Glycolic acid

Glycolic acid is the smallest alpha hydroxy acid, which is why it penetrates further than the others and why it is the one most products use.

What it does is mechanical rather than mysterious. Dead cells at the surface are held together by bonds that normally dissolve on their own; AHAs “diminish corneocyte cohesion immediately above the granular layer by detaching and desquamating the stratum corneum”. Skin that was going to shed sheds sooner and more evenly.

Why the ingredient list is nearly useless here

Because presence is not the variable. Strength and pH are, and the list carries neither.

The review is precise about how much difference strength makes: “1% AHA content can alter the pH of the outer three layers of the stratum corneum, whereas 10% can affect all 10-20 layers.” Three layers against twenty, from the same ingredient with the same name on the same list.

pH matters as much. Glycolic acid works as free acid, and above its pKa of 3.83 progressively less of it is in that form. Most home products are partially neutralised to around pH 4 to reduce irritation, which also reduces activity. Two serums can both say “glycolic acid” and differ several-fold in what they actually do.

The list will not distinguish them. This is the sharpest example of the point made in the guide to reading a cosmetics label: below 1 percent the order carries no information, and the numbers that matter are printed on the front or nowhere.

The sun sensitivity finding

This one deserves stating plainly rather than as a footnote, because it changes how the product should be used.

The review records a study in which “topical application of 10% GA for 12 weeks increased the sensitivity of the skin to UV light and enhanced the formation of sunburn cells”. The authors’ own work found that a high concentration of glycolic acid together with UVB had a synergistic effect on cell death.

So daily sunscreen is not general skincare advice bolted onto this page. It is part of the intervention, and using an acid without it is working against yourself.

The paper’s conclusion is the honest summary: “Whether AHA is a friend or foe of human skin depends on its concentration.”

Pregnancy

MotherToBaby records that “ACOG suggests that OTC topical products containing benzoyl peroxide, azelaic acid, salicylic acid, or glycolic acid may be used during pregnancy, if needed.”

Glycolic acid is on that list, which puts it among the small set of actives a pregnancy page can be positive about rather than cautious. Azelaic acid is another, and it has the advantage of not increasing UV sensitivity.

Practical reading

A stated percentage on the front is worth more than the entire ingredient list. Products that name a strength are the ones that can be compared.

Home products are not peels. Professional peels run far higher and are a different intervention with different risks. The two share a name and very little else.

More often is not better. The barrier needs to rebuild between uses, and the irritation people attribute to a “purge” is frequently just overuse, which is covered on the page for skin that reacts to everything.