The Alchemist's Guide to Comedogenicity: What Fifty Years of Science Actually Tells Us About Oils and Your Pores

If you have spent any time researching skincare, you have encountered the comedogenic scale: that tidy 0-to-5 rating system that promises to tell you whether an oil will clog your pores. It appears on countless blogs, product pages, and ingredient databases as settled fact. But the history behind that scale, and the science that has accumulated since, tells a far more complicated and more useful story than the numbers alone suggest.

In our Alchemist's Guide to Oils, we explored how fatty acid profiles determine whether an oil penetrates or seals, nourishes or disrupts. Comedogenicity is the other half of that conversation: not just what an oil does, but whether it causes problems when it stays on the skin. Understanding where the comedogenic scale came from, where it went wrong, and what modern science offers in its place is essential for anyone who wants to choose oils and formulations intelligently.

A Brief History of Pore Panic

The story begins in 1972, when dermatologists Albert Kligman and Otto Mills published a paper coining the term "acne cosmetica," describing a pattern of low-grade comedonal acne on the cheeks and chin of women over twenty, which they attributed to cosmetic products (Kligman & Mills, 1972). The concept was revolutionary: for the first time, the dermatological community formally acknowledged that the products people applied to improve their skin might be making it worse.

To test which ingredients were responsible, researchers needed a screening method. The solution was the rabbit ear assay, in which test substances were applied to the inner ear of albino rabbits, typically under occlusion, for two to four weeks. The degree of follicular hyperkeratosis (the buildup of keratin that precedes comedone formation) was then graded on a 0-to-5 scale. James Fulton and colleagues used this method to systematically rate dozens of cosmetic ingredients and published their findings in 1984, establishing the comedogenic ratings that are still widely cited today (Fulton & Pay, 1984).

The rabbit ear model was fast, inexpensive, and produced clear numerical results. It was also, as the scientific community would gradually discover, deeply flawed.

The Problem with Rabbit Ears

Almost immediately, questions arose about whether rabbit skin could predict what would happen on human faces. In 1983, Frank and colleagues asked the question directly in a paper titled "Is the rabbit ear model, in its present state, prophetic of acnegenicity?" and concluded that serious limitations existed in the conclusions that could reasonably be drawn from the model (Frank et al., 1983).

The problems were fundamental. Rabbit ears are significantly more sensitive than human skin, producing follicular reactions to substances that cause little to no comedone formation in people. This meant the assay generated a high rate of false positives: ingredients flagged as comedogenic in rabbits that were harmless on human skin. Compounding this, researchers initially failed to account for naturally enlarged follicles in rabbit ears, which were sometimes counted as induced comedones (Starzyk et al., 2025). Testing conditions were not standardized across laboratories, so the same ingredient could receive different ratings depending on where and how it was tested.

Recognizing these limitations, Mills and Kligman developed a human model in 1982, applying test substances under occlusion to the upper backs of volunteers and using cyanoacrylate biopsies to assess follicular changes (Mills & Kligman, 1982). This was an improvement, but it introduced its own constraints: back skin has different follicle density and sebaceous activity than facial skin, and it receives far less sun exposure. A substance deemed non-comedogenic on the back might still cause problems on the face.

Where Modern Science Diverges

In the decades since those foundational studies, the scientific understanding of comedogenicity has evolved substantially, and in several cases, directly contradicts the assumptions embedded in the original scale.

Finished products behave differently than isolated ingredients. The original ratings tested pure ingredients at full concentration, often under occlusion. But in a real formulation, ingredients interact with one another, and their concentrations are typically well below the levels tested. In 2006, Draelos and DiNardo published a landmark re-evaluation demonstrating that finished products containing ingredients rated as comedogenic did not necessarily produce comedones when tested on human skin as complete formulations (Draelos & DiNardo, 2006). The vehicle, the concentration, and the interactions between ingredients all matter as much as the ingredient itself.

