What 2% salicylic acid actually tells you
The number on the front is the least informative thing about an acid.
Percentages are the easiest thing to put on a label and the easiest thing to misread. A 2% salicylic acid wash and a 2% salicylic acid leave-on serum share a number and almost nothing else.
Three things the number does not carry
pH. Salicylic acid is only partly active in its acid form, and how much of it stays in that form depends on the pH of what it is dissolved in. Push the formula toward neutral and a good deal of the acid converts to a salt that behaves differently on skin. Two products at the same percentage can therefore differ substantially in how much active acid is actually present. Almost nobody prints the pH.
Vehicle. Salicylic acid is oil-soluble, which is the whole reason it is used for congested pores rather than surface texture — it can move into sebum instead of sitting on top of it. But it has to be held in something that lets it do that. The rest of the formula decides whether the acid reaches the pore or stays in the water phase.
Contact time. This is the one that matters most and gets discussed least. A wash is on the skin for perhaps thirty seconds before it goes down the drain. A leave-on stays for hours. Same percentage, wildly different exposure.
Which is why a wash is a deliberately modest thing
04 Acne Face Wash is 2% salicylic acid with the clay dropped to 5% — lower than the 8% in 01 — precisely because in this formula the acid is meant to do the work and the clay is not meant to compete with it. Two absorbent mechanisms stacked on each other is how you end up with tightness.
Panthenol is in there for the same reason. It offsets some of the dryness rather than pretending there is none.
A wash is not the strongest way to use a BHA and it is not trying to be. It is the way with the shortest contact time, which is also the way with the least room to go wrong on skin that is already irritated.
What to actually ask
If you are comparing two products at the same percentage, the percentage is the part you can stop thinking about. Ask what it is dissolved in, and how long it stays on. If a brand will not tell you either, the number on the front is doing more marketing than chemistry.
And if your skin is already sore, more acid is rarely the answer. Salicylic acid is well studied for comedonal congestion; it is not a treatment for inflamed or broken skin, and nothing here should be taken as advice for a condition a doctor should be looking at.