How to Use Comedogenicity Ratings (with Video)


You’ve probably seen a comedogenicity chart like these ones (and the one further down the page) before, rating different ingredients on their ability to cause pimples.

Supposedly you check the ingredients list of your product against the comedogenicity list. If it has highly comedogenic ingredients, it will cause pimples, if it doesn’t, then it won’t. It’s simple, systematic and foolproof, right? Unfortunately, it’s not quite that simple…

This post goes into the science behind these comedogenicity ratings, and how exactly you should use them. It also comes in video form!

Click here for the video, scroll down for the rest of the blog post…

What does comedogenicity mean?

Comedogenicity is the tendency of an ingredient or product to clog pores. Ingredients are ranked on a scale:

  • 0 – completely non-comedogenic
  • 1 – Slightly comedogenic
  • 2-3 – Moderately comedogenic
  • 4-5 – Severely comedogenic

The numbers in comedogenicity scales come from studies performed by academics, published in peer reviewed journals – this usually means they’re somewhat reliable and valid. However, like with many other skincare claims “supported by the literature”, problems emerge when you dig deeper!

What’s wrong with the comedogenicity scale?

The problem is that the studies that produced the comedogenicity ratings don’t reflect real-world usage, for a number of reasons:

Tests aren’t done in real-world conditions

In an ideal world, we’d test every single product on every single person’s face, and develop a definitive comedogenicity rating list based on that. But this would be impossible – it would cost too much, there are too many products, and getting a lot of people to only use the one product and not change their daily routine for weeks or months at a time would be a mammoth task.

Instead, what’s used in most scientific studies is a model – a situation that mimics the real world, but is simpler to carry out and control. Think crash test dummies, dyed samples of hair, pouring blue liquid onto sanitary pads, patch testing potential allergens on your arm, testing bikes on a race track.

Most of the time these models work pretty well, but sometimes they don’t reflect the real world situation, so their results can’t be applied to everyday life (they have low external validity). In the case of comedogenicity ratings, the models don’t fare so well.

The most common rabbit ear test is flawed

The most common test for comedogenicity is the rabbit ear test, pioneered in cosmetics testing by two famous dermatologists, Albert Kligman and James Fulton, in the 1970s. This involves applying a substance to the inner ear of a rabbit, and waiting a few weeks to see if any clogged pores formed. Because rabbit ears are more sensitive than human skin, they reacted to comedogenic products faster, which was more convenient.


Unfortunately, this also meant that there were lots of false positives, where ingredients that are non-comedogenic in humans would be found to be comedogenic in the hypersensitive rabbit model. Additionally, in the original tests, the scientists didn’t realise that there are naturally enlarged pores in rabbit ears. Some results counted these as acne, leading to even more false positives.

Related post: Purging vs Breakouts: When to Ditch Your Skincare

The most famous false positive is petroleum jelly (petrolatum or Vaseline), which was corrected in the late 1980s, but this was debated until the mid-1990s – that’s why the myth that Vaseline and oily products cause pimples is still so pervasive. This wasn’t the first time the rabbit ear tests were questioned – conflicting results were commonplace, and comedogenicity lists frequently disagreed with each other (and still do).

Related post: Is Mineral Oil Dangerous?

More recently in 2007, dermatologists Mirshahpanah and Maibach went so far as to say:

“[the rabbit ear] model is unable to accurately depict the acnegenic potential of chemical compounds, and is therefore only valuable for distinguishing absolute negatives.” – Mirshahpanah and Maibach, 2007

Tests on human subjects are also flawed

If rabbit ears don’t reflect what happens on human skin, then the obvious solution is to test on humans, right? Yes…but there are problems there too!

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Don’t Use Lemon Juice on Your Skin

There are more DIY skincare recipes on the internet than you can shake a stick at, and most of them really, really like lemon juice. According to these DIY tips you should be slathering it all over your face and hair. Lemon juice smells nice, and it’s pretty cheap. But is it effective? And is it safe? Let’s talk about …

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Is Sodium Hydroxide Safe in Beauty Products?

Is Sodium Hydroxide Safe in Beauty Products?

Sodium hydroxide is in a ton of beauty products. But as one of my favourite unreliable sources says (the “favourite” is sarcastic by the way, just to clarify): Sodium Hydroxide is, however, a known irritant… The National Institute for Occupation Safety and Health … recommends that consumers prevent skin and eye contact The CDC reports that “Skin contact with sodium …

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What Are Propylene and Butylene Glycol, and Are They Safe?

What are propylene and butylene glycol, and are they safe?

One of the common reader requests I get is for “toxic” ingredient breakdowns, so today I’m looking at two ingredients that are commonly on “avoid” lists: propylene and butylene glycol.

What are propylene and butylene glycol, and are they safe?

What are propylene and butylene glycol?

Glycols in chemistry are ingredients that contain two OH (alcohol) groups. Propylene glycol contains 3 carbon atoms, while butylene glycol is a little larger and contains 4 carbon atoms. In glycols, the alcohol groups are attached to different carbons.

