Exfoliating acids are sold as a single category and behave as two. The dividing line is chemical and it is genuinely useful: alpha hydroxy acids dissolve in water and work on the surface, beta hydroxy acid dissolves in oil and gets into the follicle. Everything else follows from that.
The alpha hydroxy acids
These work at the surface, loosening the bonds between dead cells so they shed more evenly. They suit dullness, rough texture, surface pigmentation and the general look of unevenness.
Glycolic acid is the smallest molecule of the group, so it penetrates furthest and irritates most. It has the longest research record, particularly for pigmentation and texture, and it is also the one most likely to cause trouble on deeper skin tones, where the irritation leaves marks of its own.
Lactic acid is larger, gentler, and humectant, meaning it holds water as well as exfoliating. For dry or sensitive skin it is usually the better starting point.
Mandelic acid is larger still and derived from almonds. It works more slowly and more evenly, which is why dermatologists often prefer it for Indian and other deeper skin tones. It is the least dramatic and the least likely to leave a mark behind.
The beta hydroxy acid
Salicylic acid is oil soluble, which means it goes where the others cannot: inside the follicle, through the sebum. That makes it the right choice for blackheads, congestion, the small closed bumps covered in tiny bumps on the face, and oily skin generally. It is also mildly anti-inflammatory, which the alpha hydroxy acids are not.
It is the wrong choice for dry, flaky skin with no congestion, where it does little and removes oil that was needed.
Which one for which problem
If the problem is texture, dullness or surface pigmentation, an alpha hydroxy acid is the fit. Mandelic first on deeper skin tones, lactic for dry or sensitive, glycolic if the skin has tolerated acids before.
If the problem is congestion, blackheads or oil, salicylic acid is the fit, and pairing it with niacinamide is sensible rather than contradictory, as salicylic acid and niacinamide together sets out.
If the problem is deep pigmentation like melasma, neither is the answer on its own. Acids work at the surface and melasma does not live there. Melasma and where cosmetics stop is the honest version of that, and acids used aggressively on melasma reliably make it worse.
How often is enough
Less often than most routines suggest. Two or three times a week is sufficient for most people on a leave-on product, and once a week is a reasonable place to start. Daily use of a strong acid is how the skin barrier gets worn down, and the early signs are easy to mistake for progress: a tight, smooth, slightly shiny face that stings when anything else goes on. Over-exfoliated skin covers what that looks like and how long the recovery takes.
Acids in a cleanser are a different matter, since the contact time is under a minute and most of it rinses away. Those are closer to a mild daily option than to a treatment.
Skin cycling is one structured way to space actives out, though the underlying point does not need a system: if skin is stinging, stop.
The sun problem
Both families increase sensitivity to ultraviolet light, alpha hydroxy acids more than salicylic acid. That sensitivity outlasts the application, so daily sun protection is not optional alongside them. Using an acid for pigmentation without sunscreen is a reliable way to end up with more pigmentation than you started with, and it happens often enough that it is worth stating plainly.
For anyone in India using acids through the post-monsoon and winter months, when the sun feels weaker and the ultraviolet is not, which SPF is enough for Indian skin is the relevant read.
What to avoid stacking
An acid, a retinoid and a strong vitamin C on the same evening is three routes to the same irritation. Pick one per night, and leave a night between.
Acids and physical scrubs on the same day is the other common mistake. One is enough.
The ingredient pairing checker covers the combinations Cham has written about, including which acid rows come with the note that Cham makes no such product.
That note applies here too. Cham makes no exfoliating acid, and nothing in the range competes with one. If an acid is working for you, niacinamide alongside it is the sensible companion rather than a replacement.
