Alpha hydroxy acids (AHAs) are a group of water-soluble acids, most commonly derived from fruits, milk, and sugar cane, that work by loosening the bonds between dead skin cells so they shed more easily. They’re one of the most widely used active ingredients in skincare, found in everything from daily moisturizers to professional-grade chemical peels. At the right concentration and pH, AHAs can smooth texture, fade dark spots, and improve signs of aging.
How AHAs Work on Your Skin
Chemically, AHAs are carboxylic acids with a hydroxyl group attached at a specific position in their molecular structure. That’s the technical definition, but what matters for your skin is what this structure allows them to do: penetrate the outermost layer of skin (the stratum corneum) and disrupt the connections holding dead cells together.
Your skin naturally sheds dead cells, but that process slows with age, sun damage, and certain skin conditions. AHAs speed it up. Their acidic nature lowers the skin’s pH, which interferes with the protein bridges (called desmosomes) that glue dead cells to each other. Once those connections weaken, the dead layer sloughs off, revealing fresher skin underneath. This process is called chemical exfoliation, and it’s why AHAs leave skin looking brighter and feeling smoother after consistent use.
The Most Common AHAs
Not all AHAs behave the same way. The key difference between them is molecular size, which determines how quickly and deeply they penetrate your skin.
- Glycolic acid is the smallest AHA, with a molecular weight of 76 g/mol. It penetrates fastest and has the most impact on cell turnover, which makes it effective but also more likely to cause irritation. It’s derived from sugar cane and is the most widely used AHA in skincare products and professional peels.
- Lactic acid is slightly larger, so it penetrates more slowly and tends to be gentler. Originally derived from sour milk, it’s often recommended for people newer to chemical exfoliation or those with drier skin, since it also has humectant (moisture-attracting) properties.
- Mandelic acid has a larger molecular weight still, making it the slowest to absorb and the least irritating of the three. It’s sometimes used in combination peels for treating dark spots and uneven tone, particularly for people who are prone to post-inflammatory hyperpigmentation.
Other AHAs you’ll see on ingredient lists include citric acid (from citrus fruits), tartaric acid (from grapes), and malic acid (from apples). These are less commonly used as the primary active ingredient but often appear in formulations alongside glycolic or lactic acid.
What AHAs Can Do for Your Skin
The most immediate benefit is smoother texture. By clearing away the buildup of dead cells, AHAs reduce roughness and dullness, often within the first few weeks of use. This same mechanism helps with several specific concerns:
- Fine lines and wrinkles: Regular exfoliation stimulates cell turnover in the deeper layers of skin, which can improve the appearance of fine lines over time.
- Hyperpigmentation: Dark spots from sun damage, acne scars, or melasma fade faster when the pigmented surface cells are shed more efficiently.
- Uneven skin tone: By removing the patchy buildup of dead cells, AHAs create a more uniform surface that reflects light more evenly.
- Keratosis pilaris: Those small, rough bumps on the backs of arms and thighs are caused by keratin plugging hair follicles. The American Academy of Dermatology lists AHAs (specifically glycolic and lactic acid) as recommended ingredients for managing this condition.
If your main concern is acne or oily, congested skin, AHAs may not be your best choice as a standalone ingredient. Because they’re water-soluble, they work primarily on the skin’s surface rather than inside pores. Beta hydroxy acid (BHA), commonly known as salicylic acid, is oil-soluble and can penetrate into pores to clear out excess oil and debris. AHAs are better suited for dry skin, surface-level texture issues, and anti-aging goals.
Concentration and pH: Why the Numbers Matter
An AHA product’s effectiveness depends on two factors working together: the concentration of acid and the pH of the formula. A higher concentration delivers more active acid, and a lower pH keeps more of that acid in its “free” (active) form. Both need to be in the right range for the product to actually exfoliate rather than just sit on your skin.
Regulators around the world generally recognize three tiers of AHA use. For daily at-home skincare, the U.S. FDA’s guidance (based on the Cosmetic Ingredient Review Expert Panel’s findings) recommends no more than 10% AHA concentration at a pH of 3.5 or higher. Salon and professional-use products can go up to 30% concentration at a pH as low as 3. Anything above that falls into the medical category, used by dermatologists with even lower pH levels.
European guidelines are more conservative for daily use, recommending glycolic acid at up to 4% with a pH of at least 3.8, and lactic acid at up to 2.5% with a pH of at least 5. These differences reflect varying regulatory approaches, but the principle is the same: stronger formulations carry more risk and need more expertise to use safely.
When shopping for an AHA product, concentration is usually listed on the label. pH rarely is. As a general rule, products from established skincare brands formulated for daily home use will fall within safe pH ranges. If you’re buying from a less familiar source, a set of pH test strips (available at most pharmacies) can tell you where a product falls.
Sun Sensitivity and Side Effects
The most important thing to know about using AHAs is that they increase your skin’s sensitivity to UV radiation. By removing the outermost layer of dead cells, you’re exposing newer, less protected skin to the sun. This makes sunburn and sun damage more likely. Daily sunscreen use is essential while using any AHA product, and the FDA specifically notes that AHA products should either be formulated to protect against sun sensitivity or include directions telling consumers to use daily sun protection.
Common side effects at appropriate concentrations include mild stinging or tingling during application, slight redness, and temporary dryness or flaking as your skin adjusts. These typically diminish within the first one to two weeks of regular use. If irritation persists or worsens, reducing the frequency of application (every other day instead of daily, for example) often helps more than switching to a lower concentration.
People with darker skin tones should approach AHAs with some extra caution. Chemical exfoliation can trigger post-inflammatory hyperpigmentation, where the skin responds to irritation by producing excess pigment. This risk is higher in darker complexions. Starting with a gentler AHA like lactic or mandelic acid at a lower concentration, and increasing gradually, reduces the likelihood of this reaction.
How to Start Using AHAs
If you’ve never used an AHA before, start with a low-concentration product (around 5% to 8% glycolic acid, or a lactic acid formula) applied two to three times per week in the evening. Your skin needs time to build tolerance. After two weeks without significant irritation, you can increase to nightly use if your skin responds well.
AHAs work best on clean, dry skin. Apply them after cleansing but before heavier products like moisturizers or oils, which can create a barrier that reduces absorption. Wait a minute or two for the product to absorb before layering anything on top. In the morning, apply sunscreen with at least SPF 30 regardless of whether it’s sunny outside, since UV exposure happens through clouds and windows too.
Avoid using AHAs at the same time as other potent actives like retinoids or vitamin C serums, at least when you’re first starting out. Layering multiple exfoliating or irritating ingredients increases the chance of redness, peeling, and sensitivity. Once your skin is comfortable with one active, you can introduce a second on alternating nights.