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In an ongoing effort to justify my blog name, let’s talk about ACIDS. I LOVE ACIDS. Your skin is naturally acidic, so most of the stuff you put on your face should fall on that side of the pH scale anyway, but this week we’re going to talk about a very special type of acid: the chemical exfoliant workhorses of our skincare toolkit, hydroxy acids.

What Are Hydroxy Acids?

In terms of skincare, hydroxy acids function as exfoliants, meaning they slough off the dead cells on the outer layer of your skin. They also have a lot of other great and not-so-great effects, depending on the type of acid.

If you took a chemistry class in the last couple of years, maybe this is relevant to you: hydroxy acids are carboxylic acids with one or more hydroxyl groups attached. If the hydroxyl group is one carbon away from the carboxyl group, it’s called an α- (alpha-) hydroxy acid; if the two groups are two carbons away from each other, it’s a β- (beta-) hydroxy acid.

Non-chem nerds can tune back in. Essentially, there are two types of hydroxy acids: alpha and beta. Can you guess what the next two headings are going to be?

Alpha-Hydroxy Acids (AHAs)

Alpha-hydroxy acids are derived from plant, fruit, and milk sugars. The most popular AHAs are glycolic, lactic, malic, and mandelic acids. Glycolic acid is derived from sugarcane, and lactic acid is made from sour milk. (Sometimes it feels like even when skincare ingredients aren’t made of, like, starfish extract and snail mucus, they’re still always kind of weird.)

Okay, maybe a little less weird than starfish extract.

AHAs are water-soluble and are used in two ways: in concentrations of 4-10%, they’re daily mild exfoliants; in concentrations above 20%, they’re used as intense chemical peels. (Chemical peels can be great for people suffering from acne, including closed comedones; they encourage more shedding and peeling of the top layer of skin, revealing new, healthier skin underneath.)

For their primary function as exfoliants, AHAs work by disrupting the ionic bonds between cells in the epidermis. As these bonds are broken, the dead skin cells on the surface of your skin loosen and slough off, making skin look brighter, smoother, and more even while reducing the appearance of fine lines and wrinkles. They also increase cell turnover and inhibit excess melanin production, which means they can fade hyperpigmentation and acne scars.

But they’re not done there! (I really like AHAs, friends.) As I mentioned in my post about anti-aging, glycolic acid (the smallest AHA and therefore the one that penetrates the skin most easily) can increase collagen production in the dermis. Dermal collagen is what gives your skin its bounce and firmness; there aren’t many things that can make you grow more of it, but AHAs are on the short list.

AHAs can also increase hyaluronic acid production in your dermis and epidermis. Hyaluronic is an extremely hydrophilic (water-loving) molecule that draws water towards itself – it can hold 1,000 times its weight in water – so more of that stuff means your skin is plumper and more hydrated. Because of that, AHAs are great for people with dry skin.

Okay okay okay, just one more, I promise, but this one is a DOOZY: because AHAs increase cell turnover, they can straight-up reduce the formation of tumors. I’m not kidding! From this article:

“The protective effect of glycolic acid was a 20% reduction of skin tumor incidence, a 55% reduction in tumor multiplicity (average number of tumors/mouse), and a 47% decrease in the number of large tumors (>2 mm)”


There are two major downsides to AHAs. First, they’re very photosensitizing, meaning if you use one, you’re more prone to sun damage, including sunburns and long-term UV damage. If you use any type of AHA with any type of frequency, ALWAYS wear sunscreen when you go outside. Second, they can be pretty irritating, especially higher concentrations. If you start using an AHA, use it once every three days at first, then once every two days, letting your skin fully adjust to each stage before using every day.

Beta-Hydroxy Acids (BHAs)

I say “acids”, plural, but in terms of skincare, there’s only one beta-hydroxy acid: salicylic acid. It’s derived from willow bark (same as aspirin!) and is similarly magical.

Future salicylic acid!

Unlike AHAs, salicylic acid (SA) is oil-soluble, which gives it unique powers. It exfoliates by dissolving the fatty intracellular matrix in your stratum corneum, the outermost layer of your skin. That matrix holds together the dead skin cells you want to get rid of, so a BHA will help you shed them naturally.

BHAs, being fat-soluble, can also penetrate your skin by traveling down your sebaceous follicles (where your skin’s oil comes from). Once there, it helps dissolve and loosen the gunk in your pores. Salicylic acid is antimicrobial, anti-inflammatory, antifungal, and anti-comedogenic. If you have acne, those are all things you want in your pores, so SA is a perfect choice for people dealing with pimples.

Salicylic acid isn’t nearly as irritating as AHAs; in fact, since it has anti-inflammatory effects, it can actually be kind of soothing. It can be moderately drying, though, so always follow up with a moisturizer or five.

More good news: salicylic acid is actually photoprotective, meaning it reduces the effects of sun damage. You should still wear sunscreen all day all the time, of course, but that’s still pretty neato burrito, as the youths say.

Poly-Hydroxy Acids (PHAs)

PHAs are a pretty recent innovation in skincare. Basically, they do the same thing as AHAs, but they’re less irritating. Gluconolactone is the most popular PHA right now, and some studies indicate it’s photoprotective, instead of photosensitizing like alpha-hydroxy acids. Something to keep an eye on.

pH and Hydroxy Acids

AHAs and BHAs are acids. This concludes the “is she stating the obvious because she thinks we’re idiots or because she’s suffered a recent head trauma?” portion of this blog post.

