Chances are you already know that acids are something you want in your skincare routine. You know (or, at least now you do) that they exfoliate and help keep skin glowing, clear, and even. However, remember that acids are a category of ingredients: There are many different types that do many different things. To help you make sense of them, we’re going to break them down for you so you can make better decisions about which one is right for you and your skin. First up: alpha-hydroxy acids.

What are they?

AHAs, as they are commonly called, refer to a group of acids that are known for their wide variety of benefits. They can help to smooth fine lines, soften texture, diminish dark spots, and clarify pores. They help your skin live its best life.

AHAs tell your skin cells to shed — or, essentially, to hit “refresh.” They come in a few variations that all work to renew skin, but have differing strengths (and weaknesses). “The skin is a brick wall,” says New York City dermatologist Dendy Engelman, M.D. “The skin cells, or keratinocytes, are the bricks and the mortar  — the glue that holds it all together — comprises ceramides, lipids, and cholesterol. Chemical exfoliants work to break down the cellular bonds so dead skin cells can be removed from the top layer of the skin.” By removing old, dead skin and revealing a fresh surface, the topmost layer of the skin can drastically improve its appearance, helping you achieve that revered Korean gwang (a.k.a. glow).  

How do they work?

As mentioned above, there are a variety of acids that comprise the AHA family. While they all essentially do the same thing, it’s how they’re made up and their interaction with the skin that differentiates them. There are four main types: glycolic, lactic, malic, and the relatively new (at least, to skincare) mandelic.

Glycolic acid is the golden child of the AHA family. However, its small molecular size is a double-edged sword. On one hand, it allows it to be an incredibly efficacious ingredient with fast results. “It is a great anti-aging ingredient as it boosts collagen and elastin production,” Dr. Engelman explains. Though it may seem surprising, glycolic acid is in fact, also a humectant, meaning it pulls moisture from the air into the skin, keeping it hydrated for longer. Science!

That’s why we put it in our OG products, the Blueberry Bounce Gentle Cleanser and our since-discontinued Watermelon Glow Sleeping Mask. In Blueberry Bounce, the glycolic acid works alongside lactic acid (more on that in a minute) to help “unglue” dead skin, washing that dulling build-up right down the drain. What’s great about Blueberry Bounce is that you can also use it as a clarifying mask — apply it to dry skin and leave it on for five minutes for a more intense (yet still gentle!) exfoliation.

That said, this speedy absorption can make it more likely to irritate or inflame sensitive skin, which can make it hard to have in high concentrations in leave-on products. With that in mind, the Watermelon Glow AHA Night Treatment, a totally new formula inspired by the original sleeping mask, contains a concentrated, pH-balanced 2.5% AHA complex, which offers the powers and the efficacy of 10% glycolic acid — while still remaining gentle. It also slowly releases throughout the night for gradual benefits while you snooze. And finally, this unique complex is also paired with hyaluronic acid and niacinamide, which both soothe skin and replenish hydration to keep your complexion comfortable.

Lactic acid is the runner-up for most popular AHA. It makes a great power couple when used with glycolic, but on its own it’s also pretty incredible. It’s a great choice for sensitive skin because it’s a “larger molecular structure than glycolic, which means it penetrates into your skin slower, therefore resulting in less of a chance of inflammation,” says medical aesthetician Jordana Mattioli. So in non-doctor speak, that means it doesn’t quite have the same instant exfoliation gratification as glycolic, but when you have easily irritated skin, that’s a good thing — gradual exfoliation over a longer period of time is less likely to cause damage. Lactic has also been shown to have antimicrobial benefits (it kills germs and bad bacteria) and can inhibit tyrosinase production (translation: It’s helpful in reducing hyperpigmentation). If that wasn’t enough, this acid is also a humectant, making it ideal for normal-to-dry skin.

One of the reasons AHAs are sometimes referred to as “fruit acids” is thanks to ingredients like malic acid, which is often derived from fruit like lemons, oranges, and apples. Malic acid, Mattioli explains, isn’t quite as potent as some of its other AHA counterparts, “so it’s often combined with other AHAs like lactic acid.” Another benefit? Its humectant properties make it great for normal to dry skin. 

Finally, there’s mandelic acid, which Mattioli notes is rapidly growing in popularity here, though it’s been popular in K-Beauty and other Asian beauty formulations for a while now. It’s derived from bitter Almonds and has “potent antioxidant and antibacterial properties, making it great for normal to oily skin prone to breakouts,” she says. And anyone who has had a breakout (so, everyone ever) knows, there’s a fine line between treating a pimple and aggravating it with too strong a product.

Mandelic acid, with its antioxidant properties, is also a top-notch anti-aging ingredient as it protects the skin from free radicals and strengthens collagen, one of the building blocks of healthy, happy skin. It’s basically the dream product for adult-acne sufferers who are dealing with the dual drama of breakouts and wrinkles.

The beauty world is not short on choices, so you’ll find acids are included in almost every type of product you can imagine, from powerful masks to lightning fast peels and everyday cleansers. They can be used daily, though you’ll want to start slow when first incorporating them into your skin-care routine. And last but not least: While SPF is essential for everyone, day in and day out, using acids actually increases our skin’s sensitivity to the sun, so it’s extra imperative to be diligent about using sunscreen.