If you know a bit about skin care, comedogenicity probably seems uncomplicated: ingredients or products that cause comedones (aka clogged pores) are comedogenic, while those that keep skin in the clear are not. “Pores already have enough debris, hair follicles, dead skin cells, and bacteria to clog on their own,” says dermatologist Corey L. Hartman, MD, the founder and medical director of Skin Wellness Dermatology in Birmingham, Alabama. Add a cream, some makeup, or oil, and that can be a recipe for surefire congestion.

This all sounds simple, but it’s anything but. To understand why that is, let’s rewind to the 1970s. Back then, dermatologists knew that some skin-care ingredients clogged pores, but there was no centralized reference. That all changed when Albert Kligman, MD, the co-inventor of Retin-A, tested potential comedogenicity by applying cosmetic ingredients to rabbits’ ears, then measuring whether they clogged pores.

“The point of doing those tests was that if anything is comedogenic on a rabbit ear, it would likely be comedogenic on human skin,” says Krupa Koestline, a clean cosmetic chemist and the founder of KKT Consultants. Using results from his experiments, Dr. Kligman developed a comedogenicity scale to assign individual ingredients a 0-through-5 rating. The higher the number, the more potential an ingredient had to clog pores.

Early research on comedogenicity came at an inhumane price.

But Dr. Kligman’s research came at a terribly inhumane price. Along with testing on animals, he performed human experiments on primarily Black incarcerated men in Philadelphia. Memorably and chillingly, he described his first entrance into Holmesburg Prison thusly: “All I saw before me were acres of skin…It was like a farmer seeing a fertile field for the first time.”  Shampoo, eye drops, deodorants, perfume—along with other more damaging experiments—were tested on the backs of these men until 1974 when hundreds of his subjects sued Dr. Kligman, and the public outcry caused his human experiments to be shut down. Decades later, and despite his legacy of disgrace, his comedogenicity scale remains the standard reference. But why?

Essentially, nobody has come up with anything more precise—and a universal scale doesn’t make sense anymore, in part because research now shows that various ingredients affect people differently. In other words, your friend may not react to a comedogenic ingredient, but you might (and vice versa). “It really depends on your skin type,” says Dr. Hartman. Take coconut oil, which earns a highly cloggy 4 on the comedogenicity scale. “If you have very dry skin and minimal pores, then go for it,” he says. “You’re probably the kind of person who’s never had an acne breakout in your life, so you probably need extra moisture and the barrier protection that an occlusive oil can provide.” On the other hand, if you’re prone to breakouts or have oily skin, coconut oil is likely to cause issues.

Comedogenic ratings measure a raw ingredient, not its final form.

Another crucial way the scale isn’t as cut-and-dry as it may suggest: Comedogenic ratings measure a raw ingredient, not an ingredient in its final form. “Even if I make a product with a whole bunch of comedogenic ingredients, it’s not necessarily going to be a comedogenic product,” says Koestline, the cosmetic chemist. She explains that a raw oil may clog pores, but its molecular structure can change in a formulation, which renders it non-comedogenic in the final product.

Still, some acne-prone people find that strictly avoiding potential pore cloggers is the only way to avoid developing painful breakouts. In-demand Los Angeles esthetician Biba De Sousa says that comedogenic beauty products cause the majority of her clients’ problem. “Roughly 95 percent of my clients who come to me with acne, no matter how severe, have cosmetic acne,” she says. “My mission is to make it known that treating it is possible.”

That’s why she reviews each client’s product regimen, screening it for potential pore cloggers on her “no list” of more than 50 ingredients. Clients can keep using existing products that are unlikely to cause problems. Otherwise, she steers them toward pore-friendly substitutions that she deems acne-safe. (Notably, indie clean-beauty brand Merit sought De Sousa’s involvement in ensuring that its entire product line is free of acne-triggering ingredients—proving the value of her expertise and approval.) While she believes in the effectiveness of this approach, De Sousa agrees that comedogenicity is more nuanced than the scale alone implies. “Comedogenic ingredients are not all comedogenic for one specific person,” she says.

“Comedogenic ingredients are not all comedogenic for one specific person.” —Biba De Sousa, esthetician

Whether you adhere to the gospel of the comedogenic scale or not, some ingredients and textures are more likely to cause congestion. For example, Dr. Hartmann tells his acne-prone patients to avoid products that feature coconut oil, cocoa butter, or lanolin as a primary ingredient. “They’re not ‘bad’ ingredients, but certain people who have acne-prone or oily skin may run into trouble,” he says. If this sounds like you, he recommends incorporating retinol or an alpha-hydroxy acid to help keep pores clear. “I think that’s the best way to regulate pores to make sure build-up doesn’t occur,” he says. To be on the safe side, swap facial oil for a moisturizing serum with hyaluronic acid, glycerin, or squalane.

In addition, Koestline recommends doing a patch test before trying any new products. “Apply a product under your arm to see if it causes any acne or clogs any pores,” she says. If it causes problems there, it’s likely to cause problems on your face. For acne-prone people, she suggests looking into emulsions—typically, the stuff of lighter moisturizers. “Emulsions are very forgiving,” she says. “Water and oil droplets are micronized when you make an emulsion, so it is very hard for something to clog pores at that point.”

Beyond that, when you find a product that works for you, stick with it. Because while we should consistently be updating our standards and moving on from old science that doesn’t mean much by way of today’s more-advanced formulations, what we can stand behind is great products that drive the results you want.

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