Alpha hydroxy acids (AHAs) work by dissolving the bonds that hold dead skin cells together on your skin’s surface, causing those cells to shed faster than they would on their own. This accelerated shedding reveals fresher skin underneath, improves texture, and over time stimulates deeper changes like increased collagen production. But the process is more precise than simply “scrubbing off” dead skin, and understanding it helps you use AHAs more effectively.
The Chemistry Behind Exfoliation
Your outermost layer of skin, the stratum corneum, is made of flat, dead cells held together by protein structures called desmosomes. These structures rely on calcium ions to maintain their grip. When you apply an AHA to your skin, the acid chelates (essentially grabs and removes) calcium ions from those cell-to-cell connections. Without calcium, the desmosomes and other adhesion points lose their structural integrity, and the dead cells detach and fall away.
This isn’t just a physical loosening. Once calcium is stripped from the adhesion proteins, those proteins become vulnerable to natural enzymes already present in your skin. These enzymes can now break down the exposed proteins, further accelerating the shedding process. The result is a controlled form of exfoliation that clears the buildup of dead cells more efficiently than your skin manages on its own, especially as you age and natural cell turnover slows.
How Different AHAs Penetrate Skin
Not all AHAs behave the same way. The key difference comes down to molecular size. Glycolic acid, derived from sugarcane, is the smallest AHA with a molecular weight of 76 g/mol. That small size allows it to penetrate skin the deepest and fastest, giving it the strongest impact on cell turnover. It’s also the most likely to cause irritation for the same reason.
Lactic acid and mandelic acid are larger molecules that penetrate more slowly. This makes them gentler options. Lactic acid has a secondary benefit: it acts as a humectant, drawing moisture into the stratum corneum. That added hydration softens the skin and can reduce the kind of compensatory thickening that happens when skin gets too dry. Mandelic acid, the largest of the common AHAs, is often recommended for sensitive skin or for people prone to post-inflammatory darkening, since its slow absorption is less likely to trigger irritation.
Effects Beyond Surface Exfoliation
AHAs don’t just clear dead skin. At the right concentration, they trigger biological changes deeper in the skin. Research on glycolic acid has shown that it increases the proliferation of fibroblasts, the cells responsible for producing collagen and other structural proteins. Collagen production rises in a dose-dependent manner, meaning higher concentrations produce more collagen, up to a point. This is why consistent AHA use over months can reduce the appearance of fine lines and improve skin firmness, not just texture.
For acne-prone skin, AHAs address a specific problem: follicular hyperkeratinization, where dead cells accumulate inside pores and form plugs. Glycolic acid corrects abnormal cell behavior inside the pore lining, clearing existing blockages and helping prevent new ones. Lactic acid works similarly by breaking down the dead cell buildup that clogs pores, which also allows other active ingredients to penetrate more effectively when layered afterward.
How Quickly You Can Expect Results
Texture improvements happen faster than most people expect. In clinical testing of an AHA-based skincare regimen, surface roughness improved by about 21% after just seven days. That initial change reflects the clearing of built-up dead cells on the surface.
Deeper improvements take longer. In a 22-week study using 8% glycolic acid or lactic acid, roughly three-quarters of participants reported noticeable improvement in the appearance and smoothness of photoaged skin. Collagen-related benefits like reduced fine lines and improved firmness fall on this longer timeline, since collagen remodeling is a slow biological process. Plan on at least two to three months of consistent use before judging whether an AHA is working for concerns beyond surface texture.
Why AHAs Increase Sun Sensitivity
One well-documented side effect of AHA use is increased vulnerability to UV damage. After four weeks of regular application, skin sensitivity to UV-induced reddening increases by up to 18%. More significantly, sensitivity to UV-induced cellular damage (the kind that contributes to premature aging and skin cancer risk) doubles on average, with wide variation between individuals.
This happens because the thinner, freshly exposed skin has less of a protective buffer against UV radiation. The practical takeaway is simple: daily sunscreen is non-negotiable when using AHAs. Even a sunscreen with an SPF as low as 2 would more than offset the 18% increase in sunburn risk, so any modern broad-spectrum sunscreen provides ample protection.
Concentration and pH Matter
An AHA product’s effectiveness depends on two factors working together: the concentration of the acid and the pH of the formula. AHAs need to be in their “free acid” form to penetrate skin and do their work. At a higher pH (less acidic), more of the acid converts to its salt form, which doesn’t exfoliate. This is why a 10% glycolic acid product at pH 4.5 may be less effective than a 5% product at pH 3.5.
At low concentrations and moderate pH levels, AHAs gently promote exfoliation and may even have anti-inflammatory effects. At high concentrations with a low pH, they act as chemical peels that aggressively disrupt the skin barrier, which can cause redness, swelling, burning, and irritation. The FDA has specifically noted that high-concentration AHA products carry a risk of these adverse reactions. For daily home use, products typically range from 5% to 15%, while professional peels go much higher and require careful monitoring.
Choosing the Right AHA for Your Skin
If your primary concern is dullness, uneven texture, or fine lines and your skin tolerates acids well, glycolic acid at 8% to 10% delivers the most noticeable results. Start with a lower concentration if you’ve never used acids before. For dry or mildly sensitive skin, lactic acid offers effective exfoliation with the added benefit of hydration, making it less likely to cause the tight, stripped feeling glycolic acid sometimes produces.
People with reactive or easily irritated skin, or those with darker skin tones concerned about post-inflammatory hyperpigmentation, often do better with mandelic acid. Its larger molecular size means it works more gradually, reducing the risk of the irritation that can trigger dark spots. Regardless of which AHA you choose, introducing it slowly (every other night, then building to nightly use over two to three weeks) gives your skin time to acclimate and helps you gauge your tolerance before committing to daily application.