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.

True, our skin naturally exfoliates dead cells daily, but over time — primarily as a result of age and with lack of sun protection, that shedding process tends to slow down and may stop altogether. This results in dull, dry, or flaky skin; clogged, enlarged pores; bumps, wrinkles, loss of firmness, and uneven skin tone. So, a little help from some exfoliating products is often needed for glowy skin. Knowing exactly what these exfoliating acids are, how they differ, and exactly what their glow-worthy benefits are can help you attain the skin of your dreams.

What are AHAs and BHAs?

AHA stands for alpha hydroxy acid. BHA stands for beta hydroxy acid. The purpose of both AHAs and BHAs is to exfoliate the skin in a non-abrasive manner by “ungluing” the bonds holding dull, dead skin on the surface. Once those bonds are broken — gently and evenly — skin naturally sheds its spent cells.

While neither type of hydroxy acid is “better” than the other, they target different needs and skin types. Plus, many exfoliants combine both ingredients, allowing you to tackle multiple skin concerns at once (more on that later).

AHAs are water-soluble acids made from sugarcane, milk, or sugary fruits. Since AHAs are water-soluble, they peel away the dead skin cells on the surface level of your skin, revealing the fresh new cells underneath.

BHAs are oil-soluble acids made from willow tree bark, wintergreen leaves or sweet birch bark. Since BHAs are oil-soluble, they don’t only work on skin’s surface to remove dead skin cells but they also go deep inside the pores to remove excess oil, so BHAs are more often preferred for normal to oily skin.

How does AHAs work?

AHAs exert benefits both to the outer layer of the skin (epidermis) and the deep layer (the dermis). In the epidermis, AHAs have an exfoliative effect, increasing the shedding of dead skin cells retained at the surface. They do this by gently cleaving the retained bonds between dead skin cells. Once cut loose, those cells can then fall off, or exfoliate.

AHAs also help stimulate collagen production, making it ideal for those wanting to reduce the appearance of fine lines. AHAs do this both by increasing collagen synthesis by fibroblasts (the collagen-producing cells in the skin), and by decreasing degradation of the existing dermal matrix. AHAs been proven effective in reducing the visible signs of sun damage and wrinkles (FYI: AHAs don’t replace sunscreen).

How does BHAs work?

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 AHAs

  • 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 almond extracts. As it is weaker than lactic acid, it’s usually combined with other acids.
  • 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.

Types of BHAs

  • 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 composed 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 dramatic results).

How AHAs and BHAs Are Similar

  • 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. 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

  • 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.”

Which One Should You Use?

Now that you’re familiar with the similarities and differences between AHAs and BHAs, what does that mean for your skincare routine?

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.

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.

Can AHAs and BHAs Be Combined?

Yes, you can use AHA and BHA together. But since both AHAs and BHAs are exfoliants, they can be very irritating to the skin if combined. Many products make use of both AHAs and BHAs, though they can cause skin irritation if doses are too high. It is recommended to look for a moisturizer that already has a combination of AHA and BHA for a balanced formula.

You could also use AHAs and BHAs on alternating days.

Another strategy is to use these acids on certain parts of your face only. For example, you can apply an AHA to dry areas and a BHA to oily areas if you have combination skin.