Deep Guide to Hydroxy Acids - how do they work and how to choose the right products for your skin

Why should you be using hydroxy acids?

Fun fact: Cleopatra bathed in sour milk to achieve softer and smoother skin. That's because sour milk contains lactic acid, a naturally occurring alpha hydroxy acid.

As Cleopatra can testify, hydroxy acids have been around since ancient times. They're naturally occurring in fruits, nuts, and milk, and are widely used in topical products today — hence, the safety information on them is also well researched and understood.

Hydroxy acids are generally well tolerated by most people, and clinical studies demonstrate their effectiveness empirically. The benefits of using hydroxy acids are wide ranging — from wrinkles, to pigmentation, to acne, and even moisture retention.

These are just a few reasons why everyone can benefit from using a form of hydroxy acid in their skincare routine — even those with sensitive skin!

Benefit 1 — Smoother, more even complexion through exfoliation

Complete skin (epidermal) renewal occurs roughly every 28 days in young adults, while the same process takes 40 to 60 days for those with more mature skin types (Rodan et al. 2016). Like many biological processes, skin renewal and natural exfoliation weakens as we age — dead skin cells aren't shed as easily, leaving skin looking rough, pores enlarged, and complexion dull.

Hydroxy acids act as chemical exfoliants, loosening the bond between skin cells and facilitating their removal. This complements the natural exfoliation process, revealing smoother, brighter, and younger-looking skin (Kornhauser et al. 2010). Regular removal of dead cells can also encourage quicker renewal of the deeper underlying skin cells — mimicking the renewal process in younger skin.

With potent hydroxy acid products (such as 10% of actives in serums and creams) you can feel the effect of exfoliation as quickly as after the very first use. The chemical peels you can get at your dermatologist's office use the same hydroxy acids — but in much higher concentrations. See Appendix A for more details.

Benefit 2 — Firmer, smoother skin through collagen and moisture support

Though the mechanisms are not fully understood, alpha hydroxy acids such as glycolic, lactic, and citric acid appear to activate signaling pathways that boost natural hyaluronic acid content and collagen production — meaning plumper, more youthful skin.

Technically speaking: Glycolic acid has been shown to activate underlying cellular processes that accelerate collagen synthesis by fibroblasts and modulate matrix degradation through keratinocyte-released cytokines (Okano et al. 2003). This collagen-building ability increases with growing concentrations of glycolic acid, while avoiding inflammation when kept at a pH of 4 (Narda et al. 2020). In studies on sun-damaged skin, glycolic acid increased hyaluronic acid content in the epidermis and dermis, as well as collagen synthesis and epidermal thickness (Bernstein et al. 2001). Citric acid has also been shown to lead to similar benefits (Bernstein et al. 1997). Double-blind, placebo-controlled studies with lactic acid show improved photo-damaged skin appearance in a similar manner (Stiller et al. 1996).

Benefit 3 — Fading hyperpigmentation and evening skin tone

Exfoliating with hydroxy acids (glycolic and lactic acid in particular) has been proven to improve hyperpigmentation. Increased cell turnover is believed to help renew hyperpigmented skin and rapidly disperse existing pigment (Kornhauser et al. 2012).

Beyond exfoliation, there may be a secondary mechanism: evidence suggests that glycolic and lactic acids inhibit tyrosinase activity (Kornhauser et al. 2012). The enzyme tyrosinase plays a key role in melanin synthesis — so inhibiting it reduces melanin production and can alleviate hyperpigmentation. In cases where pigmentation sits in deeper skin layers, chemical peels can reach it faster, but skincare ingredients that act on tyrosinase — including hydroxy acids — can slowly fade it from the source.

Benefit 4 — Clearing acne by removing pore-clogging debris

For those prone to acne, salicylic acid (popularly known as BHA) is the go-to. Because salicylic acid is fat-soluble, it can penetrate and unclog congested pores full of sebum — something water-soluble AHAs like glycolic acid cannot do as effectively.

Salicylic acid is also powerful against acne due to its antibacterial properties — it works by downregulating virulence factors for bacterial replication (Kornhauser et al. 2010). While salicylic acid is the star for acne, other hydroxy acids like glycolic have also been shown effective (Babilas et al. 2012), which is why many acne treatment formulations combine salicylic acid with an AHA for better performance.

Benefit 5 — Supporting the skin's acid mantle

The skin's surface and stratum corneum have a naturally acidic pH between 4 and 6, and this acidity (the "acid mantle") is essential for maintaining healthy skin (Lukic et al. 2021). Two key benefits of a healthy acid mantle:

(1) It dampens harmful bacterial activity — combats acne.
(2) It boosts the skin's ability to hold onto water — combats dryness.