Confusingly, the names “propylene glycol” and “butylene glycol” can refer to several slightly different substances, since there are a few choices of carbon atoms for the OH groups to be attached to.

Propylene glycol usually refers to propane-1,2-diol (formerly known as 1,2-propanediol). The less commonly used propane-1,3-diol is also sometimes called propylene glycol, but usually in cosmetics it’s called “propanediol”. Propanediol is become more popular since propylene glycol’s been on all these watchlists.

What are propylene and butylene glycol, and are they safe?

It’s a similar story for butylene glycol. “Butylene glycol ” usually means butane-1,3-diol, but sometimes it’s also used to refer to the related butane-2,3-diol.

What are propylene and butylene glycol, and are they safe?

What do propylene and butylene glycol do in products?

Alcohol (OH) groups on ingredients usually make them good humectant moisturisers that can hold onto water and keep your skin or hair hydrated. For example, glycerin has almost the same structure as propylene glycol, but with an additional alcohol group. Propylene and butylene glycol are both humectant moisturisers.

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The Pros and Cons of Accutane (Isotretinoin) for Acne

The Pros and Cons of Accutane (Isotretinoin) for Acne

This is a guest post from Facing Acne.

In the history of acne treatments, there has never been a medication that has caused more controversy than Accutane. There is also no treatment that has ever produced such startling results. For this reason, Accutane has both its passionate defenders and its equally passionate detractors. The choice of whether to try Accutane is a very difficult one. All the factors must be properly weighed, and if you are considering this option, you should be fully aware of all of the possible side effects as well as the nearly miraculous efficacy of this drug in clearing even the most severe cases of acne.

The Pros and Cons of Accutane (Isotretinoin) for Acne

A hyperaggressive treatment like Accutane should only be considered when other, more conventional treatments have been exhausted with unsatisfactory results. With so many options on the market, it is likely that one of them will fit your skin type and help give you relief from acne. (Related: Does Proactiv really work?) Many people have questions about the more common treatments and it can be overwhelming trying to sort through all of the options. But if you are a long-time acne sufferer, it is worth the effort. Here we take a closer look at Accutane and answer at least some of the most common questions in relation to this well-known, highly debated, and very controversial acne treatment solution.

How Does Accutane Work?

First of all, what is Accutane? Accutane is the brand name of the drug isotretinoin, which is a derivative of vitamin A. It is administered in pill form, usually for somewhere between 15 and 20 weeks. It works primarily by reducing the amount of sebum your skin produces and by helping your skin regenerate faster, thus healing existing lesions and evening out skin tone by eliminating any residual redness.

Different dosages are used depending on the severity of the condition. Higher dosages have a greater rate of effectiveness and a lower relapse rate, but also come with a heightened risk of side effects.

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Are unsaturated oils bad for your skin?


An article by Beautyeditor (now rebranded as The Skincare Edit) recently came to my attention, thanks to some readers who pointed me in its direction. In it, she blames polyunsaturated oils for aging, tells you to avoid them in your skincare AND diet, and tells you to use saturated fatty acids (in particular squalane) instead.

Are unsaturated oils bad for your skin?

As a big fan of oils containing unsaturated fatty acids (in particular rosehip oil which is my SOS skin saviour), I had to dig into this. Here’s a closer look at the science behind oils…

The facts about unsaturated oils

Let’s start with what Beautyeditor/The Skincare Edit got right. Unsaturated fatty acids are indeed less stable than saturated fatty acids, which means they’ll have a shorter shelf life.

Fats and oils are collectively known as triglycerides. Unsaturated triglycerides oxidise more easily than saturated triglycerides because they contain more double bonds, which are more reactive than single bonds. You’ll know an oil’s been oxidised when it goes rancid and smells a bit gross. The oils will react when exposed to air, light, heat and free radicals (though adding antioxidants like vitamin E to the oil will slow this down).

Triglyceride molecules consist of 3 fatty acids (blue) linked to a glycerin molecule (purple):


It’s the fatty acids that can vary and contain double bonds. Fatty acids are divided into saturated (no double bonds), monounsaturated (1 double bond) and polyunsaturated (2 or more) fatty acids.


Here are some common fatty acids in each category:

  • Saturated: lauric, myristic, palmitic, stearic
  • Monounsaturated: oleic (omega-9), palmitoleic
  • Polyunsaturated: linolenic, eicosapentaenoic, and docosahexaenoic (omega-3), linoleic (omega-6)

The fatty acids can vary, so you can get a triglyceride that contains, say, 2 saturated fatty acids and 1 polyunsaturated fatty acid. Most natural fats and oils aren’t entirely saturated or monounsaturated or polyunsaturated, but very few have an even balance of all three.

  • Mostly saturated fats and oils include coconut oil, butter, palm oil, beef fat
  • Mostly monounsaturated fats and oils include avocado oil and olive oil
  • Mostly polyunsaturated fats and oils include fish oil and most of the common skincare oils: safflower, sunflower, rosehip, almond, hemp, and grapeseed oils

Why might unsaturated oils be bad?

