Exfoliants
AHA, BHA and PHA, chosen by how skin responds rather than by strength
The useful question is not which acid is strongest. It is which one is soluble in the thing you are trying to reach.
Chemical exfoliants get sorted by strength in almost every guide, which is the least useful axis available. Solubility decides where an acid can go, and pH decides how much of it is active when it gets there. Percentage comes third.
Solubility
Alpha hydroxy acids are water soluble. They work on the surface, on the bonds between cells in the outermost layer. Glycolic is the smallest and penetrates fastest; lactic is larger and gentler; mandelic is larger again.
Salicylic acid is oil soluble. That single property lets it get into a pore lined with sebum, which no AHA can do at any strength. A 2% salicylic and a 10% glycolic are not competing products; they are addressing different places.
Polyhydroxy acids are larger AHAs. The size limits penetration, which is why they are the usual starting point for skin that reacts.
pH, the number nobody prints
An AHA’s activity depends on how much sits in its free acid form, which depends on the formula’s pH. A 10% glycolic at pH 4.5 is milder than a 7% at pH 3.5. Almost no brand publishes a pH, so the percentage on the front is half of a two-part number. When a product does publish one, that is a disclosure worth rewarding.
Contact time
A 2% salicylic cleanser and a 2% salicylic leave-on are different exposures by an order of magnitude. Any comparison that calls them equivalent because the percentages match is comparing labels rather than products.
What over-exposure looks like
Tightness, flaking, stinging on application of anything else, and a barrier that reacts to products it used to tolerate. None of these are signs the acid is working. The usual correction is frequency, not strength, and a barrier cream while things settle.