Comedogenic ingredients: why the lists get it wrong (and how to use them anyway)

Some dermatologists recommended products to me. Then I went to another dermatologist, who told me those same products were comedogenic. I started checking my products on a website, but all I could do was copy and paste the whole ingredient list and read through it myself. The numbers look precise. They are less precise than they look.

Where the numbers come from

The classic ratings come from the rabbit ear test, developed in the 1970s by dermatologists Albert Kligman and James Fulton. A substance was applied to a rabbit's ear and checked weeks later for clogged pores. Rabbit ears are more sensitive than human skin, so some things looked worse than they are. Human tests followed, but they were often small and often done on the back, not the face.

Why the rating is only a starting point

• Concentration changes everything. One lanolin derivative rated 4 to 5 when tested pure and at 50 percent, and 1 at 2.5 percent. Most cosmetic ingredients sit at low concentrations, so the number for the pure ingredient often does not apply.
• The finished product matters more than the ingredient. Ingredients interact, and what carries them changes how they behave. A list cannot reliably predict a formula.
• Skin is not uniform. The same product can behave differently on two faces.

How to use a list without being ruled by it

• Check the first five ingredients, where the amounts are highest. Ingredients far down the list are usually present in small amounts.
• Be more careful with products that have several strongly rated ingredients high on the list, and with pure oils and butters.
• Patch test a new product on a small area for a few days before using it on your whole face.
• Do not throw away something that works for you because of one number.

The better test: your own log

The only evidence that really counts is how your skin responds. Write down the day you start a product and note new spots near the places you apply it. If breakouts keep appearing two or three weeks after you start, you have a lead worth testing, whatever the label says.

That copy and paste routine is why Skinory has a scanner. Point the camera at a label in the store, or paste the ingredient list, and it flags the ingredients with a high rating. It also checks the product against your own breakout timeline, because both pieces of information matter.