1,4-Dioxane Ban for Beauty Products? (With video)

Dioxane video

If you’ve ever looked up sodium laureth sulfate (SLES) and sodium lauryl sulfate (SLS) in a clean beauty app, like me, you might be surprised to find that SLES is dirtier than SLS. These apps are terrible and based on pseudoscience: SLS and SLES are both safe in your products. But if you’re a skincare nerd, you’ll know that SLS …

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Will Benzene in Sunscreens Give You Cancer? (with video)

In case you haven’t already heard, the 2021 edition of “This Anti-Cancer Product Is Giving You Cancer” is that there’s been benzene found in a bunch of sunscreens. How much should we be freaking out about this? I did an Instagram post on this already, but I thought I’d elaborate a bit more. The video is here on YouTube, scroll …

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Should you avoid aluminium in deodorants? The science (with video)

I’ve recently been approached by several natural deodorant companies to talk about the health risks of aluminium-containing antiperspirants. As much as I’d like that sweet sweet cheque… I can’t. Because the science just isn’t there. So instead, I’m going to debunk the aluminium scaremongering. (Note to deodorant companies: There are valid reasons to switch to natural deodorants. Aluminium ingredients can …

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Clean Beauty Is Wrong and Won’t Give Us Safer Products

Think Dirty App

If you use beauty products, it’s impossible to not have heard of clean beauty. It’s a revolution. You want your products to be clean – you don’t want to use dirty products! You want your products to be good for your health and good for the environment. There are beautiful celebrities telling you about how they detoxed and cleaned out …

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How Bad Is Alcohol in Skincare, Really? The Science (video with KindofStephen)

Alcohol (ethanol, ethyl alcohol, SD alcohol or alcohol denat) is one of the most controversial ingredients in skincare. You might’ve heard that it speeds up aging, kills skin cells, and creates inflammation. But what do the studies actually tell us? Stephen (@kindofstephen) and I have been working on this topic for a long time, and we hadn’t decided on the …

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Silicone Mythbusting (with Video)

Silicone Mythbusting Video

This post is sponsored by Grant Industries. Silicones are one of the Big Bad ingredients in skincare, make-up and haircare. They’ve been demonised by natural brands, and there are more warnings about them than you can count. So I was delighted when Grant Industries (ingredient manufacturer, maker of Granactive Retinoid, physical sunscreens and of course, silicones) asked me to do a …

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How to Use Comedogenicity Ratings (with Video)

rabbit-ear-test-comedogenicity

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.

rabbit-ear-test-comedogenicity

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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