Comedogenic rating scale: where the 0-5 numbers come from

The comedogenic rating scale is a 0-5 score attached to individual cosmetic ingredients, never to finished products. It came out of a rabbit-ear assay used in dermatology labs from the 1970s onward, and no regulator anywhere certifies it: every published rating table is a private compilation with an author and a cut-off date.

This page sets out where the numbers came from, what the assay could and could not observe, why the same ingredient can score 5 in one column and 1 in another, and what the reference behind this site’s own checker actually contains.

What a 0-5 rating is recording

A comedogenic rating records how much follicular plugging one ingredient produced in a standardised skin assay, graded from 0 to 5. It is a laboratory observation about a single material, tested largely on its own and usually near full strength. It is not a measurement of a finished formula on a human face.

The scale is ordinal, not proportional. A 4 is not twice a 2, and nothing in the underlying data supports averaging the ratings of an ingredient list into a single product score, which is why the pore-clogging ingredients checker prints each rating beside its match instead of summing them.

Where the 0-5 scale came from

The scale has a traceable research lineage, and knowing it explains most of its limits. It was built to screen raw materials quickly and cheaply, in an animal model chosen for speed, before anyone expected the resulting numbers to be read by consumers holding a moisturiser in a shop aisle.

Why the rabbit ear reports more than a human face

The rabbit ear assay over-reports relative to human facial skin because the follicles of the rabbit’s external ear are unusually reactive: materials that produce visible follicular keratosis there frequently produce nothing measurable on human skin. The model was built for sensitivity, so it errs toward flagging, and a screening test that errs toward flagging is not a prediction.

Sensitivity was the point. A screen is supposed to catch candidates and hand them to a slower method, and the slower method — biopsy of human back follicles — was applied to only a fraction of the materials that now carry a number. The number survived; the second stage largely did not.

A comedogenic rating is the output of a screening assay, not a forecast about a face. It tells you a material was worth a second look, and in most cases the second look was never funded.

Concentration moves the number more than the ingredient does

Concentration is the variable the scale leaves out, and it is usually the largest one. The same material tested neat and tested at a few percent in a carrier can land four grades apart, so a rating copied from a table describes a test condition nobody’s bathroom shelf reproduces.

Ingredient lists are ordered by descending proportion down to one percent, after which the order carries no information at all. An ester rated 5 sitting twenty-fourth on a label is a different proposition from the same ester sitting third, and no rating table can distinguish the two. That is a limit of the scale, not a defect of any particular list.

It also explains a recurring argument. Coconut oil is rated 4 and isopropyl myristate is rated 5 in the reference below, yet both appear in products that many people use without incident, because the tested condition and the shelf condition are not the same condition.

Nobody certifies a comedogenic list

There is no FDA, EU or global organisation that certifies or regulates comedogenic ingredient lists. No agency issues one, approves one, or audits the ones in circulation. Ingredient naming is standardised — the EU published the current glossary of common ingredient names as Implementing Decision (EU) 2022/677 — but no annex of any such document assigns a comedogenic rating.

The practical test for any pore-clogging tool is therefore not how confident its verdict sounds but how much of its own working it shows. A checker that names the tables it summarises, prints the list in full on the same page and stamps it with a revision date is making a claim you can audit in a minute. A checker that returns a clean bill of health without saying how many names it managed to read is making a claim about your product out of a gap in its own vocabulary.

The reference this site publishes

The pore-clogging reference behind the checker on this site holds 88 entries, of which 86 carry a rating of 1 or higher and appear in the printed table below. It is a summary of published comedogenicity tables, kept as names and ratings only, with a version date shown wherever a result is produced.

Two entries rated 0 are retained deliberately rather than deleted. Argan oil and shea butter are widely assumed to be pore-clogging and are rated 0 on the tables this reference draws from, so keeping them in the data at 0 means the checker returns nothing for them. Dropping the entries would leave the same silence with no record of why, and a tool that quietly omits the popular myths cannot be checked against the myth.

When a comedogenic number is the wrong tool

A comedogenic rating is the wrong tool whenever the question is about a specific finished product on specific skin. The scale ranks isolated materials under laboratory conditions; it has nothing to say about concentration, formulation, how long a product sits on skin, or the difference between comedones and other kinds of bump.

The other common mismatch is diagnostic. Small uniform papules across the forehead, chest or back often belong to Malassezia folliculitis rather than comedonal acne, and comedogenic ratings are silent on that question because it turns on fatty-acid chain length instead of follicular plugging. The fungal acne safe checker applies a stated rule for that, and the rule is written out on the page.

Nothing here is medical advice, and this site publishes no verdict on any named brand’s product. Persistent or painful breakouts belong with a dermatologist, who can separate comedonal acne, folliculitis and rosacea — three things an ingredient list cannot tell apart.

The full reference, printed with its version date

Below is every ingredient the pore-clogging checker can flag, with its 0-5 rating and its job in a formula, sorted alphabetically. The version date belongs to the list rather than to this article: when the reference changes, the date above the table changes with it.