Alpha hydroxy acids (AHAs) dissolve the bonds holding dead skin cells together on your skin’s surface, revealing fresher, smoother skin underneath. They also boost collagen production deeper in the skin and help fade dark spots over time. These water-soluble acids, derived from fruits, milk, and sugar cane, show up in cleansers, toners, serums, and peels at concentrations typically ranging from 5% to 25%.
How AHAs Exfoliate Your Skin
Your outermost layer of skin is made of dead cells held together by calcium-dependent bonds called desmosomes. AHAs work by chelating (pulling away) calcium ions from those bonds. Once the calcium is removed, the connections between dead cells weaken, and the cells slough off. This is chemical exfoliation: no scrubbing, no physical abrasion, just a controlled loosening of cells that were ready to go.
This process does more than just clear away surface buildup. By removing the top layer of dead cells, AHAs signal your skin to produce new cells faster. The result is improved texture, a more even tone, and a subtle glow that comes from having fresh, healthy cells at the surface instead of a dull, compacted layer of old ones.
Collagen and Anti-Aging Effects
AHAs don’t just work on the surface. Glycolic acid in particular has been shown to stimulate collagen production in the deeper layers of skin. In a study using human skin samples, researchers found that glycolic acid increased total collagen levels at every concentration tested, from 8% to 25%. The effect was dose-dependent: an 8% concentration boosted collagen by about 3%, while a 25% concentration increased it by roughly 10% after just five days of daily application.
Glycolic acid also increases levels of hyaluronic acid in both the outer and deeper layers of skin, boosts the rate at which skin cells regenerate, and improves the quality of elastic fibers. Together, these effects help reduce the appearance of fine lines and give skin a firmer, plumper look over time. This is why glycolic acid shows up so frequently in anti-aging products.
Common Types and How They Differ
Not all AHAs behave the same way. The key difference comes down to molecular size: smaller molecules penetrate deeper and work more aggressively, while larger molecules stay closer to the surface and cause less irritation.
Glycolic acid has the smallest molecular weight of the common AHAs (72 daltons). It penetrates deeply, making it the most effective option for collagen stimulation, texture improvement, and wrinkle reduction. That deep penetration comes with a trade-off: it’s the most likely to cause redness and irritation, especially on sensitive skin. It works best for normal to oily skin types.
Lactic acid is slightly larger (90 daltons) and doesn’t sink as deep, which makes it gentler. What sets it apart from other AHAs is that it acts as a humectant, meaning it draws moisture into the skin. This makes lactic acid uniquely good for people dealing with dryness, rough texture, or conditions like keratosis pilaris (those small, rough bumps on the backs of arms). Board-certified dermatologist Jeffrey Fromowitz describes it as “more gentle and more humectant, making it ideal for hydrating dry skin.”
Mandelic acid is one of the largest common AHAs (152 daltons). Its bigger molecular structure means it absorbs slowly and stays near the surface, making it the least irritating option. This matters especially for darker skin tones. People with more melanin are more prone to post-inflammatory hyperpigmentation, where any irritation or inflammation leaves behind dark spots. Because mandelic acid is less likely to trigger that inflammatory response, it’s a safer choice for treating uneven tone and acne without creating new pigmentation problems.
Citric acid (192 daltons), tartaric acid (150 daltons), and malic acid (134 daltons) also belong to the AHA family. They appear in products less frequently and often play supporting roles, contributing to pH adjustment or antioxidant activity alongside the primary acids.
What AHAs Treat Best
AHAs are water-soluble, which means they work on the skin’s surface rather than penetrating into pores. This makes them well suited for surface-level concerns: mild hyperpigmentation like age spots or melasma, acne scars, uneven skin tone, fine lines, enlarged pores, and rough texture. If your main concern is dullness or sun-damaged skin that looks tired and uneven, AHAs are a strong fit.
They’re less effective for active acne caused by clogged pores. That’s where BHAs (beta hydroxy acids, like salicylic acid) come in. BHAs are oil-soluble, so they can cut through sebum and get inside pores to clear out the dead skin cells and oil causing breakouts. If you’re dealing with blackheads, whiteheads, or oily, acne-prone skin, a BHA will target the problem more directly. Many people benefit from using both, with AHAs addressing surface texture and BHAs managing pore congestion.
Concentration and Safety Limits
Over-the-counter AHA products generally range from about 5% to 10%. The Cosmetic Ingredient Review Expert Panel, which evaluates ingredient safety for the cosmetics industry, concluded that glycolic and lactic acid products are safe for consumers when the concentration is 10% or less and the pH is 3.5 or higher. Products need to either be formulated to protect against sun sensitivity or include directions telling you to use daily sun protection.
Professional peels use higher concentrations, sometimes up to 30% or 50%, and require trained application. At those levels, deeper exfoliation and stronger collagen stimulation are possible, but so is significant irritation and skin damage if misused.
Sun Sensitivity and How Long It Lasts
AHAs make your skin more vulnerable to UV damage. By removing the outermost layer of dead cells, you’re exposing newer, less-protected skin to sunlight. Research on 10% glycolic acid found that short-term application increased sensitivity to UV light, measured by both a lower threshold for sunburn and an increase in UV-damaged cells.
The good news: this photosensitivity is reversible. In the same study, no differences in sun sensitivity were detectable after just one week of stopping treatment. Still, daily sunscreen is non-negotiable while you’re using AHAs. Even on cloudy days, UV exposure on freshly exfoliated skin can cause the exact hyperpigmentation and sun damage you’re trying to fix.
How To Start Using AHAs
If you’ve never used an AHA before, start with a lower concentration (around 5% to 8%) and use it two to three times per week rather than daily. This gives your skin time to adjust without overwhelming your moisture barrier. Signs that you’re overdoing it include persistent redness, stinging, flaking, or skin that feels tight and dry. These indicate barrier disruption, and the fix is simple: back off, let your skin recover, and reintroduce the product less frequently.
Apply AHAs to clean, dry skin in the evening. Because they increase photosensitivity, nighttime use gives your skin hours to absorb the product before any sun exposure. Follow with a moisturizer to help maintain hydration, especially if you’re using glycolic acid, which can be drying at higher concentrations. In the morning, apply a broad-spectrum sunscreen with at least SPF 30.
Your skin type should guide which AHA you choose. Glycolic acid for oily or resilient skin that can handle deep penetration. Lactic acid for dry or mildly sensitive skin that needs exfoliation plus hydration. Mandelic acid for sensitive skin, darker skin tones, or anyone who has reacted poorly to other acids in the past. Starting gentle and building up is always smarter than starting strong and dealing with irritation.