SKIN | What the hell is an acid mantle?


The ‘acid mantle’ is a term you might have heard mentioned if you’re a skincare science enthusiast, but chances are you don’t really know what it is and how it works. It came up in my skincare class at school, to the extent that ‘you have an acid mantle on your skin’. That’s it. So I didn’t understand it properly until I started researching for this article. It sounds like somewhere you might visit with Ms Frizzle on the Magic School Bus. Is there a skincare-based Magic School Bus episode?

This is an article I wrote for xoVain but it got dropped due to some changes happening to the site, which explains the slightly different tone to normal. Let me know if you’re into it!


We’re going to need a smaller measuring tape.

We’d have to shrink the Magic School Bus down pretty small to find the acid mantle. It’s found on the stratum corneum, or the outermost layer of the epidermis, which is itself the outermost layer of your skin. The whole epidermis is about as thick as a piece of paper, and the stratum corneum is 10-40 microns thick. This is not very much. It is fragile and delicate, and we must love and protect it.

The acid mantle is made from the sebum (oil) and sweat secreted from your pores, as well as dead skin cells. It doesn’t sound that great. It sounds like the type of stuff you’re trying to remove from your face every day. But this layer helps your skin maintain the right pH, and pH is very, very important for preventing bacteria growth and moisture loss. (Quick note here: when we’re talking about the pH of skin, we’re talking about the pH of the moisture on the skin. You can’t really measure the pH of a solid.)


This photo is meant to represent ‘layers’. Shooting photos for science-y posts is hard.

A healthy pH level for skin is somewhere between 4 and 5.5 (you might remember the Johnson’s pH 5.5 range from the 90s – what happened to that?). Water is neutral, and has a pH of 7. Soap has a pH of around 10, so it is significantly more alkaline than your skin.

Propionibacterium acnes, or P. acnes for short, is the bacterium largely responsible for acne. Pimples. Zits. ‘Blemishes’, if you’re using beauty blogger terminology. P. acnes thrives at pH values between 6 and 6.5, and its growth is seriously inhibited at pH values lower than 6.

The correct pH is also important for your acid mantle to protect from external irritants, environmental damage, and moisture loss. Basically, if the pH of your skin is too high, you’re going to lose water at a faster rate (this is called transepidermal water loss, or TEWL) and your skin is going to be dry and flaky.


If you haven’t figured it out yet, soap is a major enemy to your acid mantle. I really, really hoped that no one was putting soap on their face anymore, but I guess there are a few popular face soap options out there, including African Black Soap, and Dr Bronner’s. Now, I would say no shade if you’re using soap on your face and you’re into it, but like, a bit of shade.

Using soap increases the pH of your face for four to eight hours after washing, which is pretty significant if you’re washing your face twice a day. Scientific studies have shown that acne sufferers who washed their faces with soap twice daily actually got more pimples. Those who used cleansers based on synthetic detergents with pH levels closer to normal skin pH? Their skin improved.


Please don’t take my chemical exfoliants away from me.

So if putting something really alkaline on your skin is bad, then using an acid must be good. Right?

Well, mostly. If you’re using products that contain AHAs or salicylic acid in your skincare routine, they’re probably there to exfoliate. A bit of exfoliating is fine (great, in fact), but you’ve got to take care not to overdo it. As chemical exfoliants break down dead skin cells, they can also break down and strip away your acid mantle. If your acid mantle is gone, you’re going to have problems with that TEWL I mentioned earlier. Your skin will lose moisture, it may become more sensitive, and generally look and feel pretty dull and dehydrated.

Your solution here is to restore your skin’s barrier once you’re done with the chemical exfoliation. I know some chemical exfoliants say to leave on overnight or whatever for more effectiveness, but generally after 30 minutes they’ve done all they can. Make sure you always follow up with a gentle moisturiser!


I’ve got one last skincare trend I want to mention and this one is pretty fucking stupid. It’s using Milk of Magnesia as a primer for oily skin types. Milk of magnesia breaks down waxes and oils, so on first thought it makes sense. I haven’t tried it because I know it’s a bad idea, long term, but apparently it really mattifies your skin. Well, DON’T DO IT. Milk of Magnesia has an even higher pH than soap. Save it for when you need a laxative.


This is the face of someone who has a lot of “What is the pH of [product]” Google searches ahead of her.

I hope you’ve enjoyed this ride on the Magic Hyacinth Girl Bus to the land of the acid mantle. Has it changed how you’ll approach your skin? I often forget to moisturise in the evening, and then figure ‘Well, my skin is oily, it can handle it’, so that will definitely change.

The science in this article comes from here, here, here, and here.

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  • Katie Hall

    TIL I need to up my skincare game!

  • Cesy

    I had a complete 180 on my skincare when I got some expert advice. I had pretty much wiped out my acid mantle with foaming cleansers and over exfoliating. I am all about maintaining my acid mantle now with gentle products. It seems to be working.

    • That’s good to hear! Did you see a dermatologist?

  • anjaok

    your hair looks SOOO good Morgan!!

    • Thanks girl! I tried curling it with my straightener. I think I’ve still got a way to go before I master that!

  • Great post Morgan! I bet it took a lot of research..thanks for sharing the wealth with us.

  • This was so interesting. You’d think they would have taught me this in makeup school but NOPE. Sounds like you’re learning heaps of awesome things!

    I love Korean skincare because of the layering up of the moisture. Acid mantle party. So much great xoxo

    • I really want to get in to Korean skincare, I just wish it was stocked locally so I could try it out first!

  • Steph Hicks

    Soooo interesting! I always knew using soap to wash your face was bad but never knew the reason why so thank you for enlightening me 🙂 I too have oily skin and always think I don’t need to moisturise at night…is that bad??

    • You should try to always moisturise, even if its just a light layer! I like to use a fluidy moisturiser that doesn’t feel too thick.