Exfoliating away dead skin cells has long been the secret to glowing, healthy-looking skin. But unlike the harsh, gritty scrubs of the past, there's a new generation of exfoliants that get the job done using chemical, not physical methods.
AHAs and BHAs—a.ka. alpha- and beta-hydroxy acids—are the two most common families of exfoliating acids that you'll see in skincare products, and they're often found together.
So, how do they actually work? Which type is best for your skin? And can you get better results if you use both of them?
If you've been asking these questions, this tutorial is for you. You will learn the difference between AHAs and BHAs, how to choose the right one for your skin (and whether you should use both), and the best products to try now.
What Are AHAs and How Do They Work?
AHA stands for alpha-hydroxy acid—a type of acid that is derived from sugarcane, milk or fruit. AHAs peel away the dead skin cells on the surface of your skin, revealing the fresh new skin cells underneath.
And how exactly do they do that? It was long ago established that AHAs break down the "glue" between skin cells, which allows them to be sloughed off. But now several studies have hypothesized that it is through their impact on calcium ions that they weaken these adhesions.
This study was the first to discover that glycolic acid reduces the concentration of calcium ions in the epidermis. Later research found it does so by creating acidic conditions within cells, which allows calcium ions to flow in to the point where they overload the cells and trigger cell death (apoptosis). Another study confirmed that lactic acid initiates the same process, suggesting that AHAs share the same mechanism of action.
Types of AHAs
These are the different types of AHAs that you'll see in skincare products:
- Glycolic acid: The most common AHA, derived from sugarcane. It is also the strongest, due to its small molecule size, but that makes it the most irritating, too.
- Lactic acid: The second most common AHA, derived from milk. It's a gentler alternative to glycolic acid, and can be appropriate for sensitive skin.
- Mandelic acid: A mild AHA derived from bitter almonds. As it is weaker than lactic acid, it's usually combined with other acids. Notably, Dr. Loren Pickart considers mandelic acid to be a neurotoxin (see study)!
- Malic acid: A mild AHA derived from apples. Like mandelic acid, it won't do enough on its own, so you'll typically see it in combination with stronger AHAs.
- Tartaric acid: A weak AHA derived from grapes. Instead of acting as an exfoliant, it is more often used to stabilize other acids' pH levels.
- Citric acid: A weak AHA derived from citrus fruits. It is similar to tartaric acid in that it regulates pH. It is also used as a preservative.
- Phytic acid: A weak AHA derived from rice, seeds and grains. It is more commonly used as an antioxidant.
What Are BHAs and How Do They Work?
BHA stands for beta-hydroxy acid, a type of acid that is derived from willow tree bark, wintergreen leaves or sweet birch bark. Just like AHAs, BHAs exfoliate your skin surface—but they also work deep inside your pores, too.
On the surface level, BHAs help to remove dead skin by loosening the protein bonds between skin cells. They also have anti-inflammatory and photoprotective properties.
Inside your pores, BHAs have several mechanisms of action. They exfoliate the pore lining so that oil can flow out more freely, preventing the build-up of dead skin and sebum that leads to clogged and stretched-out pores.
They also reduce the amount of oil your skin produces, and slow down the rate of skin-cell shedding. (Certain skin conditions, including acne, are associated with hyperkeratinization—meaning the body is shedding skin cells too fast.) For existing comedones, BHAs help to dissolve oil and keratin plugs.
Types of BHAs
The main BHA exfoliants you'll see in skincare products are:
- Salicylic acid: The most common BHA, and also the strongest. However, it is not as irritating as glycolic acid (the strongest AHA) because of its large molecule size and anti-inflammatory nature.
- Betaine salicylate: A BHA comprised of salicylic acid and betaine (a hydrating amino acid derived from sugar beets). It's a gentler alternative to salicylic acid, and according to a study by the manufacturer, is equally effective. (A 4% concentration of betaine salicylate is said to be equivalent to 2% salicylic acid.)
