (2025) Recent peer-reviewed review of the whole field, covering literature from 1972 to 2025. Finds that no standardised comedogenicity test exists and that "non-comedogenic" is an unregulated label. It is a synthesis of the legacy literature, not a new measurement, and independent of Fulton only in authorship, since the corpus it surveys includes his. Cited for the field-wide caveat that any single legacy score, including this one, should be read against.
Others in the same family
Silicones behave similarly enough that the evidence for one is often wrongly read across to the others. These are its structural relatives, not a guess.
Last reviewed 2026-08-12 · How we decide