AHA, BHA, or PHA: how to actually choose

Exfoliating acid is not one thing. AHA, BHA, and PHA describe three different chemical families that work on different parts of skin, at different depths, for different reasons. Picking one because it is popular, without knowing what it is actually built to do, is how a lot of people end up with a red, flaking face and no idea why.

AHAs: glycolic acid, lactic acid, mandelic acid

Alpha hydroxy acids work on the surface. They loosen the bonds between dead skin cells so they shed more evenly instead of building up into rough or dull-looking patches, and with consistent use they support a more even tone and a smoother feel. They are water-soluble, so they act where they land rather than traveling down into a pore.

• Glycolic acid has the smallest molecule of the three, which lets it penetrate fastest and generally makes it the strongest and most likely to irritate. It also has the deepest research history of any AHA.
• Lactic acid is a larger molecule, so it works a little more gently while still supporting smoother texture and more even tone, and it doubles as a humectant that pulls in some water as it works.
• Mandelic acid is the largest and slowest of the three, which makes it a reasonable starting point for anyone new to acids or prone to redness, though it still delivers a real surface effect over time.

BHA: salicylic acid, for oil and congestion

Salicylic acid is the ingredient most people mean when they say BHA, and it behaves differently from any AHA because it is oil-soluble. That lets it move into the oil inside a pore instead of just working across the surface, which is why it is the standard choice for visible congestion, blackheads, and an oily T-zone. It also has some calming behavior of its own, which is part of why it turns up in so many blemish-focused formulas rather than general brightening ones.

PHA: gluconolactone, the gentle option

Polyhydroxy acids like gluconolactone work on the same basic principle as an AHA, loosening the surface bonds between dead cells, but the molecule is larger still, so it penetrates more slowly and stays closer to the surface. That makes it the most forgiving option for sensitive skin, redness-prone skin, or anyone who reacts to glycolic acid or salicylic acid but still wants some exfoliating benefit. It is also a humectant, so it tends to leave skin feeling hydrated rather than stripped, which is unusual for an exfoliant.

How often each one actually earns a spot

• Glycolic acid or salicylic acid at a real strength: two to three nights a week is a sensible starting frequency, built up slowly rather than jumped into.
• Lactic acid or mandelic acid: often tolerated three to four nights a week once skin is used to it, since both are gentler by nature.
• Gluconolactone: can often be used nightly by skin that cannot handle the others at all, though how skin actually looks and feels still matters more than a fixed schedule.
• Any of them alongside retinol: pick separate nights. Running an acid and a retinoid on the same night asks skin to handle two forms of turnover at once.

Why more is not better

Skin has a limited tolerance for turnover in a given week, and every acid draws from the same budget regardless of which letter is in front of it. Layering a glycolic acid toner, a salicylic acid serum, and a gluconolactone cream in the same routine does not triple the benefit. It mostly triples the irritation while the actual gains flatten out fast past a certain point. One acid, matched to what your skin actually needs, oil control, surface texture, or gentle maintenance, used at a sensible frequency, will outperform a stacked routine that never gets the chance to calm down between uses.