Here's what you need to know about pH in skincare: avoid products with an alkaline pH of 8 or greater (including some soaps and body washes). Even water (neutral pH 7) can potentially disturb your skin's pH, so using an acidic face wash or following cleansing with an acidic product is a smart habit. In the past, toners played this role — but if you use Vitamin C serums or hydroxy acid products, they serve the same purpose.

Convinced yet? Here's what you should look for in a hydroxy acid product

Glycolic acid, lactic acid, and salicylic acid are the top hydroxy acids with the most research to support their effectiveness. We recommend looking for these three ingredients in particular.

How often and when should you apply?

Hydroxy acids can be found in cleansers, masks, serums, and creams and are generally safe to use as often as you'd like. In general, the longer hydroxy acids sit on your skin, the deeper the penetrative exfoliation — so leave-on products like serums and creams deliver more impact than rinse-off formats.

Hydroxy acid products have been tested in combination with Vitamins C and E, as well as niacinamide, and this combination has been found to be effective and well tolerated (Tran et al. 2014). Hydroxy acids have also been tested with retinoids — and while retinoids can be irritating — the combination is very effective (Bertin et al. 2008).

As mentioned, our skin is naturally acidic — which is why acidic skincare products are generally well tolerated. We recommend avoiding skincare products with an alkaline pH of 8 or higher; if you do use an alkaline product, follow it immediately with an acidic product to restore the skin's pH.

Things to keep in mind when using hydroxy acids

Be mindful of sun exposure

When you exfoliate away dead cells on the skin's surface, UV rays are able to penetrate deeper into the skin — so hydroxy acid use comes with increased sun sensitivity. An interesting exception is salicylic acid: studies suggest it may actually be sun-protective, as it absorbs UV (acting somewhat like sunscreen). Derivatives of salicylic acid are even being explored as UV protectants (Kornhauser et al. 2012).

Watch out for signs of over-exfoliation

When exfoliating properly, you should not experience dry skin. If your skin has become dry or tight, you're likely over-exfoliating. Those with sensitive skin might notice burning, stinging, itching, scaling, or other reactions to a hydroxy acid.

Skin sensitivity is correlated to poor ("leaky") skin barrier function, which allows excess absorption of external elements — and too much absorption of hydroxy acids can ironically lead to irritation (Ding et al. 2019).

Since a healthy acid mantle is necessary for a good skin barrier, try seeing if you can tolerate a hydroxy acid face wash first. Since cleansers don't stay on the skin's surface for long, hydroxy acid cleansers won't be absorbed deeply and are less likely to cause irritation. Niacinamide serums — typically slightly acidic, in line with skin's natural pH — can also be a good way to introduce your skin to acidic products.

Is salicylic acid safe for pregnancy?

Long story short: yes. However, we recommend skipping any chemical peels until after your pregnancy, and if you're hesitant to use salicylic acid, you can get most of the same benefits from glycolic acid.

The general concern is that because salicylic acid is fat-soluble, it has more potential to penetrate deeper into skin and enter the bloodstream. While salicylic acid skincare products are deemed safe, chemical peels (performed at medical spas or doctor's offices) involve much higher concentrations and penetrate far deeper — so those should be avoided during pregnancy.

The concerns over salicylic acid are largely based on aspirin studies (fun fact: aspirin is a form of salicylic acid). In high doses, aspirin can be teratogenic in animals, and the general recommendation is to avoid aspirin during pregnancy. However, topically applied skincare products are very unlikely to cause any issue due to (1) low concentration (2% or lower in most products) and (2) the limited ability of any topically applied product to reach the bloodstream.

Wrapping it all up — Hydroxy acids are great!

If you're looking to treat pigmentation, acne, or aging skin, hydroxy acids should definitely play an important role in your skincare routine. Look for glycolic acid, lactic acid, or salicylic acid — three of the most common, effective, and extensively researched hydroxy acids.

Those with sensitive and/or chronically dry skin may be suffering from a damaged skin barrier. We recommend starting out slow with rinse-off products like hydroxy acid-infused cleansers, and gentle non-irritating ingredients such as niacinamide to help restore the barrier. Once restored, more potent products with higher concentrations of hydroxy acids, Vitamin C, and retinoids can be added as well.

Appendix A — Exfoliation vs. Peel

To summarize: exfoliation = shallow, peels = deep.

The hydroxy acids in your skincare products are the same acids found in medical chemical peels — but chemical peels use much higher concentrations (Arif 2015, Chilicka et al. 2020, Garg et al. 2009, Sharad 2018). Concentrations of 10% to 40% can be used by trained professionals in salons; over 40% should only be used by medical professionals.

These peels are intended to go beyond exfoliating the surface layer — they penetrate the deeper, living layers of the epidermis, and some strengths reach as deep as the dermis. The goal is to remove deep imperfections such as acne scars, deep wrinkles, or hyperpigmentation (Gozali et al. 2015). But keep in mind: chemical peeling carries risk, so always put yourself in the hands of a professional and don't try this at home.

