AHA, BHA, PHA: what the letters actually mean

Exfoliating acids are labelled with initials far more often than they are explained. The initials describe chemistry, not strength.

AHA — alpha hydroxy acids

Water-soluble acids. Glycolic acid and lactic acid are the two you will see most often. Because they are water-soluble they work at the surface of the skin.

Glycolic acid has the smallest molecule of the common AHAs; lactic acid is larger.

BHA — beta hydroxy acids

Salicylic acid is the one in near-universal use. The important difference is that it is oil-soluble, so unlike an AHA it can travel into the oil in a pore rather than staying on the surface.

That property is why BHA turns up so often in products aimed at congested or oily skin.

PHA — poly hydroxy acids

Gluconolactone and lactobionic acid are the usual examples. The molecules are substantially larger than AHAs, so they penetrate more slowly. PHAs are frequently chosen where a gentler option is wanted.

What the letters do not tell you

Initials alone do not tell you how strong a product is. Concentration and pH both matter, and a low-concentration AHA can be far milder than a high-concentration one.

They also do not tell you what to combine. Layering several acids, or using acids alongside other active ingredients, is where irritation usually starts — and irritated skin is not exfoliated skin, it is damaged skin.

Sun sensitivity

Exfoliating acids can increase how sensitive your skin is to sunlight. Daily sunscreen is not optional alongside them.

A note

This article is general information about skincare practice, not medical advice, and it does not describe what any particular product will do for you. Cosmetics are not medicines. Patch-test anything new, follow the manufacturer's directions, and speak to a doctor or pharmacist if you have a skin condition, are pregnant or breastfeeding, or use prescription treatments.