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Skin Care Acids 101: What Do Different Acids Do For Skin?

From Azelaic to Tranexamic: A Comprehensive Guide to Every Skin-Care Acid Out There

Young woman cleaning her skin in bathroom mirror with a cotton pad. About 25 years old, African female with curly hair.

There's no denying that the term "skin-care acids" sounds scary, but we promise there's no reason to be fearful of them. When used correctly, acids — we're talking alpha hydroxy acids, beta hydroxy acids, and poly hydroxy acids, to name a few — can be one of the most powerful ingredients in your routine (it's why they're often referred to as "actives"). However, choosing the wrong one for your skin type or accidentally overusing them can have adverse effects.

There are no two ways about it: navigating the world of at-home skin-care acids is really, really confusing, primarily because there are so many different kinds with hard-to-pronounce names. That's why we've called in the experts — six to be exact — to help you figure out what each acid does and what skin type it works best for.

Before diving into each acid and its use, let's first look at what a skin-care acid is, how at-home acids differ from professional ones, and where acids fit into your routine.

What Exactly Is a Skin-Care Acid?

A skin-care acid is an active ingredient that targets specific skin issues, which many of them do through exfoliation. They can come in the form of a cleanser, toner, serum, peel, and spot treatment, although you can sometimes find them added into creams and oils as well. When it comes to all the different kinds of acids used in skin care, you can really break them up into two categories: hydroxy acids and everything else.

Hydroxy acids work by dissolving dead skin cells from the surface of your skin, "helping to make your skin less dull by chemically exfoliating it," Amelie Seghers, consultant dermatologist at London's Cadogan Clinic, told POPSUGAR. The level of exfoliation differs depending on the type and strength of the acid in a product's formula. Acids that are not part of the hydroxy acid family still offer a whole host of benefits; while some do offer a little exfoliation, they also unclog pores, treat hyperpigmentation, reduce fine lines, and prevent breakouts.

Hydroxy acids are divided into three subcategories: alpha hydroxy acids (water-soluble exfoliation), beta hydroxy acids (oil-soluble exfoliation), and poly hydroxy acids (gentle, water-soluble exfoliation). Each acid in these categories helps to exfoliate the skin (among many other benefits, but we'll get into that later). AHA, BHA, and PHAs differ from each other due to their molecule size. In simple terms: an AHAs have smaller-size molecules that are able to deeply penetrate the skin, producing quicker results but also causing potential irritation to some skin types. PHAs, on the other hand, are made up of larger molecules, which don't penetrate the skin as deeply and therefore are less irritating for sensitive skin types.

To make things more confusing, there are plenty of other exfoliating acids that don't fit into these categories, including dicarboxylic acid, which supports antimicrobial activity, according to The Inkey List cofounder Mark Curry, as well as an antioxidant that brightens your complexion and reduces hyperpigmentation. There's even an acid that doesn't exfoliate at all but rather hydrates.

What's the Difference Between At-Home Acids and Clinical Acid Peels?

At-home acids are a lot less concentrated than professional acids used in a dermatology clinic. Stronger concentrations typically produce more dramatic results than at-home peels as they penetrate deeper into the skin. But this doesn't mean the stronger the better when using acids at home. If your skin is sensitive, choosing a strong acid will likely only increase irritation (unless instructed otherwise by a dermatologist, of course) resulting in a compromised skin barrier (the outer layer of our skin, which protects against moisture loss), which essentially means red, sore, and peeling skin.

How to Incorporate an Acid Into Your Skin-Care Routine?

Fitting an acid into your routine should be done with patience and caution; "low and slow" is a phrase often used in the industry when it comes to introducing any active ingredient. Once you've determined which acid is best for your skin type, it's best to start with a low percentage and begin using it a couple times a week, and then slowly increase frequency — and if necessary, percentage — as your skin tolerates it.

It's important to remember that no skin-care product should be burning your face. It's not a sign of something happening or working. If you get irritation from a product, stop using it immediately. Oh, and one last thing before we get into the different types of acids: acids can make your skin more sensitive to the sun, so if you use them, you must wear sunscreen every day — yes, even when it's cloudy.

