Exfoliating acids speed up the shedding of pigmented cells sitting on the surface of your skin. For marks that are already deposited, that is a useful mechanism. Faster shedding means faster replacement with cells that have normal melanin content.
They are also the ingredient category most likely to land you in the cycle where the treatment creates the problem it was supposed to solve. If you have ever used a glycolic toner and felt your skin look brighter the next morning, then gradually noticed things getting worse the more often you used it, you have been in the exfoliation trap.
AHAs: glycolic and lactic acid
Both dissolve the bonds between dead surface cells, accelerating shedding. This is turnover from the outside in (compared to retinoids, which work from the inside out).
Glycolic acid has the smallest molecule. Penetrates deeper, faster. More potent, more irritating. Effective at 5 to 10% for home use. The more aggressive option and the one that carries more risk on reactive or melanin-rich skin.
Lactic acid has a larger molecule. Slower, more even penetration. Gentler, with humectant properties so it hydrates while it exfoliates. For pigment-prone skin, this is almost always the better starting point because slower penetration means less risk of uneven exfoliation and irritation-driven rebound.
Mandelic acid is even larger-molecule and even gentler. Worth knowing about if glycolic and lactic both irritate.
Two to three times a week, not daily. Leave-on formulations over wash-off products (a glycolic cleanser on your skin for 30 seconds is doing very little). Start at the lower end and watch your skin's response before going higher.
The exfoliation trap
This is the pattern. You use an AHA. Your skin looks brighter. You use it more often. Or bump up the concentration. Each application is satisfying because you are physically removing the dull, pigmented surface.
But each application is also an irritation event. On skin prone to pigmentation, repeated irritation triggers the inflammatory signalling that drives pigment production. Short-term brightening masks long-term damage. Your skin looks temporarily better after each use while melanocyte activity is actually being ramped up underneath.
Women stay in this cycle for months. Exfoliating more frequently. Seeing diminishing returns. Not connecting increasing pigment with increasing acid use.
The signals that you have crossed the line: persistent tightness, stinging on application, new pigment in areas that were previously clear. If any of those are happening, the AHA is doing more harm than good.
BHAs: salicylic acid
Different job. Salicylic acid is oil-soluble, which means it gets into the pore itself. For acne-related PIH specifically, that matters because the inflammation driving the pigment often starts inside the pore. If congestion and inflammation are still active in there, new pigment keeps getting deposited no matter what you do at the surface.
BHA at 0.5 to 2% keeps acne-prone pores clear and reduces intra-pore inflammation. It is not a brightening ingredient. It is the ingredient that stops acne-driven pigment from being produced in the first place. Narrow role, but for that specific situation, hard to replace. For pigment that is not acne-related, it is not the right tool.
What to watch for on reactive and melanin-rich skin
Lactic acid or mandelic acid over glycolic. Always. Slower penetration, less rebound risk. Two to three times a week maximum. And be ruthlessly honest with yourself about where the line is. The temporary glow after exfoliation can mask damage that is building underneath. If your skin is getting more sensitive over time, not less, the acid is not helping anymore.
BHA at 2% is generally well-tolerated across skin tones because it works below the surface rather than on it.
PHAs (gluconolactone, lactobionic acid) are the gentlest option. Very slow, surface-level exfoliation with minimal irritation. Not powerful enough for stubborn pigment, but they fill a niche for skin that cannot tolerate any AHA at all.
Where acids stop
Exfoliating acids work exclusively at the surface layer. They clear pigmented cells. They do not reduce melanin production, they do not influence signalling, and they do not reach dermal pigment. Once the surface is clear, if the production signal is still running from below, fresh pigmented cells replace the ones you just exfoliated away.
This is why AHAs alone so often plateau. Excellent at clearing the top. Silent on what is driving production underneath.
Exfoliating acids do real surface work. They also carry real risk on the skin that needs the most careful approach. Less acid, more consistently, with honest awareness of where the line is for your skin. That is almost always the right call.
