Because only the undissociated — free acid — fraction acts as the permeant, and pH decides how much of it exists. Glycolic acid has a pKa of approximately 3.83: at pH 3.8 roughly 50% is free acid, at pH 4.5 roughly 18%. An 8% formulation therefore carries about 4% free acid at pH 3.8 and about 1.4% at pH 4.5.
Published 30 September 2026 · Last reviewed 30 September 2026
The terms, defined
pKa is the pH at which a weak acid is exactly half dissociated. Below its pKa the undissociated, electrically neutral form predominates; above it, the ionised form does.
Free acid is the undissociated form of an alpha-hydroxy acid — for glycolic acid, the neutral molecule rather than the glycolate anion. Because the stratum corneum lipid matrix is hydrophobic, the neutral form partitions into it far more readily than the charged one.
The Henderson–Hasselbalch relation links the two: the ratio of ionised to neutral species is set by the difference between the formulation pH and the acid's pKa. One pH unit above the pKa leaves roughly 9% neutral; one unit below, roughly 91%.
Desquamation is the controlled shedding of corneocytes from the skin surface, which depends on the enzymatic degradation of the corneodesmosomes that hold them together.
Neutralisation, in the context of a professional peel, is the deliberate termination of the acid's action by raising the pH at the skin surface.
The number on the label is not the number that acts
A product declares a total concentration: 8% glycolic acid. That figure counts both species — neutral molecules and glycolate anions together. It says nothing about the proportion of each, which is fixed by the formulation pH.
Cambier and colleagues (2026) set out the arithmetic for glycolic acid, whose pKa is approximately 3.83.
| Formulation pH | Proportion present as free acid | Free acid delivered by an 8% formulation |
|---|---|---|
| 3.8 (≈ pKa) | approximately 50% | about 4% |
| 4.5 | approximately 18% | about 1.4% |
Two products may both declare 8% and differ by nearly threefold in the quantity of active species they present to the skin. This is the same physical chemistry that governs L-ascorbic acid, where the constraint runs the other way — towards a lower pH — and which we set out in our note on why topical vitamin C needs a pH below 3.5 and, in full, in our open-access report on the pH constraint on L-ascorbic acid and the oxidative stability of tetrahexyldecyl ascorbate (CC BY 4.0).
Why regulators specify a pair of values, never one
This is why every serious guidance on alpha-hydroxy acids couples a concentration ceiling to a pH floor. A concentration limit alone is not a limit on anything: the same percentage at a lower pH is a materially different exposure. As reported by Cambier and colleagues (2026), the three frameworks most often cited read as follows.
| Body | Maximum concentration for at-home use | pH condition |
|---|---|---|
| European Union — SCCNFP/SCCS opinion | up to 4% | pH ≥ 3.8 |
| United States — FDA and CIR guidance | no more than 10% | pH ≥ 3.5 |
| Australia — NICNAS/ASCC | up to 20%, or up to 10% around the eyes | pH ≥ 3.5 |
The European position is the most conservative of the three, and it is conservative on both axes at once: a lower ceiling and a higher pH floor. A 4% product at pH 3.8 presents roughly 2% free acid — half the free acid of an 8% product at the same pH, and well under a quarter of what a 10% product at pH 3.5 presents. Anyone comparing a formulation developed for the United States with one developed for the European market should compare free acid, not label percentage. The conditions of use recorded above are those reported in the cited review; the binding text for any given market is the regulation itself, which should be consulted directly before a formulation decision is made.
Professional peels occupy a different regime entirely: 20–70% glycolic acid at a pH of approximately 1.0–2.5, applied for 2–5 minutes for a superficial peel and up to 4–6 minutes for a medium-depth one, then terminated with a 5–10% sodium bicarbonate solution. The short contact time and the deliberate neutralisation are not incidental details; they are what keeps a concentration of that order bounded in its effect.
What the acid does once it is in
The mechanism is acidification of the intercellular environment. Tang and Yang (2018) describe it as follows: the acidic nature of alpha-hydroxy acids lowers local pH, inhibits transferases and kinases, and interferes with the formation of ionic bonds, all of which contribute to desmosome resolution and stimulate desquamation. Corneocyte cohesion is diminished, and the surface layers detach.