Alpha Hydroxy Acids

Alpha Hydroxy Acids, or AHAs as they're commonly referred to, are "natural water-soluble acids made from plant or animal products such as sugar, milk, and fruit," said Ife J Rodney, MD, founding director of Eternal Dermatology + Aesthetics in Columbia. "AHAs work by helping to break the bonds between surface skin cells in order to increase peeling and cell turnover. They also stimulate collagen production in deeper skin layers, which results in more youthful, radiant-looking skin with a more even skin tone and texture."

AHAs are best for people wishing to target early signs of aging, uneven skin tone, and clogged pores.

Glycolic Acid: Glycolic acid is one of the most common AHAs. It's naturally derived from sugarcane and has the smallest molecular size, making it one of the strongest acids as it can penetrate deep into the skin. This means that it can have impressive results when used correctly. "Glycolic acid gives the best result on smoothing fine wrinkles, treating pigmentation, and general exfoliation," said Kristina Semkova, consultant dermatologist at the Cadogan Clinic. Due to its molecular size, it can cause "irritation for sensitive skin, so use it cautiously," Dr. Semkova added. Not building up your skin's tolerance can result in a compromised skin barrier.

Dermatologist-Approved Glycolic Acid Products:

Lactic Acid: As you probably guessed, lactic acid is derived from milk. It has a larger molecule size than glycolic acid, and while that means it's not as powerful as glycolic acid, it does mean it's a better option if you have sensitive skin. "Lactic acid is less active than glycolic but is kind to the skin and can be used daily to exfoliate and hydrate the skin with minimal side effects," explained Dr. Semkova. "Not only does the lactic acid unclog pores and turn over the cell layers, but it also acts as a humectant [at low concentrations] to seal moisture into the skin," said Dr. Rodney. Lactic acid is also great for reducing acne-causing bacteria.

Dermatologist-Approved Lactic Acid Products:

Mandelic Acid: If you have sensitive skin but still want to try an AHA, then mandelic acid might be your best option. It's derived from bitter almonds and has a larger molecule than other AHAs, meaning "it penetrates the skin slowly and causes less irritation," said Dr. Rodney. This is what makes it the best choice for sensitive skin. Additionally, "it has an antibacterial effect and unclogs the pores, making it a good treatment for acne and rosacea-prone skin," said Dr. Semkova.

Dermatologist-Approved Mandelic Acid Products:

Malic Acid: Malic acid is derived from apples and is "less researched than glycolic or lactic acid and isn't as effective as an exfoliant on its own," Mary Sommerlad, a London-based consultant dermatologist, told POPSUGAR. Instead, malic acid is often "used as an additional ingredient to help other more active exfoliants work more efficiently and with less irritation." This acid is particularly great for acne-prone skin and people wishing to target hyperpigmentation.

Dermatologist-Approved Malic Acid Products:

Tartaric Acid: Tartaric acid is an antioxidant that is part of the AHA group. "It's the predominant antioxidant in wine, and in skin care, it is often derived from grapes and tamarind", Dr. Sommerlad explained, adding that "although tartaric acid is part of the AHA family, it's not as potent an exfoliator as more commonly used AHAs like glycolic acid or lactic acid. Instead, it's mainly added to skin-care ingredients to regulate the pH levels." Dr. Seghers echoes this: "There are not many products with tartaric acid alone as often you have a combination of AHAs together because they work synergistically. This helps to reduce side effects compared to when you only use one of the AHAs and have to go up much higher in concentration to achieve the same efficacy."

Dermatologist-Approved Tartaric Acid Products:

Citric Acid: As the name suggests, "citric acid is an acid derived from citrus fruits such as lemons or lime," said consultant dermatologist Dr. Shaaira Nasir. Part of the AHA group, citric acid helps to unclog pores, reduce pigmentation, hydrate your skin, and make skin look more even in tone. "It's also used in skin care to adjust the pH level, preventing a product from being too alkaline," Dr. Nasir added.

