AHA, BHA, PHA: which acid for which skin

Three families of acid, and the one you need depends on what is wrong — not on which is strongest. Most of the acid damage I see comes from people who picked the most powerful option available rather than the one matched to their skin.

AHA — for the surface

Water-soluble, works on the top layer. Glycolic has the smallest molecule and penetrates deepest; lactic is larger, gentler and hydrates as it exfoliates; mandelic is larger still and the mildest of the three.

• Use for: dullness, rough texture, fine lines, uneven tone, pigment.
• Best suited to: normal, dry and mature skin.
• Start at: 5–8% glycolic or 5–10% lactic, twice a week.
• Note: increases sun sensitivity. Sunscreen is not optional alongside these.

BHA — for inside the pore

Salicylic acid is oil-soluble, which is the whole point: it dissolves through sebum and works inside the pore rather than on the surface. It is also anti-inflammatory, so it calms the redness around a spot as well as clearing it.

• Use for: blackheads, congestion, oily skin, acne, bumpy texture.
• Best suited to: oily and acne-prone skin.
• Start at: 1–2%, two to three times a week.
• Note: can be used as a targeted treatment on one area rather than the whole face.

PHA — for skin that cannot take the others

Gluconolactone and lactobionic acid have much larger molecules, so they work slowly and stay near the surface. That makes them the right answer for skin that stings at everything else.

• Use for: gentle exfoliation, dullness, dryness.
• Best suited to: sensitive skin, rosacea-prone skin, a barrier still recovering.
• Note: they are humectants as well, so they hydrate rather than dry.

The rules that prevent damage

1. One acid at a time. Not a glycolic toner, a salicylic cleanser and a weekly peel. That combination is how barriers break.
2. Not on the same night as a retinoid. Alternate nights.
3. Twice a week is enough for most people. Daily acid is for skin that has earned it over months.
4. Stinging that lasts is a warning, not a sign it is working. A few seconds of tingle is normal; burning is not.
5. Stop entirely if skin becomes shiny, tight and reactive. That is over-exfoliation, and it looks deceptively like “glowing” for about a week before it turns.

If you cannot say which problem you are solving, you do not need an acid yet. Exfoliation fixes texture and tone. It does not fix dehydration, redness or a damaged barrier, and used on those it makes all three worse.