How AHAs Exfoliate
How AHAs Exfoliate — water-soluble alpha hydroxy acids, such as glycolic and lactic acid, that loosen the bonds holding dead cells together at the skin's surface so they shed more readily. By weakening these connections, they encourage smoother texture, more even tone, and brighter-looking skin over time. Being water-soluble, they act mainly on the surface and are often chosen for dryness, dullness, and uneven pigmentation.
Key points
- AHAs reduce cohesion between corneocytes in the outer skin layer, promoting gentle shedding.
- Glycolic acid is small and penetrates readily, while lactic acid is larger and often gentler and more hydrating.
- Regular use can improve surface smoothness, radiance, and the appearance of uneven tone.
- AHAs can increase sun sensitivity, making daily sunscreen important during use.
Where each step takes place
| Step | What happens | Where |
|---|---|---|
| Partitioning | Uncharged free acid enters the tissue | Outermost corneocyte layers |
| Local acidification | A proton is donated, lowering pH around the cells | Intercellular lipid matrix |
| Ionic bonding fails | Attraction between cell envelopes falls away | Between adjacent corneocytes |
| Rivets weaken | Desmoglein 1, desmocollin 1 and corneodesmosin cleave more easily | Corneodesmosome junctions |
| Earlier desquamation | The skin's own enzymes separate cells sooner and more evenly | Stratum disjunctum |
| Optical change | A flatter surface reflects light more uniformly | Skin surface only |
The chain of events, in full
An AHA reaches the skin as two forms in equilibrium: undissociated free acid and its dissociated salt. Only the free acid crosses readily into the stratum corneum, being uncharged and able to move through the lipid matrix between cells. There it releases a proton, and the local drop in pH undermines both the corneodesmosomes holding corneocytes together and the ionic attraction between their envelopes.
Nothing is dissolved. What changes is the force needed to pull two cells apart, so the kallikrein-related peptidases that normally cleave corneodesmosomes finish the job earlier. The effect is properly called reduced corneocyte cohesion rather than removal of a layer: the threshold for shedding is lowered, not forced.
Why free acid matters more than the number on the label
An AHA is a weak acid, so how much sits in the penetrating free-acid form depends on where the formula's pH falls relative to its pKa — a little below 4 for glycolic acid, slightly higher for lactic. At a pH equal to the pKa, roughly half is free acid; a unit above it, most is neutralised into a salt still listed at the same percentage but barely able to enter skin.
Molecular size sets the second limit: glycolic acid is the smallest common AHA and diffuses fastest, while lactic and mandelic acid stay shallower. Smoothness therefore shifts within days, while anything depending on repeated turnover takes weeks. A thinner outer layer also scatters less UV, the usual explanation for AHA sun sensitivity.
Where the mechanism stops
AHAs have documented effects deeper than the stratum corneum, but from exposures daily products do not reach. Glycolic acid at peel strength — high concentration, low pH, timed contact, neutralised under supervision — has been reported to raise dermal glycosaminoglycans and alter collagen expression. That is the honest basis for saying AHAs do more than exfoliate, but it is not evidence that a leave-on product at a few per cent remodels the dermis.
Two further boundaries. The mechanism says nothing about pigment production: AHAs shed pigmented cells sooner, which reads as brightening, but they do not inhibit tyrosinase. And nothing about follicular contents, since water-soluble acids do not partition into sebum — that belongs to How BHAs Work in Pores. Persistent or patterned pigmentation is a question for a clinician.
Frequently asked
What is the difference between glycolic and lactic acid?
Glycolic acid has a smaller molecule and penetrates more deeply, while lactic acid is larger, tends to be gentler, and also has humectant properties.
Do I need sunscreen when using AHAs?
Yes; AHAs can make skin more sensitive to UV light, so daily broad-spectrum sunscreen is recommended while using them.
How long before an AHA makes a visible difference?
Smoothness often changes within a few days. Tone and the look of fine lines depend on repeated turnover cycles, and are better judged over four to twelve weeks.
Does a lower percentage still exfoliate?
Yes, provided the pH keeps enough of it in free form. A 5% glycolic acid at pH 3.5 can deliver more free acid than a 10% one buffered near pH 5.
Do AHAs thin the skin over time?
They thin the dead outer layer, which is the intended effect and reverses within days of stopping. Regular use has not been shown to thin the living epidermis; lasting tightness or stinging points to over-use.
Is an at-home AHA doing the same thing as a professional peel?
The same chemistry at a very different dose. A clinic peel uses far higher concentrations at lower pH for a short, neutralised contact; a leave-on product repeats a much shallower effect over weeks.
Related topics
This is a foundational entry in the SYNC Skin Encyclopedia and is expanded over time. Educational information only — not medical advice.