Dermatologist-Approved Citric Acid Products

Beta Hydroxy Acids

"Beta Hydroxy Acid (BHA) is another acid commonly used in dermatology and cosmetics," said Dr. Rodney. "Unlike AHAs, which are water-soluble, BHAs are oil-soluble, which allows them to penetrate our skin's oil glands to remove debris and dead cells from deep within the pores." There is actually just one BHA and that's salicylic acid.

Salicylic Acid: Salicylic acid is the BHA used in skin care and is derived from willow bark. Due to the ingredient's large molecule size, BHAs are best for oily and acne-prone skin, explained Dr. Ifeoma Ejikeme, skin expert and medical director of Adonia Medical Clinic. Begin by using it twice a week and gradually increase if you don't have any reactions. "Salicylic acid is a good exfoliator and helps with treating stubborn pigment left behind after healing spots, in addition to promoting the turnover of dead skin cells," Dr. Semkova said. When combined with AHAs, salicylic acid makes a great exfoliator for acne-prone skin that already shows signs of aging. Additionally, it has antibacterial and anti-inflammatory properties. However, Dr. Semkova also explained that "salicylic acid and aspirin belong to the same family, so the acid shouldn't be used if you have a known allergy to aspirin."

Dermatologist-Approved Salicylic Acid Products

Poly Hydroxy Acids

"Poly Hydroxy Acids (PHAs) are the third, lesser-known sibling of the hydroxy acids (AHA and BHAs)," said Dr. Seghers. "Unlike AHAs and BHAs, which are smaller [in molecule size], PHAs are bigger and therefore cannot penetrate as deep into the skin. They work by sloughing off the most superficial layer of skin, helping with textural issues, fine lines, and brown spots without being as irritating, so they are ideal for those with sensitive skin," said New York City-based, board-certified dermatologist Shereene Idriss. Even some people with eczema and rosacea can use PHAs, although it's always best to get individual, expert advice from a dermatologist to make sure you don't exacerbate your condition by introducing acids.

Lactobionic Acid: "Lactobionic acid is a PHA derived from lactose, often used as a liquid exfoliator," said Dr. Sommerlad. Although they're not the same thing, lactobionic acid both sounds and acts a lot like lactic acid. "Due to its larger molecule size, lactobionic acid penetrates the skin less and therefore is a much more gentle exfoliant than AHAs," Dr. Sommerlad said. In addition to exfoliation, lactobionic acid is also a good humectant, which essentially means it helps retain moisture keeping your skin hydrated. Lactobionic acid is also great for treating rough skin on the body and keratosis pilaris, which is sometimes referred to as "chicken skin".

Dermatologist-Approved Lactobionic Acid Products:

Gluconolactone Acid: Gluconolactone acid is a sugar acid and, like lactobionic acid, is best for dry and sensitive skin types as it helps to exfoliate and hydrate your skin at the same time. Turns out, it's also great at keeping skin looking and feeling youthful. "Gluconolactone acid combats glycation, the process that weakens collagen and elastin in skin," explained Dr. Nasir.

Dermatologist-Approved Gluconolactone Acid Products:

Galactose Acid: The final PHA is galactose acid, and it doesn't differ much from the other two. Again, it's milder and more suitable for sensitive skin types and additionally is said to help speed up skin's healing. When it comes to skin-care products, there are few products that include the galactose acid as its hero ingredient. Basically, there aren't really any products available that have galactose acid as a hero ingredient, as it's normally used in combination with gluconolactone and lactobionic acids.

Other Acids That Don't Fit Into AHA, BHA, or PHA Categories

To make things a little more tricky, there are a number of acids don't fit into any of the hydroxy acid categories. These acids don't exfoliate like AHA, BHA, and PHAs but have many other excellent properties. Some are antioxidants, others have an entirely different molecular makeup to hydroxy acids, and one is a medication that's now been introduced into the world of skin care. Ahead, get to know what the experts had to say about each of these special, yet equally as effective, acids.