- Salix alba or willow bark extract: A natural BHA derived from willow bark. The salicin content converts into salicylic acid, but it is much weaker (so it won't give you as dramatic results).
How AHAs and BHAs Are Similar
You may have heard that AHAs are best for exfoliating, brightening and anti-aging, while BHAs are only suitable for people with acne.
Fortunately, that simply isn't true. These are all the benefits that both AHAs and BHAs have in common:
- Exfoliating and smoothing: AHAs and BHAs are both effective at removing surface dead skin cells and creating a soft, smooth texture.
- Brightening: Both AHAs and BHAs have been found to reduce the thickness of the stratum corneum, the top layer of skin consisting of dead skin cells. This allows your skin to reflect more light and look more radiant.
- Fading pigmentation: Since they both encourage the shedding of old, discoloured dead skin cells, AHAs and BHAs work to fade dark spots and even out skin tone.
- Firming and reducing wrinkles: At higher concentrations, AHAs and BHAs have been shown to increase the density of collagen in the dermis (see studies here and here). That means they can both help to reduce fine lines and wrinkles and improve skin firmness over time.
- Hydrating: AHAs and BHAs are both humectants—ingredients that help your skin to attract and hold more moisture.
- Clearing and preventing acne: AHAs and BHAs both help with acne by exfoliating the dead skin that can lead to clogged pores.
How AHAs and BHAs Are Different
There are, however, some important differences between AHAs and BHAs:
- BHAs penetrate more deeply: AHAs are water-soluble, so they only exfoliate the skin's surface. But BHAs are oil-soluble, so they can pass through sebum and get deep into the pores.
- BHAs reduce oil production: While AHAs don't have an effect on sebum, BHAs actually slow down its secretion to help control oily skin.
- BHAs are more effective for acne: Although both acids can help with mild acne by sloughing off dead skin cells, BHAs also work their magic underneath the skin surface. By deep-cleaning the pores, BHAs not only clear existing breakouts but also help to prevent them long-term. I consider BHAs to be the most effective topical treatment for acne.
- BHAs "shrink" pores: Technically, you can't change the size of your pores—but they can look bigger when they are filled with debris. While AHAs don't affect pores, BHAs can help them to look smaller by keeping them clean.
- BHAs are less irritating: Any acid can be drying and irritating if you use it at the wrong concentration or pH, or if you apply it too frequently for your skin. However, AHAs (especially glycolic acid) are more often associated with irritation, redness and inflammation. BHAs are gentler due to their larger molecule sizes, anti-inflammatory benefits and lower required concentrations. According to the late Dr. Albert Kligman (who conducted many studies on BHAs), AHAs need to be used in concentrations of at least 8% in order to be effective, whereas BHAs only need a concentration of 1.5-2%.
- AHAs cause photosensitivity: It's well-known that AHAs increase your skin's sensitivity to the sun, making it more vulnerable to sun damage and premature aging. While sunscreen is essential always, BHAs actually have some photoprotective effects.
- AHAs cause skin wounding: As I mentioned above, AHAs are skin-wounding agents because they encourage cells to self-destruct through apoptosis (programmed cell death). BHAs are non-wounding agents, as they simply loosen the attachments between skin cells—which is a more physiological process. Is apoptosis or cell death something to worry about? Maybe, if you're using strong AHAs on a regular basis. Apoptosis is also induced by toxins such as estrogen, unsaturated fatty acids and radiation, and this paper describes it as "a cellular endpoint of the stress response."
Should You Use AHAs, BHAs or Both?
Now that you're familiar with the similarities and differences between AHAs and BHAs, what does that mean for your skincare routine? Here are my thoughts:
If you have acne: BHAs are the way to go. Salicylic acid is proven to reduce the number and severity of acne lesions, and is superior to benzoyl peroxide. Look for a concentration of 2% salicylic acid or its equivalent, 4% betaine salicylate. Keep in mind that you may experience initial purging (which is a good thing!).
