AHA, BHA or PHA: which acid suits your skin

Three acronyms on three bottles, and no obvious way to tell which one you need. The good news is that the difference comes down to one thing you can actually reason about: how big the molecule is.

Bigger molecules travel more slowly into the skin. Slower means gentler, and usually also less dramatic. That is the whole trade-off.

The three, side by side

Glycolic acid, the fastest

Glycolic acid has the smallest molecule of the common AHAs, which is why it works quickly and why it stings the most. It loosens the bonds between dead cells at the surface, so they shed more evenly. That is what people mean by a brighter, smoother complexion.

It also makes skin temporarily more sensitive to UV, so daily SPF is part of using it.

Salicylic acid, the one that goes into the pore

Salicylic acid dissolves in oil, so it can get into the sebum sitting inside a pore rather than working on top of it. That is why it comes up for blackheads and congestion, and why it is the odd one out here. The article on salicylic acid goes into it properly.

Gluconolactone, the slow one

Gluconolactone is a PHA, a polyhydroxy acid. The molecule is considerably larger than glycolic acid, so it penetrates more slowly and more evenly. Less depth per application, and much less stinging.

It has two side jobs the others do not. It holds water, so it hydrates while it exfoliates. And it works as a mild antioxidant. For anyone whose skin has never tolerated acids, this is usually the one that changes their mind.

The mistake almost everyone makes

Not choosing the wrong acid. Using several at once.

An exfoliating cleanser, then a toner with acids, then a weekly mask, and retinol somewhere in the week. Each product is reasonable on its own. Together they are a daily peel, and the result is the tight, stinging, reactive skin people then blame on one product.

A workable rule: one exfoliating step in your routine, two or three times a week to start. If you want to use two different acids, put them on different evenings.

And never in the same evening as retinol. Alternate.

How to tell you have overdone it

Skin that feels tight after cleansing, stings from products that were always fine, looks shiny in a taut rather than an oily way, or has small rough patches. That is a damaged barrier, and the answer is to stop exfoliating and rebuild with lipids. The article on ceramides covers how.

Where ours sits

Milky Miracle combines glycolic acid with gluconolactone in a cleansing milk. That format matters: a cleanser is on your skin for under a minute, so the exfoliation is gentle by design. It suits people who want the effect of an acid without committing to a leave-on product, and it works as a first acid.

For congestion inside the pore, Radical Refresh is the one with salicylic acid.

Frequently asked questions

Which should I start with?

Oily skin with blackheads: BHA. Dull or uneven skin: AHA. Sensitive skin, or you have reacted badly to acids before: PHA.

Do I have to use SPF with them?

With AHAs, yes, and it is sensible with all of them. Shedding the top layer faster leaves skin more exposed.

Can I use acids and vitamin C?

Yes, on different parts of the day. Vitamin C in the morning, acids in the evening.

Is a higher percentage better?

No. Above a certain point you get more irritation rather than more result, and how often you use it matters more than how strong it is.

Sources

• Regulation (EC) No 1223/2009 on cosmetic products.
• Green BA, Yu RJ, Van Scott EJ, Clinical and cosmeceutical uses of hydroxyacids, Clinics in Dermatology, 2009.