Comedogenicity: what those ratings actually mean

Almost every guide to acne-prone skin points to a comedogenicity scale from zero to five. Fewer explain where the numbers come from.

Where the numbers come from

The original ratings were produced in the 1970s and 1980s by applying pure ingredients to the inside of a rabbit's ear, which clogs far more readily than a human face. The tests also used the neat ingredient, not a finished product in which the same substance might be present at a fraction of a percent.

That does not make the numbers worthless. It makes them a rough signal rather than a measurement.

Why position matters

Coconut oil first in the list and coconut oil last in the list are, in practice, different products. So SkinIndex does not only tell you that a watchlist ingredient is present; it tells you where in the list it sits. A match in the first six positions counts double in our classification.

What we do instead

We make no promise about breakouts, in either direction. We tell you three things: which watchlist ingredients are in the product, where they sit in the list, and whether it contains fragrance or drying alcohol. Those produce a classification whose rules are published in full.

The rest is individual. Two people can use the same formula with different results, and an ingredient list cannot predict how it will go for you.