If you have oily skin: Only BHAs will reduce your oil production. It may take some experimenting to find the best dose for your skin. I suggest between 1-2% salicylic acid, or 2-4% betaine salicylate.
If you have dry or sensitive skin: Lactic acid is the best AHA for these concerns, as it's one of the gentlest and most hydrating acids. Look for a concentration between 5-8% percent to start, moving up as high as 10% if tolerated. However, a mild BHA would be an equally appropriate option, such as 0.5-1% salicylic acid or 1-2% betaine salicylate.
If you have pigmentation: Both AHAs and BHAs will help, but I think BHAs are your best bet—especially if you have darker skin. Unlike AHAs, they won't trigger post-inflammatory hyperpigmentation, which is a risk for many ethnicities. Since BHAs also give you some photoprotection, you'll be at less risk of creating new pigment, too. Go for a higher concentration if you can, such as 2% salicylic acid or 4% betaine salicylate. To target discolourations even further, use it in conjunction with niacinamide (see this tutorial for layering tips!).
If you have wrinkles: Glycolic, lactic and salicylic acids have all been shown to produce changes in dermal thickness and the depth and number of fine lines and wrinkles. For best results, you'll want around 8% (or more) glycolic or lactic acid, or 1-2% percent salicylic acid. Again, just keep in mind that AHAs can make your skin more inflamed and vulnerable to sun damage, which can exacerbate signs of aging.
If you have "normal" skin: If you're blessed with well-behaved skin and don't have any major concerns, all you need from your acid is mild exfoliation and brightening. I suggest trying a gentle lactic acid (around 8%), salicylic acid (around 1%) or betaine salicylate (around 2%).
If you want to combine acids: If your skin tolerates it, you can layer AHAs and BHAs together in one routine—or use a product that contains both—but there's no real reason to do so. According to Dr. Kligman: "Salicylic acid does everything AHAs do and more, and does it more effectively." So if you have a good BHA, you don't need an AHA as well.
The Best AHA Exfoliants
The Ordinary Lactic Acid 5% + HA
The Ordinary Lactic Acid 5% + HA is one of the mildest formulas on the market, ideal for skin that is new to acids. It has 5% lactic acid along with hyaluronic acid, glycerin and propanediol for extra moisture. There's also a 10% version when you're ready for something stronger. See my brand review here.
ClarityRx Brighten It 10% Lactic Acid Solution
ClarityRx Brighten It 10% Lactic Acid Solution is a water-based mixture that contains 10% lactic acid in purified water. With only two ingredients, it is perfect for anyone who wants to avoid unnecessary or questionable ingredients, such as fragrance, oils, alcohol or silicones.
Biossance Squalane + Lactic Acid Resurfacing Night Serum
Biossance Squalane + Lactic Acid Resurfacing Night Serum is another 10% lactic acid formula, but in a lightweight overnight serum. Its moisturizing base includes coconut fatty acids, squalane oil and glycerin.
Youth To The People Kombucha + 11% AHA Exfoliation Power Toner
Youth To The People Kombucha + 11% AHA Exfoliation Power Toner combines 8% lactic acid with 3% glycolic acid for a total 11% concentration of AHAs. It's also got kombucha, a prebiotic, to support your skin's microbiome.
COSRX AHA 7 Whitehead Power Liquid
COSRX AHA 7 Whitehead Power Liquid is a mild glycolic acid treatment containing 7% of the active ingredient. This minimalistic K-beauty option also features 2% niacinamide for extra brightening, and panthenol and hyaluronic acid to moisturize your skin.
Summer Fridays Soft Reset AHA Exfoliating Solution
Summer Fridays Soft Reset AHA Exfoliating Solution boasts one of the highest percentages of AHAs, at 16%. Although we don't know how much is glycolic versus lactic acid, it is primarily comprised of the latter, along with multitasking niacinamide.
