Porecelain

Flawless skin starts with the fine print.

Hydrogenated Polyisobutene

The 1 that follows this ingredient around is a real printed number, but it is printed under a different spelling — "hydrogenated polyisobutane" — and the mapping of that row to this ingredient is probable rather than proven. It sits in section VI of Fulton's table, "Oils*", the one section carrying a footnote that results depend on the source of the raw material. The one other comedogenicity experiment we identified is attributed to two different ingredients in two places in the same report.

Clear Low confidence Legacy score 1 on a 0-5 scale · Fulton 1989 · low reliability

Synthetic emollient (common lightweight mineral-oil alternative); low comedogenic risk.

What it is

A synthetic branched-chain hydrocarbon. The Cosmetic Ingredient Review's 2008 assessment gives the chemistry plainly: hydrogenated polyisobutene and polyisobutene are both homopolymers of isobutene, and at the terminal unit the difference between them is one bond — "Polyisobutene has a double bond in its end unit, whereas the final carbon is fully hydrogenated in Hydrogenated Polyisobutene." That sentence is about the end unit, not about the whole of what separates two commercial materials, which also differ by molecular-weight distribution and by grade specification. The International Cosmetic Ingredient Dictionary defines the hydrogenated form (CAS 68937-10-0) as a branched-chain aliphatic hydrocarbon. They are two separate INCI names with two separate CAS numbers, and that distinction does real work further down this page. It is not one substance so much as a range of them. CIR records that these ingredients "are produced in a wide range of molecular weights", and gives numbers: Panalane L-14E, the lowest-molecular-weight grade named in the report, has a number-average molecular weight of 370, with higher grades above that. Trade names in the report include Panalane, Parleam, Polysynlane, Permethyl, Creasil ISO and Fancol Polyiso. What reaches a label is one grade out of a family, and the label does not say which. What a formulator uses it for, in CIR's own words, is that it "functions as a skin-conditioning agent-emollient and nonaqueous viscosity-increasing agent". The report adds a description worth quoting exactly, because it is not quite the one that circulates: it "has an additive function, which improves the feel of and generally upgrade the spreading and penetrating properties of lotions based on mineral oil and petrolatum." It is widely described online as a replacement for mineral oil. The primary document we have read describes it as something added to mineral-oil and petrolatum lotions to improve them. We are reporting what the document says and leaving the market positioning alone, because we have not audited it.

Why we rate it Clear

We rate it Clear at Low confidence. Nothing on this page overturns a scare number, because no scare number attaches to this ingredient in the charts we checked. What the page can do is show you how thin the ground under a low number is, which matters more than it sounds: a reassuring figure sourced from nowhere is the same failure as an alarming one. A ROW EXISTS, AND IT IS SPELLED DIFFERENTLY. Fulton's 1989 rabbit-ear survey in the Journal of the Society of Cosmetic Chemists is where essentially every comedogenic number in circulation comes from, and we have the table transcribed by eye off the journal scan rather than read off a chart. It contains a row we take to be this material's — probably, not provably. The row reads "Hydrogenated polyisobutane" and the entry is 1 on his 0-5 scale. Not polyisobutene. Polyisobutane. That mapping is a judgement, so we are going to state it as one rather than bury it. Our own material-identity table maps "hydrogenated polyisobutene" to the printed row "Hydrogenated polyisobutane", and the reasoning is this: isobutane is a saturated alkane and does not polymerise, so "polyisobutane" is not a monomer-derived name for anything, while the hydrogenated product genuinely is a saturated branched alkane — which is what the paper's spelling describes. We think the row is ours. Fulton reports no CAS number, no supplier, no trade name and no grade, so it cannot be proved, and anyone who quotes this 1 without noticing that the paper spells it differently has not opened the paper. AND THE ROW SITS IN AN ASTERISKED SECTION. Fulton's Table I is divided into numbered sections, and this row is in section VI, headed "Oils*". The asterisk is defined once in the paper and it reads "Results depend on source of raw material." It is attached to the heading, not to individual cells, so it governs every row underneath it — including this one, and including the coconut butter and cocoa butter 4s that get quoted as though they were flat numbers. On his own published key, 0-1 "is not considered significant", 2-3 is "borderline", and 4-5 is "considered positive". So the honest statement of Fulton's finding is: a 1, which is below his own threshold of significance, on a material whose section he flagged as source-dependent, at roughly 10% in propylene glycol, on the inner ear of three New Zealand albino rabbits, five days a week for two weeks. THERE IS A SECOND DOCUMENT, AND IT IS MORE INTERESTING THAN THE FIRST. The Cosmetic Ingredient Review's Final Report on Polyisobutene and Hydrogenated Polyisobutene, published in the International Journal of Toxicology in 2008, has a Comedogenicity section. It reports an unpublished 1987 contract-laboratory study by Product Safety Labs on "Permethyl 104A/105A Aliphatic Hydrocarbon (Hydrogenated Polyisobutene)". Three adult New Zealand white rabbits, undiluted test material after an eight-day adaptation period, applied to the internal base of the right ear daily on five consecutive days per week for three weeks, with the left ear untreated as the negative control, scored for hyperkeratosis and comedone formation. The result is not the clean negative it gets summarised as. In the report's own words: "There were no signs of hyperkeratosis or comedone formation during weeks I and 2. At week 3, two treated ears showed signs of hyperkeratosis. One ear remained clear. Histological examination revealed no signs of follicular hyperkeratosis on the treated, untreated, or control ears of any rabbits." Two of three ears showed something at three weeks; the histology found nothing in any animal. That is a negative result resting on histology in three rabbits, which is a reasonable thing to report and a weak thing to lean on. AND THE REPORT CANNOT DECIDE WHICH OF ITS TWO INGREDIENTS THAT STUDY BELONGS TO. This is the finding this page exists for, and it is visible in three places in one document. The abstract, on page 83, says: "Polyisobutene was not comedogenic in a rabbit ear study" — naming the other ingredient. The Comedogenicity section labels the test article "Permethyl 104A/105A Aliphatic Hydrocarbon (Hydrogenated Polyisobutene)". The summary, describing an eye-irritation study on the same trade name, calls it "Permethyl 104A/105A Blend Aliphatic Hydrocarbon (trade name Polyisobutene)". And the report's own trade-name list puts Permethyl 104A under POLYISOBUTENE, not under the hydrogenated form. Elsewhere it describes a product "containing 10% Hydrogenated Polyisobutene (Permethyl 104)". We are not accusing anyone of an error we cannot see the resolution of — the trade name may cover materials under both INCI names, and CIR does not say. What is established is narrower and still worth knowing: the one comedogenicity experiment we identified in the modern literature on either of these two ingredients is attributed to both of them, in the same report, and the abstract most people would read attributes it to the one this page is not about. So the rating rests on two rabbit results, one graded 1 below its own author's significance threshold under a different spelling, and one ungraded and ambiguously attributed. Both point the same way. Low confidence is what that is worth.