As acids, each type has its own ideal pH – the pH at which it does its very best work. This is called its pKa value; in science terms, when the pH of an acid is equal to its pKa, exactly 50% of the compound is in its acid form, and 50% is in its neutral salt form. In non-science terms, it’s when the acid feels completely comfortable and balanced. It’s an introvert that gets one big social event a week; it’s getting back from a camping trip and then going for brunch with the girls. Completely, 100% balanced.

So ideally, the pH of an AHA or BHA we’re using is the same as its pKa. (Salicylic’s pKa is 2.97, and the AHAs are all in the 3.4-3.9 range.) It doesn’t have to be exact, but as the pH rises above its pKa, less and less of the acid is available in its free acid form, and more and more is in its useless (to us, at least) salt form. From the great blog Lab Muffin, here’s a chart of how much of salicylic, glycolic, and lactic acid is available for exfoliation at various pHs:


Something I want to make a note of, though: just because it’s stronger, doesn’t mean it’s better for your skin. Especially with AHAs, there’s a tradeoff – the more exfoliating it is, the more irritating it is. It’s all about finding a balancing act. You don’t need 100% of the free acid on your face, because you want to keep your face and not burn it off. Deeper penetration is not always better. (That’s what she said that’s what she said that’swhatsshesaid)

To simplify:

AHAs: best for daily use in concentrations of 4-10% and below a pH of 4

BHAs: best for daily use in concentrations of 1-2% and below a pH of 3.5

Usage: Since these acids need a particular pH to do their best work, try not to apply any other products that would disrupt that pH for at least 20 minutes after you use an AHA or BHA.

How Can I Introduce Hydroxy Acids?

Slowly. Pick one to start with. If your skin is rough, dry, or you’re dealing with hyperpigmentation and acne scars, start with an AHA; if you have oily skin or are struggling with active acne, a BHA might be a better jumping off point.

Either way, introduce them slowly – every other day, max, until your skin adjusts, then go up to every day. Once your skin is totally used to one kind of hydroxy acid, you can consider adding the other. Slowly. Hydroxy acids can be serious business.

Some products I’ve either tried and enjoyed, or have heard good things about:

Paula’s Choice 2% BHA Liquid ($28): my favorite BHA so far. It has a semi-oily formulation, which is actually a good thing for my skin because it keeps it from drying out.

Cosrx BHA Blackhead Power Liquid ($18): I’ve heard amazing things about this stuff. Cosrx as a brand is killer at making simple, no-frills, highly effective products. If you have sensitive skin, this might be a better starter product – it uses betaine salicylate, a milder spinoff of salicylic acid.

Silk Naturals 8% AHA Toner ($9): this toner uses lactic acid, which is gentler than glycolic. I like it fine. It’s pretty bare-bones, but I’ve seen decent results, and you can’t beat the price.

Cosrx AHA 7 Whitehead Power Liquid ($17): it uses 7% glycolic acid and, again, I’ve heard amazing things. (I’m trying to wait until I finish my Silk Naturals to try a new AHA. Waiting is the worst and I hate it.)

Makeup Artist’s Choice Mandelic Acid Peels: I love these things. I use the 25% and 40% mandelic peels every couple of weeks, and the results are phenomenal – three minutes on my face and I get smooth, bouncy, even skin with minimal irritation. (Mandelic acid is one of the biggest AHA molecules, so it doesn’t penetrate as far and is less irritating.) Use this link to get 25% off your MUAC order.

Sources: 1, 2, 3, 4, 5


  1. Another stellar blog post! Your posts are really easy to understand and process, which I really appreciate! I’ve read other posts and even though they are speaking English and using analogies, I still do not understand what they are saying… haha You rock! I can’t wait to get back on the acids wagon! (Slowly, slowly, of course!) Also, adore the TWSS reference, too!!

      1. No but for real though! I vaguely remember reading about AHA’s potential as a collagen builder, but I don’t think I ever saw it spelled out quite as clearly as you did. And I’m a person who needs things spelled out sometimes.

        Plus, PHAs? I gotta try some of those sometime.

      2. That makes me so happy! The science of skincare makes my brain feel sparkly so I’m really glad I can share that with people in a way that connects. And for REAL about PHAs! Everything AHA does, minus irritation and photosensitivity? SIGN ME THE FUCK UP.

  2. Hello there, I was looking for a long time for something like this post, and it explains all I wanted to know about acids, thank you so much!! I have a question though, you have recommendations for every acid except for Poly-Hydroxy Acids (PHAs), have you heard of any product with it that someone has raved about? I am super curious because AHAs have been really irritating for me, and I’m looking for something nice to my dry skin. Thanks in advance for any help on it!

    1. Thanks! No, I haven’t tried any PHA products, nor have I heard much about any on the market. They’re still so new that there’s not much out there. I’ll try to hunt some down, though!

      1. Do you find the Zelens PHA pads drying at all given alcohol is the second ingredient? I know alcohol is not necessarily the devil for all, just curious about your experience with it in these pads which otherwise look interesting!

  3. Hi mandelic acid has pka 3.4 . So does that mean you want to find a product that is close to 3.4 ph level as possible ? Is 3.0 good or too low , 3.8 good or too high ?

    Second what is ideal free acid level to exfoliate ? Thank you !!!

  4. As far as order is concerned, do AHAs go before BHAs or the other way around? The conflicting opinions are real; a convincing scientific case for either, however, as of yet is not… after reading your post on waiting times, I would be very interested to hear your position. Thanks greatly!

  5. Hello Queen, what about CAPRYLOYL SALICYLIC ACID? It is oil-soluble do you think it will need a PH to be fully potent in anhydrous products?

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