What They Are
AHA stands for alpha hydroxy acids. It is a group of water-soluble acids obtained most often from fruit, milk or sugarcane. What unites them is the way they act on the skin surface.
Dead cells on the surface hold onto one another through bonds that normally break down on their own. With age and in some skin types that process slows and cells linger. AHAs loosen those bonds and help the cells shed.
Why water-soluble matters
Because they do not mix with oils, AHAs work mainly on the surface and barely enter pores. So for congested pores, salicylic acid, which is oil-soluble, usually does a better job.
Which Acid for Which Skin
Glycolic acid has the smallest molecule and penetrates fastest. It gives the quickest result but also irritates most often. It suits more robust skin with pronounced uneven texture.
Lactic acid is larger, acts more gently and hydrates at the same time, which makes it convenient for dry skin. Mandelic acid has the largest molecule and is the gentlest of the three - a sensible choice for sensitive skin and for beginners.
Concentration and pH
Home-use products usually contain between five and ten percent acid. That is the range where the result is visible while the risk stays manageable without a professional.
Concentration means nothing without pH, though. Acids work in an acidic environment, usually between 3 and 4. A high-concentration product at neutral pH will do little. Comparing products by the percentage on the box alone is therefore misleading.
Salon peels are a different matter
Professional peels use considerably higher concentrations at lower pH and are applied by a trained specialist who controls contact time and neutralises the acid. This is not a procedure for home and should not be imitated with consumer products.
How to Use Them
In the evening, on clean dry skin, twice a week to begin with. If after three weeks skin is calm, move to every other evening. Daily use is possible at low concentrations but is not a goal to aim for.
Do not combine them on the same evening with retinol or another acid. Alternate, and leave evenings for restorative care only.
Sunscreen Is Not Optional
AHAs make skin considerably more sun-sensitive, and for some time after use. Without daily protection you risk more pigmentation than you started with. Why that happens is explained in our article on sun damage.
Signs You Are Overdoing It
Shiny, taut skin, stinging when you apply products you normally tolerate, persistent redness, flaking that does not stop. That is a compromised barrier, not deeper cleansing.
If you recognise these signs, stop acids for one or two weeks and stick to cleansing, moisturising and protection. Return at a lower frequency than the one that got you here.
When You Will See a Result
Smoother skin often within two or three applications. More even tone and fading of post-blemish marks takes two to three months. Deep lines are outside what acids address, whatever the concentration.
Toner, Serum or Rinse-Off Peel
Toners hold low concentrations and tolerate more frequent use, but act more superficially. Serums stay on the skin and are stronger, so they need more careful introduction.
Rinse-off peels have brief contact with the skin and are a convenient option if you react to anything left on overnight. Starting with a rinse-off format is a sensible way to learn how your skin takes acids.
Which Season
Acids can be used year-round, but summer demands more discipline with sun protection. If you spend long periods outdoors, reduce frequency during the strongest months rather than stopping altogether.
In winter skin is often drier and the barrier under more strain, so the same frequency can turn out to be too much. How to adjust a routine by season is covered in our seasonal skincare guide.
Who Should Avoid Them
Skin with broken areas, burns or active irritation. Skin immediately after a salon procedure, unless your specialist has said otherwise. If you have a diagnosed skin condition, whether to use acids is a dermatologist decision.
With very sensitive skin you do not necessarily have to give up, but start with the gentlest acid and the lowest frequency. Once a week is still use, and it still produces a result over time.
