How products are tested for pore-clogging (and what the tests miss)

When a label says a product “won’t clog pores”, somebody, somewhere, presumably checked. But how? You can’t see a pore plugging in real time, and waiting months for blemishes on hundreds of faces isn’t practical for every new moisturizer. So dermatologists and cosmetic chemists built shortcuts, and their story explains a lot about what a “non-comedogenic” claim can and can’t tell you.

Where it started: the rabbit ear

In the early 1970s, dermatologists described acne cosmetica, a low-grade, stubborn acne in adults that seemed to be caused by their cosmetics. The next question was which ingredients were to blame, and the tool that answered it was the rabbit ear.

The inside of a rabbit’s ear has large, accessible follicles, the tiny tubes in skin that hair grows from and that sebum, your skin’s natural oil, flows through. Researchers applied a test substance there and watched whether the follicles filled with keratin, the protein of dead skin cells, forming plugs called comedones (a blackhead is an open comedone, a whitehead a closed one).

A widely used version, published in 1989, went like this: dilute the ingredient to about 10 percent in propylene glycol, apply 1 mL to the inner ear once a day, five days a week, for two weeks, then measure how wide the follicular plugs are, by eye and under a microscope. The result was graded on a 0-to-5 scale: 0 to 1 was minimal, 2 to 3 borderline, and 4 to 5 reproducibly positive. Isopropyl myristate, a common emollient known to plug follicles, served as the positive control every run was compared against.

The method was cheap, fast and repeatable, and it produced clear rankings for screening raw materials. Those rankings are the ancestors of the “comedogenic scale” lists still circulating online.

Why rabbits misled us

The trouble showed up as soon as anyone compared rabbits with people. In 1982, a review in the Journal of the American Academy of Dermatology cited “the many problems in performance of the test method, the absence of correlation with experience in the human”, and the “serious limitations” of any conclusions drawn from it. The same year, the dermatologists behind much of the original rabbit work published their own human test and reported flatly that the rabbit model is more sensitive than the human: substances only weakly comedogenic in rabbits were probably safe for people, with the possible exception of acne-prone skin.

Several things make rabbit ears a poor stand-in. Their follicles plug more readily than ours. The test used near-pure ingredients, at concentrations and with a daily dosing no finished product delivers. And a rank on a 0-to-5 scale says nothing about how an ingredient behaves as a small fraction of a lotion with twenty other ingredients.

In Canada, since December 22, 2023, it has been illegal to test cosmetics on animals in Canada to meet a safety requirement, or to sell a cosmetic whose safety depends on new animal test data. For anything new, the rabbit ear is off the table here.

The modern method: patches on the upper back

The human test described in 1982 is, with modifications, still the standard. The logic is simple: find people whose skin forms comedones easily, apply the product where follicles are large, and look inside the follicles afterward.

Who volunteers

Studies recruit adults with prominent follicular openings on the upper back, who form comedones readily. The original 1982 work used young Black men, chosen for their large follicles. The point is sensitivity: if pores that plug easily don’t plug, ordinary pores are unlikely to.

What happens

The product goes under an occlusive patch, a covering that traps it against the skin, on a marked area of the upper back. A 2006 study used 0.2 to 0.5 mL of product three times a week for four weeks; a 2021 study applied test creams for four weeks alongside two controls. A positive control, a known pore-clogger such as isopropyl myristate or octyl palmitate, goes on another spot to prove the test can detect plugging; an empty patch, the negative control, shows what occlusion alone does.

How plugging is measured

The clever part is the follicular biopsy, which isn’t surgical. A drop of fast-setting cyanoacrylate glue is pressed onto the skin with a slide, allowed to set, and peeled off, lifting out the top layer of skin along with the contents of the follicles. Under a microscope, a technician counts the microcomedones, comedones still too small to see with the naked eye, before and after the four weeks.

What counts as a pass

In the 2021 trial, the positive control raised the microcomedone count from about 6 to about 27 per area, an increase of more than 50 percent in every volunteer. The empty patch saw counts fall. The test products all produced less than a 50 percent increase and were reported as non-comedogenic on that basis. That threshold is a common yardstick, though no regulation fixes it.

Why the whole product matters more than the ingredients

The 2006 study tackled the question shoppers actually care about. Its authors noted that animal tests had rated raw materials “with the assumption that finished formulations containing these ingredients would also be comedogenic”, and that dermatologists had duly handed patients lists of ingredients to avoid. They then patch-tested finished cosmetics containing those very ingredients on six comedone-prone volunteers. Their conclusion: finished products using comedogenic ingredients are not necessarily comedogenic.

Why? Concentration, first: an ingredient at 100 percent in a rabbit ear and at 2 percent in a lotion are different exposures. The rest of the formula, second: emulsifiers, water content and how quickly a product absorbs all change how much of any one ingredient actually sits in a follicle. The 2021 study made the same point from the other direction: creams containing lanolin and several plant oils, ingredients on many “avoid” lists, passed the human test.

That’s why a claim backed by a test on the finished product is worth more than one based on checking the ingredient list against a chart.

What even the best test can’t tell you

A human patch test is a screen, not a promise.

• It’s small. Six, twelve or fifteen volunteers is typical: enough to catch a strong pore-clogger, not enough to capture how differently individual faces behave.
• The back isn’t the face. Facial skin has busier oil glands and meets sweat, sunscreen, makeup and hands. A patch on the back is a tidy version of real life.
• Occlusion is artificial. Sealing a product against skin for weeks exaggerates exposure, which makes the test sensitive but not a mirror of daily use.
• It measures plugging only. A product can pass and still sting or trigger the fast red bumps that come from irritation rather than blocked pores. Some labels use “non-acnegenic” for that separate question.
• Nobody sets the rules. No Canadian or American regulator prescribes the method, the number of people or the threshold. Two identically labelled products may have been tested very differently, or not at all.

If you break out easily, prefer products whose makers describe human testing on the finished formula, try one new product at a time, and give your skin several weeks to answer. If breakouts persist despite careful choices, or you have deep or painful spots, see your doctor or pharmacist; treatments exist that go well beyond what any label can offer.

Pore-clogging tests began with rabbit ears, which flagged many ingredients that turned out to be harmless on people. The better method is a four-week patch test on comedone-prone human backs, run on the finished product rather than its parts. It’s a sensible screen with known limits: small, artificial, unregulated, and silent about irritation. Read “non-comedogenic” with those limits in mind.