Alpha hydroxy acids (AHAs) work by breaking the bonds that hold dead skin cells together on your skin’s surface, allowing those cells to shed more evenly and revealing fresher skin underneath. But exfoliation is only part of the story. AHAs also boost hydration, stimulate collagen production, and increase skin thickness over time.
How AHAs Loosen Dead Skin Cells
Your skin’s outermost layer, the stratum corneum, is built like a brick wall. Dead skin cells are the bricks, and protein structures called desmosomes act as the mortar holding them together. Calcium ions help keep those desmosomes intact. AHAs work by chelating (pulling away) calcium ions from these cellular junctions. When calcium drops, the connections between dead cells weaken and the cells detach more easily.
What makes this process especially useful is that it’s targeted. Research on glycolic acid shows that desmosomal breakdown happens only in the outermost, already-loosening layer of dead skin, not in the deeper, compact layer that forms your skin’s protective barrier. At low concentrations (2 to 5%), AHAs progressively weaken cohesion between these outermost cells, producing a gradual, uniform exfoliation rather than stripping the skin all at once.
What Happens Deeper in the Skin
AHAs don’t just clear the surface. When they penetrate into the dermis, they trigger a cascade of activity in fibroblasts, the cells responsible for producing structural proteins. Glycolic acid accelerates collagen synthesis by fibroblasts, a process that appears to be guided by signaling molecules released from the skin cells above. In one study, applying a 20% glycolic acid lotion twice daily for three months increased epidermal thickness, boosted hyaluronic acid levels in both the epidermis and dermis, and raised collagen gene expression. More collagen and hyaluronic acid translate to firmer, plumper skin with better moisture retention.
Why Molecular Size Matters
Not all AHAs penetrate the same way. The key variable is molecular weight: smaller molecules pass through the skin more easily.
- Glycolic acid (from sugarcane) has the smallest molecular weight at 76 g/mol. It penetrates fastest and has the greatest effect on cell turnover, which also means it’s more likely to cause irritation.
- Lactic acid (from milk) is larger, so it penetrates more slowly and tends to be gentler. It also functions as a natural moisturizing factor, pulling water from the environment into the outer skin layer to increase hydration and flexibility.
- Mandelic acid (from almonds) is larger still, making it the slowest to absorb. This slower penetration makes it a common recommendation for sensitive or darker skin tones that are more prone to post-inflammatory pigmentation from irritation.
The tradeoff is straightforward: smaller molecules work faster and more aggressively, while larger molecules are gentler but require more time or higher concentrations to achieve similar results.
The Hydration Effect
Beyond exfoliation, certain AHAs actively hydrate the skin. Lactic acid is part of the skin’s own natural moisturizing factor, a collection of water-attracting molecules that keep the stratum corneum flexible and intact. When applied topically, lactic acid absorbs moisture from the surrounding environment into the skin’s outer layer. Clinical studies consistently show that lactic acid increases stratum corneum hydration and epidermal thickness. This is why lactic acid products often feel more moisturizing than glycolic acid products at the same concentration, even though both exfoliate.
How AHAs Compare to BHAs
AHAs are water-soluble, which means they work primarily on the skin’s surface. Beta hydroxy acid (BHA), most commonly salicylic acid, is fat-soluble. That lipid solubility allows BHA to dissolve into the oily sebum inside pores, making it more effective for blackheads, whiteheads, and acne. AHAs can’t cut through oil the same way, so they’re better suited for surface-level concerns like dullness, uneven texture, fine lines, and sun damage. If your primary issue is congested pores, BHA is typically the better choice. If you’re targeting overall skin texture and signs of aging, AHAs have a broader range of benefits.
Sun Sensitivity Is Real
AHAs make your skin more vulnerable to UV damage. FDA-supported research found that after four weeks of AHA use, sensitivity to UV-induced skin reddening increased by 18%. More concerning, sensitivity to UV-induced cellular damage roughly doubled on average, though individual responses varied widely. The good news: this effect is fully reversible. One week after stopping AHA use, researchers found no significant difference in UV sensitivity between treated and untreated skin.
The FDA recommends that any product containing AHAs carry a sunburn alert, advising daily sunscreen use, protective clothing, and limited sun exposure while using the product and for a week after stopping. This isn’t optional advice. Using AHAs without sun protection can accelerate the exact kind of damage you’re trying to reverse.
Putting It Into Practice
If you’re new to AHAs, concentration and frequency matter more than which specific acid you choose. Products in the 5 to 10% range are standard for at-home use. Starting with two or three applications per week lets you gauge how your skin responds before increasing frequency. Tingling is normal; sustained stinging, redness, or peeling means you’re overdoing it.
For sensitive skin, lactic acid or mandelic acid at lower concentrations gives you the exfoliation and hydration benefits with less risk of irritation. For more resilient skin or stubborn texture issues, glycolic acid at higher concentrations delivers faster results. Regardless of which AHA you use, applying it to dry skin (rather than damp) slows absorption slightly and can reduce the initial sting. And sunscreen every morning is non-negotiable for as long as you’re using these products.