The Best BHA Exfoliants
Benton Aloe BHA Skin Toner
Benton Aloe BHA Skin Toner is a moisturizing and exfoliating toner made with a whopping 80% aloe and 0.5% salicylic acid. That, along with its just slightly acidic pH level, makes it mild enough for virtually all skin types
Juice Beauty Blemish Clearing Serum
Juice Beauty Blemish Clearing Serum is another low-dose salicylic acid treatment. suitable for not just acne-prone skin but anyone looking for gentle exfoliation, clarifying and brightening. It features 0.75% in a gel-like serum with aloe vera and other botanical ingredients.
COSRX BHA Blackhead Power Liquid
COSRX BHA Blackhead Power Liquid is my all-time favourite acid exfoliant—a 4% betaine salicylate solution spiked with 2% niacinamide. It gently clears and prevents breakouts while brightening and fading pigmentation. See my in-depth review here.
Farmacy Deep Sweep 2% BHA Pore Cleaning Toner
Farmacy Deep Sweep 2% BHA Pore Cleaning Toner is a gentle salicylic acid toner that should be well-tolerated by most skin types. Although it has 2% of the active ingredient, it's free of drying alcohol and irritating fragrance and essential oils.
Paula’s Choice Skin Perfecting 2% BHA Liquid Exfoliant
Paula's Choice Skin Perfecting 2% BHA Liquid Exfoliant is one of the strongest BHAs, which I'd specifically recommend for oily skin and breakouts. (On other skin types, it will likely be too drying.) It has 2% salicylic acid along with the penetration enhancer methylpropanediol.
The Best AHA and BHA Exfoliants
INNBeauty Project Down to Tone Resurfacing Acid Toner
INNBeauty Project Down to Tone Resurfacing Acid Toner gives you a mix of gentle AHAs and BHAs (lactic, phytic, malic, salicylic and citric acids) in a potent but non-irritating serum. That's because it's also packed with gluconolactone, glycerin, propanediol and niacinamide, and is alcohol-free and fragrance-free.
Tarte Knockout Tingling Treatment
Tarte Knockout Tingling Treatment features a 10% acid complex made up of lactic, citric, malic, phytic and salicylic acids. It also has niacinamide for brightening and reducing inflammation, glycerin for moisture, and sulfur to reduce the look of pores.
Moon Juice Acid Potion
Moon Juice Acid Potion is a nightly acid exfoliant that balances 25% AHAs and BHAs with nourishing hydrators. While we're not told the percentage of each, the acids include glyolic, phytic, lactic and salicylic, and are combined with niacinamide, propanediol, allantoin and mushroom extract.
The Ordinary AHA 30% + BHA 2% Peeling Solution
The Ordinary AHA 30% + BHA 2% Peeling Solution is so strong, you're only meant to leave it on your skin for 10 minutes, twice a week. There 30% AHAs in here—glycolic, lactic, tartaric and citric acids—plus 2% BHAs in the form of salicylic acid. It also has hyaluronic acid, provitamin B5 and Tasmanian pepperberry (which gives it its famous blood-red hue).
Now you know the differences between AHAs and BHAs.
As you may have guessed, I side with Dr. Kligman—I think that BHAs are the more beneficial and versatile acid, no matter what your skin type. "AHAs cause smoothing of the skin by removing some of the superficial dead cells, which makes them exfoliants, but that's all they do. And they don't begin to compare with salicylic acid, which cleans out the pores."
In other words, BHAs do everything AHAs do, and more! So I hope you'll consider trying them out, if you haven't already. You may very well find that your skin is less irritated than it was with AHAs, and of course, less congested. Just be patient if you do experience more breakouts initially. Purging simply means that the BHA is doing its job of deep-cleaning, and your skin will become clearer within a few weeks.
Whether you choose an AHA, a BHA or both, the key is to find a formula that is the right strength for your skin, and not overdoing it. While stronger peels are great for instant results (like before a special event), I believe you'll get the best results from introducing a gentler, low-dose acid on a regular basis—as often as daily.
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