What Is Alpha Hydroxy Acid? | Exfoliating Skincare Explained

Alpha hydroxy acids are water-soluble exfoliants that loosen dead skin cells, with glycolic and lactic acid being the most common types in cosmetics.

Alpha hydroxy acid—usually shortened to AHA—shows up in everything from daily cleansers to peel pads, and it earns its place by dissolving the bonds that hold dead skin cells to your face. The result is smoother, brighter skin with less visible fine texture. But the same chemical action that makes AHAs effective also creates a real safety consideration: these acids can make your skin more sensitive to the sun, both while you use them and for up to a week after you stop.

That sun-sensitivity warning comes straight from the U.S. Food and Drug Administration, which regulates AHAs as cosmetic ingredients. This guide covers what AHAs are, how they work, the safety rules that matter, and the mistakes to avoid.

How Do Alpha Hydroxy Acids Work On Skin?

AHAs are a class of acids used in skincare products to exfoliate the skin. They work by loosening the glue-like substance that holds dead skin cells to the surface, letting those cells slough away more easily. Newer, fresher skin sits underneath, which is why regular use tends to improve skin texture and tone.

The FDA’s materials on cosmetic ingredients identify two predominant AHAs in consumer products:

• Glycolic acid — the smallest AHA molecule, so it penetrates fastest and tends to produce the most noticeable results.
• Lactic acid — a larger molecule that exfoliates more gently and also attracts moisture to the skin.

The FDA also lists citric acid, α-hydroxyoctanoic acid, and α-hydroxydecanoic acid as AHA ingredients used in cosmetics, though less commonly.

Are Alpha Hydroxy Acids Safe?

Yes, when used as directed — the FDA’s safety framework supports consumer products at a concentration of 10% or less and a final product pH of 3.5 or greater. These two numbers matter because stronger acids at lower pH penetrate deeper and raise the risk of irritation.

The agency’s adverse-reaction data shows why that standard exists. Between 1992 and early 2004, the FDA received 114 reports of dermatologic problems tied to AHA products, including burning, rash, swelling, pigment changes, blisters, skin peeling, itching, and chemical burns. Increased sunburn sensitivity appears on that list too.

Professional-strength formulations are another matter entirely. Salon-only peels described in the dermatology literature can run as high as 30% AHA at a pH near 3.0, applied by trained practitioners. That is not the consumer standard — over-the-counter products are expected to stick to the gentler ≤10% and pH ≥3.5 formula.

Why Sunscreen Is Non-Negotiable With AHAs

The FDA states plainly that topically applied AHA cosmetics may increase your skin’s sensitivity to the sun during use and for up to a week after you stop. That is the single most overlooked safety point in everyday AHA use, and it explains why so many exfoliating products come with a sunscreen warning on the label.

FDA guidance recommends that AHA product labeling warn users to use sunscreen, wear protective clothing, and limit sun exposure while using the product and for a week afterward. Treating that advice as optional is the most common mistake people make — a morning AHA routine without daily SPF is a routine building toward sun damage.

Common AHA Mistakes To Avoid

Three errors account for most disappointing or uncomfortable AHA experiences:

• Treating all AHAs as identical. Glycolic acid and lactic acid have different molecule sizes and different gentleness profiles — switching between them without adjusting expectations invites irritation.
• Ignoring the product’s own directions. Many AHA formulas tell you to use sun protection daily, not just on application days. That instruction exists because the sensitivity window extends a full week after your last use.
• Using professional-strength products at home. The 30% salon peel belongs to trained professionals. Consumer products should stay within the ≤10% and pH ≥3.5 range the FDA’s safety review supports.

If you are comparing formulations and wondering which AHA product fits your routine, our tested roundup of top alpha hydroxy products breaks down the options by strength and skin type.