What do acid exfoliants (AHAs) actually do?
Short answer: they smooth the surface — and one study suggests they quietly thicken the deeper skin too.
AHAs ("alpha-hydroxy acids") are the acid exfoliants: glycolic, lactic, mandelic, and citric acid. They loosen the "glue" holding dead surface cells together, so those cells shed more evenly. That's why skin looks smoother and brighter after you've used them a while.
But they may do more than surface work. In a 1996 study, people put a 25% mixed-alpha-hydroxy-acid lotion on one forearm and a plain placebo lotion on the other for about six months. Skin samples showed changes in thickness and dermal structure in that studied formula [1]. This does not establish the same result for every AHA, strength, vehicle, or leave-on product.
Which acid to pick — it comes down to molecule size:
- Glycolic, lactic, and mandelic acids differ in molecular size and in how individual formulas are buffered and delivered. Those differences may affect penetration and irritation, but they do not establish a universal ranking of effectiveness or gentleness.
How to use them safely:
- AHAs can increase sun sensitivity — the FDA's guidance for AHA products includes sun protection and a "Sunburn Alert" label telling users to use sunscreen [2]. The guidance is for AHAs, not BHA products, and it does not make nighttime use mandatory.
- Start with a lower percentage a few times a week.
- Don't pile everything on at once — AHAs plus a strong retinoid plus vitamin C on the same night is the fast track to an irritated, damaged barrier.
Honest caveat: that study used one 25% mixed-acid forearm formula, which is stronger and more specific than most leave-on products you'll buy. A gentle 5% toner or a BHA product should not be assumed to produce the same result.