Who exfoliating acids are for, and when they stop being for you

Exfoliating acids are in cleansers, toners, serums, masks and peels, and for good reason: on the right skin they produce a result you can see within weeks. The question is not whether they work. It is whose skin they work on, and what changes when the skin underneath them changes.

What they do

The outer layer of skin is made of flattened dead cells that are meant to shed on their own, evenly, a little every day. When shedding is even, the surface is smooth and reflects light. When it is uneven, cells cling in patches, pores cap over, and skin looks rough, dull and congested.

Hydroxy acids loosen the bonds that hold those dead cells to each other so they lift away sooner.¹ The two families work in slightly different places. Alpha-hydroxy acids, glycolic and lactic chiefly, are water-soluble and work on the surface.¹ Salicylic acid, the beta-hydroxy acid, is oil-soluble, so it can get into the pore and loosen the plug of oil and cells there; this is why it has been the acne acid for decades.² At higher concentrations in a clinic, both are used as peels, with well-documented results on acne, sun damage, uneven tone and rough texture.^2,3

Who they suit

Skin that has oil and barrier to spare. In practice that is:

• Oily, congested skin in the 20s and 30s. Oil and uneven shedding are the whole problem, and the barrier is thick enough to give up some lipid and replace it overnight. Salicylic acid's comedolytic action is at its most useful here.²
• Sun-thickened skin. Where years of sun have built up a rough, uneven surface, alpha-hydroxy acids have decades of evidence for smoothing it.^1,3
• Robust skin with occasional texture. Skin that can use an acid once or twice a week and be fully comfortable the next day.

The common thread is a barrier that can afford the trade.

What the trade is

An acid cannot fully separate the dead cells it should remove from the lipid that holds the barrier together. It can loosen both. The effect depends heavily on concentration: a 2018 review concluded that whether an alpha-hydroxy acid helps or harms skin depends on how much is used, and listed swelling, burning and itching among the reactions at the wrong dose.³ Alpha-hydroxy acids can also increase the skin's sensitivity to the sun, and the FDA advises sunscreen while using them and for a week afterwards.^1,4

On a barrier with plenty of lipid and a fast turnover, this is a fair trade. The skin is smoother in the morning and can rebuild what the acid took within a day or so.

When they stop being for you

The barrier underneath is what changes in midlife, and it changes for hormonal reasons, not because of anything in the bottle. As estrogen declines, skin becomes thinner, holds less water and makes fewer barrier lipids.⁵ Stress hormones reduce barrier lipids further and slow the barrier's recovery after any disruption.⁶

Put an acid on that barrier and the trade turns. There is less lipid to spare, and what is lost is replaced more slowly, so each use leaves the skin slightly more open than the last. The texture returns within days, because the uneven shedding has a hormonal cause the acid did not touch, and now the skin also stings. The same glycolic toner that was a good choice at 28 is a poor one at 48, not because the ingredient changed, but because the skin did.

Put together

Exfoliating acids are a good tool for skin with oil and barrier to spare: oily, congested, sun-thickened, robust. They are the wrong tool for skin that is losing both, which is what estrogen decline and cortisol produce. Rough texture in midlife is usually a hydration and hormone problem, and it responds to the opposite approach: water in the surface so it sheds evenly, oil balanced rather than stripped, and barrier lipids put back.