The comedogenic scale is a rating system that assigns numbers from 0 to 5 to cosmetic ingredients based on their tendency to block pores and cause comedones (blackheads and whiteheads). Zero means non-comedogenic; five means highly likely to cause congestion. It appears on product labels, in ingredient databases, and in skincare discussions everywhere.
It is also, in its most commonly cited form, not particularly reliable as a guide for individual skin behaviour. Here’s why.
The original comedogenic ratings were established in the 1970s and 1980s using a rabbit ear model — a method in which ingredients were applied to the inner surface of rabbit ears and assessed for comedone formation. Rabbit ear skin is significantly more sensitive to comedogenic effects than human skin. Ingredients that produce a strong comedogenic response in rabbits may produce little or no response in humans.
Human studies conducted since have found poor correlation between rabbit ear ratings and actual comedogenicity in human skin, and poor correlation between comedogenic ratings and real-world breakout risk. The concentration at which an ingredient is used matters. The vehicle (the other ingredients in the formula) matters. Individual skin variability matters enormously.
This doesn’t mean comedogenic ratings are useless. They are a useful directional guide — an ingredient rated 4 or 5 is probably worth avoiding in large concentrations if your skin tends toward congestion. But a 0 or 1 rating is not a guarantee, and a 2 or 3 rating does not mean the ingredient will cause problems in your skin.
The more reliable approach is patch testing, introducing one product at a time, and paying attention to what your own skin tells you. Which is, incidentally, the same approach we take when formulating for the Onyx collection — not just avoiding high comedogenic ratings, but building a base that is genuinely non-comedogenic in practice, tested on real skin, at the concentrations actually used.