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.
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, lactic acid is made from sour milk, and mandelic comes from bitter almonds.
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, as well as those seeking a powerful anti-aging treatment; 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 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.
AHAs 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 it means your skin is plumper and more hydrated. Because of that, AHAs are great for people with dry skin.
There are two major downsides to AHAs. First, they’re very photosensitizing, so 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.
Unlike AHAs, salicylic acid (SA) is oil-soluble, which gives it unique powers. It exfoliates by penetrating 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 soothing. It can be moderately drying, though, so always follow up with a moisturizer.
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.
pH and Hydroxy Acids
AHAs and BHAs are, of course, acids. 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.
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.
Something I want to make a note of, though: just because it’s stronger, doesn’t mean it’s better for your skin. 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.
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 a few 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.
If you're new to hydroxy acids, check out our Soft Touch AHA, which is formulated with 10% mandelic acid and is perfect for sensitive or reactive skin.