Appendix B — More about acid mantle

The creation of the acid mantle

The acid mantle is formed in part by filaggrin proteolysis and release of free amino acids and their derivatives — including urocanic acid and pyrrolidone carboxylic acid — into the cytosol of corneocytes of the stratum corneum. Other pathways contribute as well, including free fatty acid processing, a sodium/proton exchanger, and cholesterol sulfate ionization and hydrolysis (Lukic et al. 2021).

Sebum can release free fatty acids to the skin's surface, while sweat can release urea, lactic acid, sodium, and potassium. The acid mantle is thus formed by the combination of the acidic nature of the stratum corneum cells themselves and the acidic components released by sebum and sweat. Many components of the acid mantle also perform double duty as humectants — urocanic acid, pyrrolidone carboxylic acid, and lactic acid can all hold water.

Healthier acid mantle can dampen harmful bacterial activities

The skin provides a protective barrier for our entire body. While the outer skin is acidic, the blood is alkaline at pH 7.4, and inside living body tissue sits at a neutral pH of 7.0. This appears to have evolved so that bacteria living on the skin's surface — when they enter the bloodstream — aren't adapted to survive the alkaline environment, and die off. By having acidic skin and an alkaline interior, the body ensures bacteria living on the surface can't easily harm the interior.

For example, a higher skin pH coupled with sebum overproduction encourages the growth of P. acnes (also known as C. acnes), a bacterium implicated in acne development. Glycolic acid at various pH levels was found to kill P. acnes, with optimal bactericide activity below pH 4.0 — inactive above pH 5.0 — suggesting pH 4 or below is necessary for this activity (Valle-Gonzalez et al. 2020).

Healthier acid mantle boosts the skin's ability to hold onto water

When the acid mantle is disturbed — by ichthyosis, alkaline soaps, or even by soaking in neutral-pH water — it leads to drier skin due to impaired skin barrier function and increased trans-epidermal water loss (TEWL). This is because enzymes important for building skin barrier components operate better at lower pH, while enzymes that break down barrier components are activated at neutral to high pH.

Specifically, ceramide-generating hydrolytic enzymes (β-glucocerebrosidase and acidic sphingomyelinase) with low pH optima (pH < 5) build the lipid-rich lamellar membranes that make the stratum corneum largely waterproof, while serine proteases — induced by increased pH — degrade ceramide-forming enzymes, free fatty acids, and corneodesmosome constituent proteins (Hachem et al. 2010). Excessive serine protease activity is characteristic of atopic dermatitis.

Hydroxy acids are also known to be highly moisturizing, likely in part through supporting the acid mantle and in turn increasing skin barrier function. Improved barrier function and increased skin ceramides have been found in human studies using lactic acid (Rawlings et al. 1996). A study by Hachem et al. (2010) acidified the stratum corneum using PHAs lactobionic acid or gluconolactone, finding faster skin barrier recovery and reduced TEWL — driven by activation of ceramide-generating enzymes and by decreased serine protease activity.

The waterproof skin barrier uses a "brick and mortar" structure — bricks of flattened protein-rich corneocytes connected by corneodesmosomes, and lipid-rich mortar channels made of fatty lipids, ceramides, and cholesterol. When functioning correctly, water should not easily come in or out. Some people, for various reasons, have a "leaky" barrier where water evaporates out easily and molecules leech in readily — this is impaired skin barrier function of the stratum corneum.

Appendix C — What exactly are hydroxy acids?

Now that you know what hydroxy acids are used for, you may ask: but what are they?

In technical terms, hydroxy acids are acids that have a hydroxyl group in the molecule in addition to the acid group itself. An alpha hydroxy acid (AHA) has a hydroxyl group in the alpha position; a beta hydroxy acid (BHA) has a hydroxyl group in the beta position. Does that clarify the picture? Of course not.

In fact, even experts are confused. Salicylic acid is often called a beta hydroxy acid when really it is in its own class as a monohydroxy benzoic acid (Kornhauser et al. 2012). However, the FDA classifies salicylic acid as a BHA — albeit with an asterisk noting that "from a chemist's perspective, salicylic acid is not a true BHA. However, cosmetic companies often refer to it as a BHA and consequently, many consumers think of it as one." Basically, the label stuck — and we at Maelove are also guilty of calling salicylic acid a BHA.

In our opinion, the technical classifications don't necessarily matter, though it is useful to understand what each acid brings to the table. Some people like to think of them simply as acids naturally occurring in fruits, nuts, and milk — hence they are often called fruit acids.

The most commonly used are glycolic acid (found in sugarcane), lactic acid (found in sour milk and fermented plants), and salicylic acid (found in many flowering shrubs) (Kornhauser et al. 2010).