What the evidence doesn’t tell you

Neither result is a measurement of what this ingredient does on a human face, and we have not found one that is. We identified no human comedogenicity study of hydrogenated polyisobutene. The rest of the legacy corpus is silent on it, and we checked rather than assumed. Fulton, Pay and Fulton's earlier 1984 survey carries no row for it under either spelling; Morris and Kwan's 1983 rabbit-ear study does not include it among its thirty-two test materials; and it is not among the materials Nguyen, Dang and Maibach tested in 2007. Those are verified absences, not zeros. The CIR report contains a great deal of human data, and none of it is about pores. Single-insult and repeat-insult patch tests on products containing 4% and 1.44% hydrogenated polyisobutene, a makeup remover at 51%, a lip gloss at 66.11%, a face powder at 17.1% — these measured dermal irritation and sensitisation. Irritation is not comedogenicity and sensitisation is not comedogenicity. Neither is the Panel's "safe as used" conclusion, which is a safety judgement about a different question. A page that cited those studies as evidence about follicles would be doing exactly what this site exists to object to, and the temptation is real, because there are a lot of them and they are all reassuring. Fulton's row does not say which grade he tested, and the ingredient is a family. CIR documents the molecular-weight spread and gives 370 as the number-average for the lowest grade it names, with higher grades above. Fulton reports no molecular weight, supplier or trade name, and his section footnote says in terms that results depend on the source of the raw material. We are not going to tell you that molecular weight predicts comedogenicity — no document we have read establishes that, and inventing a mechanism to explain a rating we already hold is a failure mode we have committed before. The narrow point is that "hydrogenated polyisobutene" on a label and "hydrogenated polyisobutane" in a 1989 table are each a name for a range, and the row does not identify which member of it went on the rabbit. The 1987 study is a contract-laboratory report we have not seen. We have CIR's summary of it and nothing else: no published methods section, no scale, no statistics, no scoring key, three animals. Its own numbers include two of three treated ears showing hyperkeratosis at week three. We are reporting what CIR reports, which is what it is. The instrument is the same one the whole field runs on and it is not a good one. Fulton called his own survey "not at all definitive but simply designed to stimulate research", and warned that the rabbit ear is sensitive enough that not everything which irritates it will irritate human skin. A 2025 review in JAAD Reviews, surveying the literature from 1972 onward, finds that no standardised comedogenicity test exists and that "non-comedogenic" remains an unregulated label. Those objections apply to a 1 exactly as hard as to a 4. One document we have not read. CIR published a "Safety Assessment of the Polyene Group" that incorporates several earlier assessments. We have not opened it and we do not know whether it revisits this ingredient or adds anything on comedogenicity. It is listed here as an open gap rather than summarised from a search result. And we have not traced the chain from Fulton's row to any particular chart. The number circulating on ingredient checkers for this ingredient is a 1, and Fulton's row prints 1, and it is very likely they are the same number — but matching figures do not prove who copied whom, and we did not follow the citation. What we can tell you is that we found the row ourselves, in the paper, and that it is spelled differently there.

