The word comedo comes from the Latin comedere, meaning "to eat up", and was historically used to describe parasitic worms; in modern medical terminology, it is used to suggest the worm-like appearance of the expressed material. https://en.wikipedia.org/wiki/Comedo
A comedo is a clogged hair follicle (pore) in the skin. Keratin (skin debris) combines with oil to block the follicle. A comedo can be open (blackhead) or closed by skin (whitehead), and occur with or without acne.
What is comedogenic? Comedogenic ingredient is the one which clogs pores. It does so by increasing follicular hyperkeratosis—an increased production of keratin in hair follicles. Over time, this leads to clogged follicles and comedones. This doesn’t always happen quickly, and it can take months of using a comedogenic product before clogging is noticeable. Individual skin chemistry can determine the extent of an ingredient’s comedogenicity, so it is highly variable between people. One client may have no reaction, while another may have excessively clogged pores in a few weeks. Even ingredients that are not typically comedogenic can become so by a person’s own unique skin enzymes. Human sebum is naturally comedogenic, so even if clients who are prone to clogging avoid all likely comedogenic products, they are not necessarily guaranteed protection against comedones. If this is the case, how do skin care professionals know if ingredients are playing a role in their clients’ clogged or acne-prone skin?
Comedogenicity testing - The Rabbit Ear Test.
Comedogenic Scale.
The rabbit ear test has long been the gold standard. This test was originally used to assess the dangers of industrial chemicals in the 1950s and then entered the world of cosmetics in the 1970s. Rabbit ears were found to be more sensitive than human skin and also responded much more quickly to comedogenic ingredients. Follicular hyperkeratosis could be seen in a matter a weeks versus months.
You try out a chemical behind a rabbit’s ear – and if it causes a blackhead there you assume the same will happen in humans.
In 1979 Albert Kligman, a respected dermatologist, published a protocol explicitly intended for assessment of cosmetic ingredients. In it he created a scale of comedogenicity from 0 to 5. (0 was good, 5 was bad). Papers started to circulate using this method to generate data and drawing conclusions about just how likely to mess up your pores particular ingredients might be.
It was a seductive idea. One could run all your ingredients through a test and screen out all the ones that would give problems.
But the problems started as soon as the results started going around. Experienced formulators noticed quickly that what the data was predicting had little relationship to their personal experience. And things got worse when it turned out that there was little agreement between different papers studying the same ingredients.
The rabbit ear model was also used for predicting acne – naturally enough since blackheads and acne pustules have a strong family resemblance to blackheads. A new disease name was coined: Acne cosmetica. This was a form of acne supposedly caused by using highly comedogenic cosmetic formulations. The rabbit ear model lasted a bit longer in this context but it was still discredited in time. Even Kligmann himself came round to the idea that the rabbit ear test could produce very misleading results and was noble enough to say so in a paper published in 1996. By 2007 Howard Maibach, someone whose reputation is not too far behind that of Kligmann himself, could dismiss it.
Even if the rabbit ear model had been more reliable, it was hopeless from the start. Cosmetic formulations are not simply a sum of their ingredients. It is obvious that you can’t tell how a cake will taste from knowing a lot about flour, butter and sugar. The same is true of a skin cream or a lipstick. The Comedogenic Scale was never going to be of much use.
The trouble is that the papers are still out there and if you don’t know the context it is very easy to be misled. And it is such a nice idea as well. So to this day there are still people using the data generated from the rabbit ear model to judge how comedogenic formulations are. Internet is full of this data. At least one website has a complete chart of comedogenic ratings supposedly to help you choose the right products.
References
Albert Kligman original 1979 protocol
http://www.ncbi.nlm.nih.gov/pubmed/157151
Rabbit Ear Comedogenic Scale
http://journal.scconline.org/pdf/cc1983/cc034n05/p00215-p00225.pdf
Kligmann renounces rabbit ear model
http://journal.scconline.org/pdf/cc1996/cc047n01/p00041-p00048.pdf
Maibach dismisses it.
http://www.ncbi.nlm.nih.gov/pubmed/17687685