What does comedogenic mean? The 0–5 scale, explained
Comedogenic means likely to block pores and cause comedones (blackheads and whiteheads). The familiar 0–5 comedogenic scale grades how much follicle blockage an ingredient caused in a lab test, from 0 (no significant increase) to 5 (large comedones). Most published grades come from tests on rabbit ears, which react far more strongly than human facial skin, so a rating is a warning sign to look closer, not a prediction.
Where the scale comes from
In the early 1970s, dermatologists Albert Kligman and Otto Mills described "acne cosmetica": a low-grade, persistent acne of closed comedones they kept seeing in adult women and attributed to cosmetics (Kligman and Mills, 1972). To find which ingredients were responsible, researchers used an older animal assay: apply a substance to the inside of a rabbit's ear for two weeks and measure how much the follicles fill with keratin.
James Fulton's group ran that assay on hundreds of raw materials and finished products through the 1980s. The 0–5 scale people quote today is the one set out in his 1989 paper in the Journal of the Society of Cosmetic Chemists (Fulton, 1989). Each ingredient was usually tested at 10% in propylene glycol, applied once a day, five days a week, for two weeks, to the inner ear of three rabbits. The width of the follicle plugs was measured with a micrometer and converted to a grade:
| Grade | Average follicle plug width | How Fulton read it |
|---|---|---|
| 0 | 0.009 in or less | No significant increase in follicular keratosis |
| 1 | 0.010–0.014 in | Not considered significant |
| 2 | 0.015–0.019 in | Moderate increase; borderline |
| 3 | 0.020–0.025 in | Moderate increase; borderline |
| 4 | 0.025–0.029 in | Extensive increase; reproducibly positive |
| 5 | 0.030 in or more | Large comedones across the ear, like the positive control, isopropyl myristate |
The same paper graded surface irritancy on a separate 0–5 scale, from no irritation to epidermal necrosis. An ingredient can score low on one and high on the other.
The checker uses Fulton's own reading of the grades: 4–5 is flagged as a positive result, 2–3 is flagged as borderline, and 0–1 is reported but not flagged.
Why a rating is a guide, not a verdict
- Rabbit ears overreact. Fulton himself called the model extremely sensitive and noted that not everything that irritates a rabbit ear irritates human skin. When Mills and Kligman built a human test on the upper back, they found the rabbit "more sensitive than the human" and judged weakly comedogenic substances probably fine for most people, with the possible exception of the acne-prone (Mills and Kligman, 1982).
- Concentration matters. A raw material tested at 10% or full strength may be a fraction of a percent in a cream. That is why position in the ingredient list matters: ingredients present at more than 1% are listed in descending order of amount.
- The formula matters. Testing finished products on human backs, Draelos and DiNardo concluded that "finished products using comedogenic ingredients are not necessarily comedogenic" (Draelos and DiNardo, 2006).
- Results vary between labs and vehicles. Fulton showed that the same fatty acid or red pigment graded differently depending on what it was dissolved in. Where our sources disagree, the checker shows the range and uses the higher number.
- Most ingredients were never tested. The published tables cover a few hundred raw materials. Newer ingredients (most silicones, many plant extracts, modern emollients) have no published grade, and the checker says so instead of guessing.
How "non-comedogenic" claims are tested
A "non-comedogenic" claim on a product is the manufacturer's statement about that finished formula. Testing labs substantiate it either with the human back model descended from Mills and Kligman or with a longer clinical use study on acne-prone volunteers, which is considered the stronger evidence (Weiss and Caswell, 2017). A product can carry the claim and still contain an ingredient that rated highly as a raw material.
Questions
What does comedogenic mean?
Comedogenic means able to cause comedones: the blackheads and whiteheads that form when a hair follicle fills with sticky dead skin cells and oil. In cosmetic testing it describes an ingredient or product that produced this blockage in a laboratory test.
What does non-comedogenic mean on a label?
It is a claim by the manufacturer that the finished product did not produce comedones in the test they relied on, often a version of the human back test. It is a statement about that formula, not a guarantee for every skin.
Is a 0 on the comedogenic scale the same as safe for acne-prone skin?
No. A 0 means no significant follicle blockage in that particular test. Products containing 0-rated ingredients can still irritate or break out some people.
Sources
- Fulton JE Jr. Comedogenicity and irritancy of commonly used ingredients in skin care products. J Soc Cosmet Chem. 1989;40:321–333. SCC library
- Kligman AM, Mills OH Jr. "Acne cosmetica". Arch Dermatol. 1972;106(6):843–850. PubMed 4264346
- Mills OH Jr, Kligman AM. A human model for assessing comedogenic substances. Arch Dermatol. 1982;118(11):903–905. PubMed 7138047
- Fulton JE Jr, Pay SR, Fulton JE 3rd. Comedogenicity of current therapeutic products, cosmetics, and ingredients in the rabbit ear. J Am Acad Dermatol. 1984;10(1):96–105. PubMed 6229554
- Draelos ZD, DiNardo JC. A re-evaluation of the comedogenicity concept. J Am Acad Dermatol. 2006;54(3):507–512. doi:10.1016/j.jaad.2005.11.1058
- Weiss C, Caswell M. Non-comedogenic and non-acnegenic claim substantiation. J Cosmet Sci. 2017;68(4):253–256. PubMed 29616616