On Comedogenic and Irritancy Ratings

As a site where you can check comedogenic and irritancy ratings easily, we think it’s important to give you some context about what the heck these numbers are, where do they come from and most importantly how to interpret them.

What is comedogenic & irritancy rating?

It is a number on the 0-5 scale that describes the potential of an ingredient to cause comedones or irritate the skin surface. 0 is the best and means no comedones/irritation, while 5 means the (undiluted) ingredient has a good chance of causing problems. Sometimes the rating is a from-to value (eg. 0-2) where the result depended on the study, source of raw material or type of oil (refined or not).

Shall I throw out all my stuff that contains an ingredient with 4-5 rating?

NO. We will explain later, but wanted to emphasise this early on.

Where do these numbers come from?

The numbers come from scientific studies, though some pretty old ones. The biggest list that we know of was published in 1989 by super famous dermatologist and co-inventor of Retin-A James Fulton. It uses a Rabbit Ear Model, meaning that the ingredients where tested on the ear of rabbits, that may or may not behave similarly to human skin.

What is the problem with comedogenic and irritancy rating?

To be honest, there are multiple problems with these numbers. We already mentioned that they mostly come from a Rabbit Ear Model and the rabbit ear is more sensitive than the human skin. So it is totally possible that an ingredient forms comedones on the rabbit but not on us, humans.

Another problem is that in the rabbit assay, the ingredients are mostly tested undiluted but that is not how we use them. The concentration matters a lot. For example, acetylated lanolin alcohol at 100% is comedogenic (rating 4-5), but when diluted to 2.5% in propylene glycol it is not (rating 1).

Also, the chemistry of the finished formula can change the comedogenicity of the individual ingredients. For the better, or for the worse. An example of the latter is that the emollient fatty alcohol, cetearyl alcohol has a comedogenic rating of 2, the ethoxylated emulsifier version of it, ceteareth-20 also has a 2, but combined, they score 4. On the other hand, a 2006 study examined finished products with comedogenic ingredients and found that they were mostly non-comedogenic.

So is comedogenic and irritancy rating useless? Why do you show them?

We think the ratings have a lot of limitations but are not useless. They tend to be a bit overemphasised on the internet as if these numbers were the most important things to check on an ingredient list. They are not, but they give useful extra info about the ingredients.

If an ingredient is non-comedogenic (0-1), then you can be pretty sure that it is not the reason behind your “acne cosmetica” (the acne caused by cosmetics). False positive is more of a problem with the Rabbit Ear Model than false negative.

If an ingredient is comedogenic (4-5) and is high up in the ingredient list (so there is more of it in the product) and you are prone to comedones, you might want to reconsider or read some extra reviews from people with similar skin type. Or if you are breaking out, and trying to figure out why, comedogenic ingredients in your products are obviously more suspicious than non-comedogenic ones.

Show me some proof