Azelaic Acid: Chemically, azelaic acid is a dicarboxylic acid and is a naturally occurring acid produced by yeast that is found on the skin. For cosmetic products, it can be derived from grains such as barley, wheat, and rye, but most commonly created synthetically. "The lab-engineered version is most often used in skin care not only due its stability, but also so that it's safe for those with a gluten intolerance," noted Dr. Sommerlad. "Strictly speaking, it's not an exfoliator, the same way AHAs, BHAs, and PHAs are, but it does help stabilize the production of keratin, which can has an exfoliating effect," said London-based dermatologist Cristina Soma. "Classically exfoliators loosen the bonds that hold our skin cells together allowing them to shed. Azelaic acid, however, does not do this but modifies how we make keratin, the structural protein in our skin," which can decrease mild breakouts.

"Azelaic acid is used by dermatologists at prescription-strength for the treatment of rosacea, acne, and pigmentation," said Dr. Soma. It works well as a treatment for these conditions due to the fact it gently exfoliates the surface layers of the skin and features both antibacterial and antioxidant properties. Azaleic acid also treats hyperpigmentation by "inhibiting tyrosinase, a key step in the production of brown melanin pigment in the skin," Dr. Rodney stated.

Due to azelaic acid being quite mild (unless using prescription-strength), dermatologists recommend that some people combine it with other exfoliating ingredients like AHA and BHAs. Dr. Rodney also noted that one of its major benefits is that it's considered safe during pregnancy, which a few other hydroxy acids aren't. Regardless, you should always speak to your doctor about using the ingredient while pregnant.

Dermatologist-Approved Azaleic Acid Products:

Succinic Acid: Another form of dicarboxylic acid, succinic acid is the newest kid on the skin care block — recently thrust into the spotlight thanks to The Inkey List's new Succinic Acid Blemish treatment — although it's been used in medical circles for years. What is it and what makes it so great? Succinic acid is naturally found in amber and sugar cane and has been used for centuries in traditional medicine thanks to its antimicrobial properties. Recent studies have shown the ingredient to be a hero when it comes to its "skin re-conditioning and antioxidant properties", said Curry. "These factors could bring this ingredient up there with the likes of retinol and vitamin C." Unlike a lot of other acids, rather than targeting something specific, succinic acid has a broad, multitargeted efficacy, which basically means, it does a lot. Its antimicrobial and anti-inflammatory properties make it great at reducing acne, excess oil-production, and fine lines — all while being gentle enough for more sensitive skin types.

Dermatologist-Approved Succinic Acid Products:

Ferulic Acid: Ferulic acid is an antioxidant that is "botanically derived from the cell walls of plants like oats, apples, brown rice, and oranges, but is often created in the lab as it is cheaper and more consistent," said Dr. Seghers.

"Ferulic Acid is often used to help stabilize less-stable antioxidants and potentiate their effects," explained Dr. Sommerlad, adding that "it is oftentimes combined with L-ascorbic acid (vitamin C) to boost the effects and prolong shelf life." Dr. Seghers echoes this: "ferulic acid is a powerful antioxidant that boosts the performance of other antioxidants, which is why it's typically combined with vitamin C, vitamin E, and resveratrol, as together they work better than alone — enhancing stability and efficacy."

Although it doesn't fit into the hydroxy acid categories, it's just as affective as an ingredient. "Antioxidants help to neutralize free-radical damage from pollution (which causes a loss of elasticity, dryness, and dark spots, to name a few) and ultraviolet light (which causes premature aging and increased chances of skin cancer), which in turn, helps to slow down the process of skin aging," explained Dr. Seghers. This powerful antioxidant is safe for most skin types; however, it can cause irritation for some people with sensitive skin. Dr. Seghers recommends trying a tester behind your ear first to avoid causing potential irritation to your entire face.

Dermatologist-Approved Ferulic Acid Products:

Ascorbic Acid: Ascorbic acid — also known as L-ascorbic acid — is the most potent form of vitamin C. Both Drs Sommerlad and Seghers agree that it's a hero skin-care ingredient because it's so effective for multiple different skin concerns. A powerful antioxidant, ascorbic acid helps to reduce the effects of pollution on the skin, lightens dark marks such as sun spots, stimulates collagen, and brightens up your overall complexion. "To be effective in a skin-care routine, [ascorbic acid] must be kept in a dark bottle and away from air and sunlight (think: dark bottle with a pump or dropper bottle) and should be used in the morning after cleaning and before moisturizers and sunscreen," said Dr. Sommerlad. "Ascorbic acid works best in combination with vitamin E and ferulic acid," Dr. Seghers added.