The depth reached scales with concentration. Citing Antoniou and colleagues (2010), the same review reports that a 1% alpha-hydroxy acid content can alter the pH of the outer three layers of the stratum corneum, whereas 10% can affect all 10–20 layers. That is a large difference in the extent of the affected tissue for a tenfold difference in concentration — and it is a further reason why the pH that sets the free acid fraction matters as much as the percentage that sets the total.
The skin's own surface pH is the baseline this acts against; we have treated it separately in why the skin's own pH matters.
Where the literature does not support the simple version
Two findings cut against the comfortable account, and they belong in any honest treatment of this ingredient.
First, photosensitivity. Tang and Yang (2018) report a study in which topical application of 10% glycolic acid for 12 weeks increased the sensitivity of skin to ultraviolet light and enhanced the formation of sunburn cells. Their own laboratory found that co-treatment of a high concentration of glycolic acid (5 mM) with UVB had a synergistic effect on apoptosis in HaCaT human keratinocytes. Their conclusion is stated as a question and answered by dose: whether alpha-hydroxy acids are a friend or a foe of human skin depends on concentration. Daytime photoprotection is not an optional accessory to glycolic acid use; it is a condition of it.
Second, the structural effects reported in the positive direction are slow and modest. Cambier and colleagues (2026) report histological findings of a restored granular layer and an increase in epidermal thickness of approximately 20–30% after months of use, and note one study in which glycolic acid adjusted to pH 4 increased collagen production and epidermal proliferation without raising pro-inflammatory cytokines such as TNF-α. These are published observations about the ingredient under defined conditions, not outcomes attributable to any particular product.
On barrier recovery the same review is reassuring but precise: in healthy participants, barrier function typically recovers fully within 24 hours of a peel, and daily-use formulations produce a small increase in transepidermal water loss during use that reverts to normal on discontinuation. The recovery is documented for healthy skin; it is not a general guarantee.
A note on molecular size
Glycolic acid has a molecular weight of 76 Da, lactic acid 90 Da and mandelic acid 152 Da. All three sit far below the threshold that constrains most topical permeants, which is why the pH question dominates the size question for this family — the reverse of the situation for larger molecules, discussed in what determines whether a molecule penetrates skin. Size differences within the family are not negligible, however: mandelic acid is twice the mass of glycolic acid and markedly more lipophilic, and the two do not behave identically at equal concentration and pH.
Scope
This article describes findings published in the peer-reviewed literature on alpha-hydroxy acids. It is not a claim about any product, and nothing in it should be read as an assertion that a cosmetic product prevents, treats or modifies any physiological condition. In the European Union, cosmetic claims are governed by Regulation (EC) No 1223/2009 and the common criteria of Regulation (EU) No 655/2013, which require demonstrable substantiation; in the United States, the cosmetic–drug boundary under the Federal Food, Drug, and Cosmetic Act is determined by the claim made, not by the ingredient used. Professional peel procedures are performed by qualified practitioners and are outside the scope of consumer cosmetic use.
References
- Cambier B, Barragan Ferrer D, Huysmans L, Cambier F, Barragan Ferrer JM. Formulating Glycolic Acid: Balancing Penetration, Irritation, and Therapeutic Outcomes in Dermatology. Cosmetics. 2026;13(4):164. DOI 10.3390/cosmetics13040164.
- Tang S-C, Yang J-H. Dual Effects of Alpha-Hydroxy Acids on the Skin. Molecules. 2018;23(4):863. DOI 10.3390/molecules23040863. PMID 29642579.
- Antoniou C, Kosmadaki MG, Stratigos AJ, Katsambas AD. Photoaging: Prevention and topical treatments. American Journal of Clinical Dermatology. 2010;11(2):95–102, as cited by Tang and Yang (2018).
- TTS Clinical Scientific Team. Topical Vitamin C: the pH Constraint on L-Ascorbic Acid and the Oxidative Stability of Tetrahexyldecyl Ascorbate, v1.0. 2026. DOI 10.5281/zenodo.22491021.