Comedogenic ratings, explained

What comedogenic means

An ingredient is comedogenic if it tends to block a follicle and form a comedone — the blackhead or whitehead that precedes most acne. The scale runs 0 to 5: 0 caused no observed blockage in testing, 5 caused it reliably.

Where the numbers came from

From work by Albert Kligman and O.H. Mills in the 1970s, which assessed follicular occlusion using the rabbit ear canal, later refined with human follicular biopsy studies. Essentially every “pore-clogging ingredients” list in circulation descends from that body of work, usually several copies removed — which is why the same numbers appear everywhere with no citation attached.

What a rating cannot tell you

This is the part most lists leave out, and it decides whether a product actually breaks you out:

• Concentration is missing. Ratings come from testing raw ingredients at high concentration. A 5-rated ingredient at the end of an INCI list may be a fraction of a percent.
• Formulation changes behaviour. An oil that clogs neat can behave differently emulsified into a fast-absorbing lotion.
• Rinse-off is not leave-on. Thirty seconds of contact is not eight hours.
• Skin is individual. Comedogenicity is a population tendency, not a prediction about you.