AHAs, or alpha hydroxy acids, are a family of acids from fruit, sugar cane and milk. Glycolic and lactic acid are the ones used most in cosmetics1. BHA, or beta hydroxy acid, in skin care means salicylic acid. They look similar on a label, but they behave differently.
| Compared | AHA (glycolic, lactic) | BHA (salicylic acid) |
|---|---|---|
| Dissolves in | Water | Oil |
| Where it works | Surface of the skin | Surface, and inside the oil gland lining4 |
| How | Loosens the 'glue' between dead surface cells1 | Disrupts the bonds between dead cells and removes lipids between them4 |
| Leave-on strength in the sources | 10% or less, pH 3.5 or higher2 | 0.5% to 2%5 |
| Suits | Dull, rough surface | Oily, congested skin |
What an AHA does
The FDA says AHAs cause exfoliation, and how much depends on the type and concentration of acid, the pH and the other ingredients2. The industry-funded Cosmetic Ingredient Review panel concluded that glycolic and lactic acid products are safe for consumers at 10% or less, at a final pH of 3.5 or higher, and when the product protects from sun sensitivity or tells users to wear sunscreen2.
Sun sensitivity is the catch
In the FDA's data, after four weeks of AHA use, sensitivity to UV reddening rose by 18% and sensitivity to UV cell damage roughly doubled on average, with big differences between people. It faded within a week of stopping. The FDA suggests wearing sunscreen and protective clothing while you use an AHA and for a week afterwards2. In a study cited by Kornhauser, 10% glycolic acid applied six days a week for four weeks made 16 volunteers more sensitive to UVB3.
What a BHA does
Salicylic acid is oil-soluble, so it can get into the pore lining as well as the surface4. It has a pKa of about 2.97, which means a leave-on needs an acidic pH, roughly 3 to 4, to have enough free acid6. The US over-the-counter acne range is 0.5% to 2%5, and the EU also caps leave-on products at 2%.
For acne, the 2024 AAD guideline conditionally recommends topical salicylic acid, with low certainty7. A Cochrane review found salicylic acid may make little or no difference compared with tretinoin, with low quality evidence8. We do not claim that our BHA treats acne. It helps the look of congested pores.
Using them
- Start two or three nights a week and see how your skin responds. That is our guidance: the sources give no tested frequency.
- Do not stack an exfoliating acid with retinol on the same night when you are new to actives.
- Wear sunscreen every morning, whatever acid you use.
