AHA, BHA and PHA: Three Acids, Three Behaviours

The three acid families sold to consumers look interchangeable on a shelf and are not. An AHA dissolves the bonds between corneocytes at the surface and is chosen for texture and tone. A BHA is oil-soluble and follows sebum into the pore, which is why it is the standard for blackheads. A PHA is a larger molecule that penetrates slowly, which makes it the gentle option.

The formulation questions that follow are dose, pH, buffering and neutralisation. Get the pH wrong and an acid stops being an acid; get the buffer wrong and it stings for ten minutes after application. Both are cheap to control and expensive to discover in a complaint trend.

1. AHAs and the pH window

Glycolic, lactic and mandelic acid are effective between pH 3.5 and 4.0. At pH 3 the product exfoliates strongly and stings; above pH 4.5 the free acid fraction falls and the effect becomes cosmetic. Glycolic has the smallest molecule and penetrates fastest; mandelic is larger and slower, which makes it the choice for darker skin types where post-inflammatory pigmentation is a risk.

2. BHA

Salicylic acid is oil-soluble and works inside the pore. It is effective at 0.5 to 2 percent at pH 3 to 4, and above 2 percent the irritation risk rises without much added benefit. Betaine salicylate is a gentler alternative used in Asia at higher percentages with a lower sting profile.

3. PHAs

Gluconolactone and galactose are large molecules that penetrate slowly. They exfoliate gently and are compatible with sensitive skin, and they also act as humectants, which is why a PHA toner leaves skin feeling soft rather than tight. At 4 to 8 percent at pH 4, they are a realistic option for a daily product.

4. Buffering and sting

The sting of an acid product usually comes from the free acid fraction plus a low water content, not from the acid type. Adding a small amount of a buffer, keeping the glycol content moderate and avoiding alcohol reduces the sting without changing the exfoliation. A product that stings for thirty seconds is tolerable; one that stings for five minutes is a return.

5. Neutralisation in production

An acid formula is neutralised to the target pH after the actives are dissolved, and the pH must be checked on every batch. A batch that lands at pH 3.0 instead of 3.8 is 60 percent stronger in free acid terms and will generate complaints from the first week. In-process pH measurement is the cheapest control in the category.

6. Sun sensitivity and the warning

AHAs increase photosensitivity for a period after use, and the pack must say so. BHA carries a similar but smaller effect. A product sold without a sun warning in a market that requires one is a compliance problem, and the warning costs one line of copy.

7. Combining acids

A toner with both AHA and BHA is common and works if the total free acid is controlled. The failure mode is stacking three acids in one product and then adding a retinol in the same routine. A single acid at a defined pH, used consistently, outperforms a cocktail used irregularly.

8. Where it fits

Acid products sit across the serum range; the pH mechanism is expanded in the pH entry.