PHA vs AHA
PHA vs AHA — a comparison of two chemical exfoliants where polyhydroxy acids (PHAs) are larger, gentler molecules that exfoliate more superficially, and alpha hydroxy acids (AHAs) penetrate more readily for stronger resurfacing. PHAs are often chosen for sensitive skin, while AHAs deliver more noticeable smoothing.
Key points
- PHAs such as gluconolactone and lactobionic acid have larger molecules that stay closer to the surface, generally causing less irritation.
- AHAs like glycolic and lactic acid penetrate more easily and tend to offer more pronounced exfoliation and brightening.
- PHAs also have humectant and antioxidant properties and are often recommended for sensitive or reactive skin.
- Both exfoliants can increase sun sensitivity, so regular sunscreen use is recommended with either.
PHA vs AHA at a glance
| PHA | AHA | |
|---|---|---|
| Molecule size | Large — gluconolactone about 178 g/mol, lactobionic acid about 358 g/mol | Small — glycolic acid 76 g/mol, lactic acid 90 g/mol |
| Depth reached | Mostly the uppermost stratum corneum | Further into the stratum corneum, glycolic acid deepest |
| Hydrating behaviour | Humectant in its own right — several hydroxyl groups hold water | Mildly humectant at best; lactic acid more so than glycolic |
| Irritation profile | Little or no stinging for most people | Tingling, transient redness and dryness are common early on |
| Speed of visible change | Gradual, better judged over six to eight weeks | Faster, often smoother texture within two to three weeks |
| Who it suits | Sensitive, reactive, dry or barrier-compromised skin | Rough texture, dullness, sun damage, fine lines |
| Typical strengths | 4–15% in leave-on formulas | 5–10% leave-on; higher concentrations in professional peels |
| Tolerance in sensitive or rosacea-prone skin | Usually well tolerated, sometimes used after procedures | Often stings or flushes; patch testing is sensible |
How each acid works
Both are hydroxy acids that loosen the bonds holding dead cells at the surface, so the outer layer sheds more evenly. The difference is size, and therefore how far the molecule travels before it acts. Glycolic acid, at around 76 g/mol, is the smallest AHA in common use; gluconolactone and lactobionic acid are two to five times heavier, so far more of the dose stays near the surface.
Depth is not the whole story. PHA molecules carry several hydroxyl groups, which makes them humectants in their own right — they hold water in the outer layers while they exfoliate, so skin tends to feel cushioned rather than stripped. AHAs do more of their work just below the surface, which is why the evidence for texture and photoageing is stronger, and also why they sting.
Which to choose
For most people with resilient skin who want visible smoothing, an AHA is the more efficient choice. Glycolic acid has decades of clinical work behind it for rough texture, dullness and fine lines. Lactic acid is a reasonable middle ground — still an AHA, but larger, slower and more hydrating — and 5–10% on two or three nights a week is a common starting pattern.
PHAs earn their place where tolerance rather than potency is the limiting factor: skin that stings with most actives, a barrier that is already compromised, eczema- or rosacea-prone skin, or a routine that already includes a retinoid. They are also a sensible first acid if you have never exfoliated chemically. Where an inflammatory skin condition is being managed by a clinician, ask before adding either.
Using them together, or switching
Applying both in one sitting is rarely worth it: you raise the total acid load without a matching gain, and if irritation follows you cannot tell which one caused it. Alternating is easier to control — an AHA on some evenings and a PHA on the others, or a PHA alone through a spell when skin feels reactive.
Moving up from a PHA to an AHA usually brings a minute of tingling and some dryness in the first fortnight; once or twice weekly at first keeps that manageable. Going the other way can feel like a step backwards, because the exfoliation is subtler — judge it at six to eight weeks rather than by the first night. Sunscreen matters either way.
Frequently asked
Are PHAs gentler than AHAs?
Generally yes, because PHA molecules are larger and penetrate less, which usually makes them milder and a good option for sensitive skin.
Do PHAs work as well as AHAs?
PHAs exfoliate more gently and gradually, so AHAs often give faster, more noticeable results, while PHAs prioritize tolerability for reactive skin.
Can I use a PHA and an AHA in the same routine?
Yes, but layering both in one application adds acid load without much extra benefit. Alternating nights is easier to judge — an AHA on two or three evenings and a PHA in between, or a PHA on its own when skin feels reactive.
How long should I give a PHA before deciding whether it is working?
Six to eight weeks of consistent use is a fairer test than the first fortnight, since PHAs work gradually. Smoothness generally shows before any change in tone. If nothing has shifted by then, a low-strength AHA is a reasonable next step.
Is a PHA a sensible first acid if I have never exfoliated chemically?
For many people, yes. Starting with a PHA lets you learn how your skin responds to exfoliation with a low chance of stinging or peeling. If it stays comfortable after a month or two, moving up to a mild AHA is straightforward.
Do PHAs still make skin more sun-sensitive?
Any exfoliating acid can increase sun sensitivity, and the effect may persist for around a week after you stop. PHAs are milder in this respect than glycolic acid, but daily broad-spectrum sunscreen is still advised with either.
Related topics
This is a foundational entry in the SYNC Skin Encyclopedia and is expanded over time. Educational information only — not medical advice.