Concentration changes everything. An ingredient that scores a 5 at 100% concentration may be entirely benign at the 2-5% levels found in most finished products. Isopropyl myristate, one of the most notorious ingredients on comedogenic lists, has been used at low concentrations in acne treatment formulations without inducing comedones, and research has shown that its comedogenic activity at typical use concentrations is too weak to serve as a reliable positive control in testing (Starzyk et al., 2025).

Contact time matters profoundly. The original assays applied substances under occlusion for weeks. A cleanser that touches the skin for thirty seconds and is then rinsed away presents a fundamentally different risk profile than a moisturizer that sits on the skin for hours. This is perhaps the single most important distinction the traditional scale fails to capture, and it is directly relevant to formulation decisions about wash-off products like shampoo bars and body bars versus leave-on products like lotions and serums.

Individual variation is enormous. A 2013 case-control study of 910 patients actually found a dose-dependent inverse association between overall cosmetic use and post-adolescent acne, meaning more cosmetic use was associated with less acne, not more (Singh et al., 2013). This does not mean cosmetics prevent acne, but it powerfully demonstrates that the relationship between topical products and comedone formation is far more complex than a simple ingredient rating can capture. Skin type, hormonal status, genetic predisposition, and the overall formulation context all mediate the outcome.

The Regulatory Void

Perhaps the most troubling aspect of the comedogenic scale's legacy is how it has been commercialized. The term "noncomedogenic" appears on countless products as though it were a certification, but there is no regulatory standard behind it. The U.S. Food and Drug Administration does not define, regulate, or verify "noncomedogenic" claims. There is no required test, no threshold, and no enforcement. Any manufacturer can apply the label without conducting a single comedogenicity assay (FDA, 2024). A comprehensive 2025 review in JAAD Reviews identified this regulatory gap as a major concern, noting that the lack of standardized testing allows companies to freely label products as noncomedogenic regardless of their actual potential to cause acne (Starzyk et al., 2025).

This means the responsibility for evaluating comedogenic risk falls on the formulator and the informed consumer, not on the label.

What Actually Matters: Fatty Acids, Not Fear

If the comedogenic scale is unreliable, what should guide oil selection? The answer, as we explored in the Alchemist's Guide to Oils, lies in fatty acid profiles, and the science here is much stronger.

As discussed in our oils guide, the ratio of oleic acid to linoleic acid in an oil is one of the most functionally relevant characteristics for acne-prone skin. Linoleic acid supports ceramide synthesis and barrier integrity, while excess oleic acid can disrupt the lipid barrier and has been associated with increased comedone formation in susceptible individuals (Lin et al., 2018). Oils high in linoleic acid, such as hemp seed, rosehip, and grapeseed, tend to be better tolerated by oily and breakout-prone skin types because they work with the skin's natural lipid chemistry rather than against it.

This functional approach is more useful than a single number because it addresses mechanism. An oil does not clog pores because of an arbitrary rating; it causes problems (or does not) because of how its molecular components interact with the specific biology of the skin it contacts.

The Wash-Off Distinction: Bars, Shampoos, and Contact Time

This brings us to the most practical implication of modern comedogenicity science for formulation: the distinction between wash-off and leave-on products.

In a shampoo bar or body bar, ingredients contact the skin for seconds to minutes before being rinsed away. The saponification process itself transforms the original oils into soap molecules and glycerin; the fatty acid profile of the bar determines its lather, hardness, and conditioning properties, but the original oil no longer exists in its native form. Coconut oil, for example, which carries a high comedogenic rating as a raw oil, contributes lauric acid that produces excellent lather and has demonstrated the unique ability to penetrate and protect hair protein (Rele & Mohile, 2003). In a properly formulated wash-off product, the comedogenic risk of the original oil is largely irrelevant.