Dermatologist-Approved Ascorbic Acid Products:

Tranexamic Acid: "Tranexamic acid is a drug originally designed to reduce blood loss and is on the World Health Organiazation (WHO) list of essential drugs precisely for this reason," explained Dr. Sommerlad. "Over time, it became clear that it has additional benefits such as improving the pigmentation seen in melasma," said Dr. Seghers. "It is derived from the amino acid lysine, and is known as an antifibrinolytic. In skin care, it can either be used in topical form, which you can get over the counter, or in oral form via prescription from a dermatologist," Dr. Sommerlad added.

In skin-care formulations, tranexamic acid is now used to reduce excessive pigmentation and works by "disrupting the the formation of melanin and the way it is transferred to skin cells," said Dr. Sommerlad. "It has excellent qualities in improving the appearance of post-inflammatory hyperpigmentation and melasma with less side effects compared to hydroquinone (a skin lightening agent). Orally, it has been shown to be safe and effective in the management of melasma." Oral use of tranexamic acid must be taken with the guidance of a dermatologist as side effects can include things like hand swelling.

Dermatologist-Approved Tranexamic Acid Products:

Retinoic Acid: Retinoic acid "is the scientific name for the gold standard anti-aging product more commonly known as tretinoin," said Dr. Sommerlad. The ingredient can be a little confusing, but stay with us. Retinoic acid belongs in a group called retinoids, which is the "umbrella term for the entire family of vitamin A derivatives," said Dr. Serghers. You've probably heard of other retinoids such as retinol, retinaldehyde, and retinyl palmitate, all of which are available over the counter.

Retinoic acid, on the other hand, is only available on prescription. The reason? Well, the over-the-counter retinoids must undergo a conversion process in the skin to turn into retinoic acid so that the skin can make use of it; essentially making the ingredient less potent. "Over-the-counter retinoids have to convert in the skin to retinoic acid, which is why, depending on how many conversion steps there are, they may take longer to work," explained Dr. Sommerlad. When you get retinoic acid in its pure form, however, your skin doesn't need to covert it to make use of it. This makes retinoic acid the most powerful of all the retinoids and the quickest to get to work — and exactly why it's only available on prescription.

When it comes to retinoic acid, there are a multitude of benefits, which include regulating skin cell turnover (so helping with exfoliation), reducing oiliness, improving the appearance of hyperpigmentation, helping to reduce acne, and stimulating collagen. Although, because retinoic acid is so powerful, it does come with some potential side effects. It can cause irritation, redness, dryness, and even some flaking. Dermatologists recommend using retinoic acid at night and stress the need to wear SPF in the day. But since it's a prescription-only ingredient, your doctor or dermatologist will explain the ins and outs of use.

If you're just starting out in your retinoid journey, you might want to consider working up to retinoic acid. "While retinoic acid is the most effective, it can be less well-tolerated for retinoid newbies," said Dr. Serghers. If you aren't quite ready to go for full-on retinoic acid, or have sensitive skin, Dr. Ejikeme recommended trying retinaldehyde (also known as "retinal") first. If you use it at 0.025 or 0.5 percent, it still offers skin-improving results but is generally better tolerated for sensitive skin.

Dermatologist-Approved Retinoid Products:

Hyaluronic Acid: While it has acid in its name, unlike all of the other ingredients in this round up, hyaluronic acid isn't exfoliating in the slightest. "In the skin, hyaluronic acid is not a true acid, but rather is a naturally occurring sugar that traps water in your skin — up to 1,000 times its weight — and binds it to collagen, the result of which is plump, youthful skin," explained Dr. Rodney. "Our bodies produce less and less hyaluronic acid as we age, and our skin becomes dehydrated more easily. That is why hyaluronic acid has become such an important part of skin-care regimens," she added. Hyaluronic acid is suitable for most skin types and is often used underneath moisturizer.

Dermatologist-Approved Hyaluronic Acid Products:

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