Answer: Comedogenicity refers to the potential of a substance to cause a comedo, a plugged or inflamed pore.
Non-comedogenic means that in testing, the substance or product has not been shown to cause comedos (clogged or inflamed hair follicle). Some substances (eg. coal tar) are highly comedogenic and produce a type of allergic, acne-like reaction. There is not a clear consensus when it comes to the comedogenicity of many substances, with different tests yielding different results.
How is comedogenicity determined?
The large majority of comedogenicity testing is done on animals, often using a rabbit ear model. In this test, the substance is applied to the inside of the rabbit’s ear, which has a similar structure as human facial skin. The development of comedos is quantified to evaluate the relative comedogenicity of a substance. In some cases, human volunteers are used to evaluate substances. In these tests, the most common treatment area is the skin on the back.
What kinds of things impact comedogenicity?
Comedogenicity is a complicated process that can vary from individual to individual. One interesting observation is that human sebum is itself comedogenic. A substance can be comedogenic for several reasons. It can contribute directly to the formation of a plug in the hair follicle. This could potentially occur with a substance that triggers the coagulation of free sebum.
A substance could also be comedogenic because it triggers an allergic reaction and/or inflammation. Substances like SLS (sodium lauryl sulfate) are common ingredients in topical preparations and but can cause allergic reactions in some individuals and are generally considered comedogenic.
Additionally, a substance can serve as a direct food source for the bacteria responsible for acne, Propionibacterium acnes. P. acnes bacteria eat fatty acids as one of their primary food sources and certain substances like olive oil or other vegetable oils could potentially serve as food sources for these bacteria and encourage their growth. Increased bacteria levels in the skin can then stimulate a local immune response, inflammation and comedogenecity.
Lastly, relatively harmless substances can be converted into allergens and comedogens by the enzymes present in the skin, or even by UV light.
Common Comedogenic Substances
There are several lists of comedogenic substances available in different places on the internet. However, a review of the scientific literature reveals a serious lack of actual testing on commonly used substances. This may be because many companies do their own testing and do not publish the results, but it also casts some doubt on some of these online comedogenic substances lists.
We are currently working to compile a comprehensive comedogenic substance lists from published scientific journal articles. Until then, we have included this table from the original comprehensive comedogenicity testing done by Dr. Fulton, et al. Comedogenicity and irritancy are graded on a scale of 0 to 5, with 0 being no effect and 5 being highly comedogenic:
Comedogenicity and Irritancy of Commonly Used Ingredients in Skin Care Products. Fulton, et al. 1989.
A re-evaluation of the comedogenicity concept. Draelos, et al. 2006.
Comedogenicity of Squalene Monohydroperoxide in the Skin after Topical Application. Chiba, et al. 2000.
An Experimental Study on the Comedogenicity of Several External Contactants. Ahn, et al. 1985.