Leave-on products, conversely, remain on the skin for hours. Here, the fatty acid profile, concentration, and formulation context become genuinely important. A facial serum with 30% oleic acid-dominant oil will interact with the skin very differently than a body lotion with 5% of the same oil in an emulsion base. For leave-on facial products, especially for acne-prone individuals, selecting oils with favorable linoleic-to-oleic ratios and testing the finished formulation, not just its individual ingredients, represents a far more scientifically grounded approach than consulting a decades-old rabbit-derived rating.

The Potionologie Approach

At Potionologie, we respect what the comedogenic scale was trying to do. Kligman, Mills, and Fulton asked an important question: can the products we put on our skin cause acne? The answer is yes, they can. But the scale they created to quantify that risk has been superseded by a more nuanced understanding.

We formulate using fatty acid profiles, not fear-based ingredient lists. We distinguish between wash-off and leave-on applications, adjusting our oil selections accordingly. Coconut oil earns its place in our shampoo and body bars for its lather and its protein-protective penetration of the hair shaft, not despite its comedogenic rating, but because the context of a rinse-off product renders that rating functionally meaningless.

For leave-on products, we select oils based on their linoleic-to-oleic ratio, their chain length, and their interaction with the other components of the formula. We test formulations as finished products, because the science is clear: an ingredient's behavior in isolation tells you very little about its behavior in a well-designed formula.

The comedogenic scale gave the skincare world a language for talking about pore-clogging risk. Modern science has given us a better one. The numbers on those old lists were never wrong, exactly. They were just answers to the wrong question, tested on the wrong skin, under the wrong conditions. The right question is not "Is this ingredient comedogenic?" It is: "In this formulation, at this concentration, for this application, on this skin, does it cause problems?"

That is the question an alchemist asks. And it is the one worth answering.

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References

Draelos, Z.D. & DiNardo, J.C. (2006). A re-evaluation of the comedogenicity concept. Journal of the American Academy of Dermatology, 54(3), 507-512. https://doi.org/10.1016/j.jaad.2005.11.1058

FDA. (2024). Cosmetics labeling claims. U.S. Food and Drug Administration. https://www.fda.gov/cosmetics/cosmetics-labeling/cosmetics-labeling-claims

Frank, S.B., et al. (1983). Is the rabbit ear model, in its present state, prophetic of acnegenicity? Journal of the American Academy of Dermatology, 9(5), 655-659. PubMed ID: 6461674.

Fulton, J.E. & Pay, S.R. (1984). Comedogenicity of current therapeutic products, cosmetics, and ingredients in the rabbit ear. Journal of the American Academy of Dermatology, 10(1), 96-105. PubMed ID: 6229554.

Kligman, A.M. & Mills, O.H. (1972). Acne cosmetica. Archives of Dermatology, 106(6), 843-850. PubMed ID: 4264346.

Lin, T.K., Zhong, L., & Santiago, J.L. (2018). Anti-inflammatory and skin barrier repair effects of topical application of some plant oils. International Journal of Molecular Sciences, 19(1), 70. https://doi.org/10.3390/ijms19010070

Mills, O.H. & Kligman, A.M. (1982). A human model for assessing comedogenic substances. Archives of Dermatology, 118(11), 903-905. PubMed ID: 7138047.

Rele, A.S. & Mohile, R.B. (2003). Effect of mineral oil, sunflower oil, and coconut oil on prevention of hair damage. Journal of Cosmetic Science, 54(2), 175-192. PubMed ID: 12715094.

Singh, S., Mann, B.K., & Tiwary, N.K. (2013). Acne cosmetica revisited: A case-control study shows a dose-dependent inverse association between overall cosmetic use and post-adolescent acne. Dermatology, 226(4), 337-341. https://doi.org/10.1159/000350936

Starzyk, T., Aust, N., Schur, N., & Miller, R. (2025). Comedogenicity in cosmeceuticals: A review of clinical relevance, regulatory gaps, and future directions. JAAD Reviews, published online 2025. https://doi.org/10.1016/j.jdrv.2025.04.002