Where you’ll see it

Colour cosmetics first. The CIR report's use data, gathered from the FDA's Voluntary Cosmetic Registration Program in 2006 and an industry survey the same year, records 654 reported uses and a concentration range of 0.001% to 96%, with the top of that range in lipsticks. The individual studies the report summarises give a sense of the categories: lipstick and lip gloss, foundations and concealers, face powder, makeup remover, facial lotion. Those are 2006 figures reported to a safety panel, not a current market survey, and we have not audited what is on shelves now. On a label it is Hydrogenated Polyisobutene. Trade names you may see on a supplier document rather than an ingredient list include Panalane, Parleam, Polysynlane, Permethyl, Creasil ISO and Fancol Polyiso. Polyisobutene, without the "hydrogenated", is a different INCI name with a different CAS number — and on the evidence above, the two are easier to confuse than they look, including for the people writing the safety assessments.

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.

  1. Fulton JE. "Comedogenicity and irritancy of commonly used ingredients in skin care products." J Soc Cosmet Chem 40:321-333 (1989) The primary source behind nearly every comedogenic number in circulation, including this one. 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 under a micrometer. Read off a 400dpi render of the journal scan and transcribed by eye, not quoted from a chart. Cited here for three things a chart cannot give you: the row is spelled "Hydrogenated polyisobutane" and prints 1; it sits in section VI, headed "Oils*", whose asterisk is defined as "Results depend on source of raw material" and applies to every row beneath it; and the paper's own grade key reads 0-1 "not considered significant", 2-3 "borderline", 4-5 "considered positive". It is a survey the author described as "not at all definitive". It reports no supplier, grade or molecular weight for any row.
  2. Cosmetic Ingredient Review Expert Panel. "Final Report of the Cosmetic Ingredient Review Expert Panel on the Safety Assessment of Polyisobutene and Hydrogenated Polyisobutene as Used in Cosmetics." Int J Toxicol 27(Suppl 4):83-106 (2008) A safety assessment, read in full, and a FINAL report rather than a draft, tentative or re-review packet — we check the status line first now, having cited three comment-stage CIR documents as though they were settled. What it IS: a compilation of industry-submitted and published toxicology, assessed by a panel, concluding that these ingredients are safe as used at the reported concentrations. That is a safety conclusion and not a comedogenicity finding, and we do not use it as one. It is cited on this page for exactly three things. Its chemistry section distinguishes the two ingredients. Its use tables give 654 reported uses and 0.001% to 96%, from 2006 data. And its Comedogenicity section reports the one modern comedogenicity experiment we identified on either material — which the report attributes to the hydrogenated form in that section, to plain Polyisobutene in its abstract and again in its summary, and whose test article, Permethyl 104A, appears in the report's own trade-name list under Polyisobutene.
  3. Product Safety Labs (1987d), comedogenicity study of Permethyl 104A/105A Aliphatic Hydrocarbon — unpublished, submitted to CIR and summarised in Int J Toxicol 27(Suppl 4):83-106 (2008) An unpublished contract-laboratory report. WE HAVE NOT SEEN IT; we have CIR's paragraph summarising it, and we are listing it separately so that nobody reads our account of it as a paper we read. What CIR reports: three adult New Zealand white rabbits, undiluted material after an eight-day adaptation, applied to the internal base of the right ear five days a week for three weeks, left ear untreated as negative control, scored for hyperkeratosis and comedone formation. No signs in weeks 1 and 2; at week 3, two treated ears showed hyperkeratosis and one remained clear; histology showed no follicular hyperkeratosis on any ear of any rabbit. No grading scale is given, so this result sits on no scale and cannot be compared with Fulton's 1. It is the source of the sentence "Polyisobutene was not comedogenic in a rabbit ear study".
  4. Fulton JE Jr, Pay SR, Fulton JE III. "Comedogenicity of current therapeutic products, cosmetics, and ingredients in the rabbit ear." J Am Acad Dermatol 10(1):96-105 (1984) Fulton's earlier rabbit-ear survey, on a 0-5 scale with its own key (1-2 not significant, 3 or above considered positive) and its own asterisk convention, neither of which may be read across to 1989. Cited for an absence only: its table carries no row for this ingredient under either spelling.
  5. 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 laboratory's rabbit-ear assay, six years before Fulton 1989, whose thirty-two test materials were lanolins, vegetable oils, esters and surfactants. Cited for an absence: this ingredient is not among them, so it offers no second reading in either direction.
  6. 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 its test materials and this ingredient is not among them. Cited for that absence.
  7. "Comedogenicity in cosmeceuticals: A review of clinical relevance, regulatory gaps, and future directions." JAAD Reviews (2025) A recent peer-reviewed review of the whole field, covering literature from 1972 onward, which finds that no standardised comedogenicity test exists and that "non-comedogenic" is an unregulated label. It is a synthesis of the legacy corpus rather than a new measurement, and it is independent of Fulton only in authorship, since the literature it surveys includes his. Cited for the field-wide caveat, which cuts against a reassuring 1 as hard as against anyone's 4. It contains no result for this ingredient.

Others in the same family

Hydrocarbons 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 hydrocarbons as a family ›

Last reviewed 2026-08-12 · How we decide

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