Field guide

Glycolic Acid, Explained: The Percentage on the Label Is Half the Story

Glycolic acid is the smallest alpha-hydroxy acid, and it exfoliates by loosening the bonds between dead skin cells so they shed. The CIR Expert Panel caps consumer leave-on products at 10 percent with a pH of 3.5 or higher. What matters is not the percentage alone but the free acid content, which depends on pH: at pH 3.83, the acid's pKa, exactly half of it is in active free-acid form.

Shoppers compare glycolic acid products by percentage, and the comparison feels rational. Ten percent must beat five percent. The chemistry disagrees. Glycolic acid only exfoliates in its free, un-ionized form, and how much of it is free depends on the product's pH compared with the acid's pKa. At pH 2.5, an 8 percent glycolic product carries 7.64 percent active free acid. To get the same amount of active acid from a product at pH 4.5, you would need 43 percent glycolic acid. The number on the front of the bottle is half the story, and the pH on the back, if the brand prints it at all, is the other half.

That is why two products with the same percentage can behave like different ingredients, and why a gentle-sounding 5 percent at a low pH can sting more than a 10 percent at a higher one. Glycolic acid is also the most studied AHA in the cabinet, with real randomized trial data behind it, which makes it the right ingredient to learn the pH lesson on.

What the trials actually show

In a 22-week double-blind, vehicle-controlled trial of 74 women with photodamaged skin, an 8 percent glycolic acid cream produced at least one grade of photodamage improvement in 76 percent of users, against 40 percent for the vehicle. A 2001 study confirmed it upregulates type I collagen mRNA and hyaluronic acid synthesis in human skin. The effective range for consumer leave-on products is 4 to 10 percent, with the CIR cap at 10 percent and pH at 3.5 or higher. Professional peels run 20 to 70 percent and need a trained applicator.

The EU takes a stricter line than the US: its scientific committee suggests no more than 4 percent at pH 3.8 or higher for consumer leave-on products. Both limits agree on the shape of the rule, which is that concentration and pH are one decision, not two. The FDA adds a disclosure requirement that is easy to miss: products with AHAs must carry a sunburn alert, because the photosensitization risk is clinically proven. In FDA-reviewed studies, 10 percent glycolic acid at pH 3.5 applied daily for four weeks made skin significantly more sensitive to UVB, and sensitivity returned to normal about a week after stopping.

The pH math, in plain terms

Every acid has a pKa, the pH at which half of it is free acid and half is dissociated. Glycolic acid's pKa is 3.83, lactic acid's is 3.86. When the product pH equals the pKa, the split is 50/50. At pH 3.5, about 67 percent of the glycolic acid is free. At pH 3.8, about half. At pH 4.5, a small fraction. The Henderson-Hasselbalch equation sets these numbers, and they are not negotiable: a 10 percent product at pH 4.8 may exfoliate less than a 5 percent product at pH 3.6.

This is also why buffered products exist. At a higher pH, some of the acid sits on the skin as dissociated ions, then converts to free acid as the free acid is absorbed, which reads as a slow release. It is gentler, and it is slower, and neither fact is visible on the label. If a brand will not state the pH, assume the product is formulated conservatively and judge it by results over four to six weeks, not by the percentage on the front.

How to use it without wrecking your barrier

Start at the low end of the effective range, 4 to 5 percent, two or three nights a week, and build up only if there is no stinging, flaking, or persistent redness. Do not stack it with a retinoid on the same night while you are still learning what your skin tolerates; alternate nights instead. The irritation risk is driven more by pH than by concentration, according to the EU scientific committee's review, which means a low-pH product at a modest percentage can irritate more than a higher-percentage product at a gentler pH.

Daily SPF is not optional while you use it. The sun sensitivity is real, measured, and the reason the FDA requires the warning. And keep glycolic acid away from open skin, sunburn, and the days right after a professional peel. The ingredient works by controlled injury to the surface, and controlled is doing a lot of work in that sentence. Give it the respect of a schedule, a sunscreen, and a product whose pH you actually know.

Frequently asked questions

What percentage of glycolic acid should I use?

For consumer leave-on products, 4 to 10 percent is the effective range, with the CIR Expert Panel capping it at 10 percent and pH 3.5 or higher. Start at 4 to 5 percent a few nights a week. Professional peels at 20 to 70 percent need a trained applicator.

Why does pH matter for glycolic acid?

Glycolic acid only exfoliates in its free, un-ionized form, and the share that is free depends on pH relative to its pKa of 3.83. At pH 3.5 about 67 percent is free acid, while at pH 4.5 only a small fraction is, so a 10 percent product at pH 4.8 can exfoliate less than a 5 percent product at pH 3.6.

Does glycolic acid make skin more sensitive to the sun?

Yes. In FDA-reviewed studies, 10 percent glycolic acid at pH 3.5 applied daily for four weeks significantly increased UVB sensitivity, which returned to normal about a week after stopping. Daily sunscreen is required while using it, and the FDA requires a sunburn alert on AHA products.

Can I use glycolic acid with retinol?

Not on the same night while you are still building tolerance. Alternate nights instead, and watch for stinging, flaking, or persistent redness. Both ingredients accelerate cell turnover, and stacking them multiplies the irritation faster than it multiplies the benefit.