Hydroxy Acids in Skin Care: What They Are and How They Work

Hydroxy acids are some of the most widely used skin care ingredients and can treat a range of common skin concerns, including acne, clogged pores, hyperpigmentation, and dull, rough skin texture. Not all hydroxy acids are the same, however. There are three types of hydroxy acids, though two of them are most commonly used in skin care products: alpha hydroxy acids (AHAs) and beta hydroxy acids (BHAs).

It is important to note that different hydroxy acids are best suited for certain skin types, so the first step in choosing a hydroxy acid product is to know your Baumann Skin Type . Once you know your skin type, use this guide to understand the best type of hydroxy acids for your skin.

Hydroxy acids are a class of organic acids commonly used in skin care for chemical exfoliation. They are acids and have a low pH and work by breaking the bonds between the dead skin cells that make up the outermost layer of the epidermis, allowing them to shed away more easily. This process accelerates skin cell turnover, improves texture, and enhances the skin’s overall appearance.

Unlike physical exfoliants (such as scrubs), which mechanically remove dead skin cells, hydroxy acids work at a molecular level, dissolving the “glue” that bonds skin cells together. This makes some types gentler on the skin than physical exfoliants.

Hydroxy acids can also stimulate skin cell renewal by causing exfoliation and activating pathways that encourage new skin cells to generate. Some acids, such as glycolic acid, can penetrate into the dermis, where they can stimulate collagen and hyaluronic acid production for improved skin texture and firmness (3).

These acids are classified based on their solubility and molecular structure, which determines how deeply they penetrate into the skin and what types of skin concerns they target. The three main types of hydroxy acids used in skincare are alpha hydroxy acids (AHAs), beta hydroxy acids (BHAs), and polyhydroxy acids (PHAs).

The two most commonly used types in skin care are AHAs and BHAs. PHAs, or polyhydroxy acids, are relatively newer ingredients.

AHAs are water-soluble acids that work on the surface of the skin to remove dead skin cells. They are typically best suited for dry, sun-damaged, or aging skin because they help to improve hydration, fade hyperpigmentation, and boost collagen production.

Examples of AHAs:

• Citric acid- Found in citrus fruits, citric acid provides mild exfoliation and skin-brightening properties.

• Glycolic acid - Derived from sugarcane, this is the smallest and most potent AHA, making it effective for exfoliation and anti-aging.

• Lactic acid- Derived from milk, lactic acid is gentler than glycolic acid and has hydrating properties, making it ideal for dry or sensitive skin.

• Malic acid- This AHA is less commonly used alone but can help to enhance the effects of other AHAs. It is derived from apples.

• Mandelic acid- This larger-molecule AHA penetrates the skin more slowly than other AHAs and is derived from almonds.

• Phytic acid- Derived from grains and legumes, phytic acid is a gentle exfoliant with antioxidant and skin-brightening properties. It’s ideal for sensitive or uneven skin tone.

  Tartaric acid- Found in grapes and wine, tartaric acid offers mild exfoliation and helps enhance the performance of other AHAs.

BHAs are oil-soluble, meaning they can penetrate deeper into the pores to dissolve excess sebum and exfoliate inside the follicle. This makes them particularly effective for oily and acne-prone skin. Unlike AHAs, BHAs also have anti-inflammatory and antibacterial properties, making them a good option for reducing breakouts and redness.

Examples of BHAs:

• Salicylic acid- The most common BHA, derived from willow bark, salicylic acid is excellent for acne-prone skin because it unclogs pores, reduces inflammation, and helps to prevent future breakouts.

• Betaine salicylate- This is a gentler alternative to salicylic acid, often used in Korean skincare for mild exfoliation.

• Willow bark extract- A natural source of salicylic acid with similar pore-clearing effects but in a less concentrated form.