So far so good, but does this instability have an effect on your skin? Here’s where her argument gets dicey.

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Should You Be Avoiding Parabens? The Science

Should You Be Avoiding Parabens? The Science

Much like “organic” and “all-natural”, “paraben-free” is one of those phrases you’ll see displayed prominently on increasing numbers of skincare and beauty products. What are parabens, what health effects do they have, and should you be avoiding them? Here’s the science behind the marketing.

(I’ve written a much simpler rundown of parabens here, if you want a quicker overview.)


What Are Parabens?

Parabens are a family of preservatives commonly used to control the growth of microbes in cosmetics, toiletries, food and pharmaceuticals. They are esters of para-hydroxybenzoic acid, a naturally occurring chemical found in many fruits and plants. Their chemical structures and actions are very similar, with the “R” group changing as shown below.

Paraben Science: Should You Avoid Them in Your Products?

The most commonly used parabens are methylparaben, ethylparaben, propylparaben and butylparaben, although many others (isopropyl-, isobutyl-, pentyl-, phenyl-, benzyl-) have been used in products as well. Different parabens work best under different conditions and act against different microbes, so you’ll often see them used in combination to enhance the preservative effect.

Parabens were developed in the 1920s and these days, they’re the most widely used preservatives in cosmetics, appearing in over 85% of products. Parabens are popular for good reason: they’re inexpensive, effective in very small amounts, work well in most products, and act against a wide range of nasty microbes. They have a very long record (almost 100 years) of safe use. The only reliably linked harmful health effect is allergy, which occurs in a tiny fraction of people, and it’s often only a problem on broken skin.

Why Do Parabens Have a Bad Reputation?

Despite all these advantages, parabens have become well known as a “nasty” in the last 10 years. This came about when a few studies appeared which led people to question whether parabens were really as safe as they seemed:

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Why Has the FDA banned Antibacterial Soap?

Why Has the FDA banned Antibacterial Soap?

The U.S. Food and Drug Administration (FDA) has announced that hand and body washes containing certain antibacterial ingredients can no longer be widely sold (the final rule can be found here, and the consumer update is here). Why have they been banned?

Why Has the FDA banned Antibacterial Soap?

The Banned Antibacterial Ingredients

In high enough concentrations, antibacterial ingredients kill bacteria either by rupturing their membranes (their “skin”) or by interfering with how they work.

19 of these ingredients have been targeted:

  • Cloflucarban
  • Fluorosalan
  • Hexachlorophene
  • Hexylresorcinol
  • Iodophors (Iodine-containing ingredients)
    • Iodine complex (ammonium ether sulfate and polyoxyethylene sorbitan monolaurate)
    • Iodine complex (phosphate ester of alkylaryloxy polyethylene glycol)
    • Nonylphenoxypoly (ethyleneoxy) ethanoliodine
    • Poloxamer-iodine complex
    • Povidone-iodine 5 to 10 percent
    • Undecoylium chloride iodine complex
  • Methylbenzethonium chloride
  • Phenol (greater than 1.5 percent)
  • Phenol (less than 1.5 percent)
  • Secondary amyltricresols
  • Sodium oxychlorosene
  • Tribromsalan
  • Triclocarban
  • Triclosan
  • Triple dye

The most common ones in hand soap are triclosan and triclocarban. Any hand or body washes containing these ingredients will not be able to be sold in the US from September 2017.

Three other antibacterial ingredients are still being reviewed, but can still be used in soaps for the time being: benzalkonium chloride, benzethonium chloride, and chloroxylenol.

Why have antibacterial soaps been banned?

The key reason is: because they don’t work any better than non-antibacterial soaps to warrant the potential risks. While antibacterial soaps do tend to kill more bacteria, this hasn’t translated into tangible health benefits, like reduced rates of sickness. In the FDA’s regulatory terms, they’re no longer “generally recognised as safe AND effective (GRAS/GRAE)”.

The risks are:

Possible health effects from long-term, frequent exposure

Animal studies suggest that some antibacterial ingredients may contribute to health problems like hormonal disruption and skin cancer. A positive result in an animal study doesn’t automatically translate to humans, since there are fundamental differences between species – for example, chocolate is poisonous to dogs, and two paracetamol (acetaminophen) tablets will kill a cat. But it does signal that there could potentially be a problem, and more research is warranted (and is currently underway).

Contributing to antibiotic resistance

Antibiotics are a mainstay of modern medicine. However, a few bacteria have a natural immunity to antibiotics, and will survive and multiply. If this happens on a large enough scale, the bacteria will become resistant to that antibiotic, and you’ll need a new antibiotic. The scary thing is that antibiotic resistance is developing at a frightening speed – faster than the discovery and development of new antibiotics. Which means in the not-too-distant future, we may regress back to a scary time when skin infections were ~11% lethal.

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