PHAs are a newer type that offer gentler exfoliation than AHAs and BHAs. They work on the surface of the skin like AHAs, but their larger molecular size makes them less likely to cause irritation. PHAs can be ideal for sensitive skin types and those with conditions like rosacea.

Examples of PHAs:

• Gluconolactone- A mild exfoliant that also helps to hydrate and strengthen the skin barrier.

• Lactobionic acid- Derived from milk, this PHA has antioxidant properties and can help to hydrate the skin. This acid has also been shown to have strong anti-aging effects (4).

• Maltobionic Acid- Also derived from natural sugars, maltobionic acid helps reduce the appearance of fine lines, wrinkles, and discoloration through gentle exfoliation. It also attracts moisture, enhancing skin hydration. 

Hydroxy acids are best known for exfoliating, but they can also offer other skin benefits, depending on the type:

• Remove dead skin cells- All types help to speed up skin cell turnover.

• Improve skin texture and tone- AHAs in particular help to brighten and smooth rough or uneven skin.

• Unclog pores- BHAs are able to break up oil found in clogged pores and help prevent blackheads and breakouts.

• Reduce inflammation- Salicylic acid has anti-inflammatory properties.

• Boost hydration. Many AHAs and PHAs can have humectant properties, allowing them to attract moisture and hydrate the skin.

• Support collagen production- AHAs like glycolic acid can stimulate collagen, reducing fine lines and wrinkles.

Always introduce hydroxy acids to your skin care routine gradually, increasing the frequency of use only as your skin tolerates. Dry, sensitive skin types may only tolerate an AHA cleanser once a week, while oily, resistant types may tolerate these ingredients more frequently.

The most common side effects of hydroxy acids include:

• Skin irritation, burning, or stinging

• Redness or blotchy skin

• Dryness and peeling

• Over exfoliation can lead to acne breakouts

Some very sensitive skin types, including those with rosacea, may need to avoid AHAs and BHAs. Those with an active sunburn should also pause using hydroxy acids. Additionally, hydroxy acids should generally not be paired with other exfoliating ingredients like retinoids, or those that can cause stinging, burning, or irritation, such as vitamin C.

The best hydroxy acids for you will depend on your skin type and specific concerns. The best way to find the right one for you is to take the skin type quiz, then choose products that are approved for your skin type. In general, some good options for dry and oily types are below.

Hydroxy acids can be very beneficial for a variety of skin types and concerns. The key is to make sure you are using the right type for your skin type and that you gradually begin using hydroxy acid products to avoid side effects like redness, irritation, or dryness.

We can help! Take the quiz and we can help you find the right AHA skin care for your skin's unique needs. Or come to Reddit and ask me a question at r/skintypesolutions.

1. Baumann L. Antiaging Ingredients in Ch. 37 of Baumann's Cosmetic Dermatology Ed 3. (McGraw Hill 2022)
2. Baumann, L. Chapters2 and 82 Cosmeceuticals and cosmetic Ingredients (McGraw Hill 2015)
3. Bernstein EF, Lee J, Brown DB, Yu R, Van Scott E. Glycolic acid treatment increases type I collagen mRNA and hyaluronic acid content of human skin. Dermatol Surg. 2001 May;27(5):429-33. doi: 10.1046/j.1524-4725.2001.00234.x. PMID: 11359487.
4. Baumann, L. S. (1995). Lactobionic acid. Annu Rev Med, 46, 235-47.
5. Egli, C., Min, M., Afzal, N., & Sivamani, R. K. (2023). The hydroxy acids: Where have we been and what's new?. Dermatological Reviews, 4(6), 260-267.
6. Green, B. A., Ruey, J. Y., & Van Scott, E. J. (2009). Clinical and cosmeceutical uses of hydroxyacids. Clinics in dermatology, 27(5), 495-501.
7. Bowes, L. (2013). The science of hydroxy acids: mechanisms of action, types and cosmetic applications. Journal of Aesthetic Nursing, 2(2), 77-81.