What makes an acid an AHA
Alpha hydroxy acids are a group of water-soluble acids, many of which occur naturally in foods or are made in labs for use in cosmetics. They share a similar chemical structure, which is where the “alpha hydroxy” name comes from. In skincare they are used mainly for one purpose: loosening dead cells on the surface so skin looks smoother and brighter.
Because AHAs dissolve in water rather than oil, they mostly act on the surface rather than inside pores. That is the main practical difference from salicylic acid, which suits oily, clogged skin better.
The common members of the family
Glycolic acid
Glycolic acid has the smallest molecule in the group and is generally described as the strongest and most irritating of the common AHAs. It has its own profile in our article on glycolic acid, so here it is enough to say it is the usual benchmark that others get compared to.
Lactic acid
Lactic acid is a larger molecule than glycolic acid and is typically regarded as gentler. It is also commonly said to have some moisturizing effect, and it often shows up in body products aimed at rough or bumpy skin. People with dry skin often find it easier to tolerate than glycolic acid, though this is a tendency, not a guarantee.
Mandelic acid
Mandelic acid has a larger molecule again, so it tends to work more slowly and be milder. It is often chosen by people who want to try an AHA but are cautious about irritation, and it is also sometimes used for uneven tone. Its slower action is the reason it is usually seen as a gentle starting point.
Others you may see
Citric acid, malic acid and tartaric acid also appear on labels. In many products they are present mainly to adjust acidity rather than to exfoliate, and their effect varies with the formula. You do not need to memorize them. If a label lists one low down the ingredient list, it is unlikely to be doing much exfoliating.
Size, strength and irritation
As a rough rule, a smaller molecule can move into skin faster, so it can feel more potent. This is why glycolic acid usually feels stronger than lactic or mandelic acid at similar concentrations. But strength depends on more than the molecule:
- the concentration on the label
- the acidity of the finished product
- whether it is a leave-on or a rinse-off
- your own skin, which may be thicker, thinner, or already irritated
If you are curious why acidity matters, our article on skin pH goes through it. For now, keep in mind that a product’s label alone does not tell you how it will feel.
What AHAs are typically used for
AHAs are commonly used to help with:
- dull-looking skin
- rough, flaky texture
- uneven tone, including marks left after breakouts
- the look of fine surface lines, in a limited and gradual way
They are not a treatment for skin conditions, and they will not remove deep wrinkles, scars or deep pigment. If marks bother you, hyperpigmentation is a bigger topic, with sun protection at the center of it.
Sun sensitivity
This is the point that deserves the most attention. AHAs remove some of the outermost layer of skin, and regulators and dermatology bodies have noted that AHA products can make skin more easily sunburned while you use them and for a period afterward. The effect is not unique to one acid, but it is a good reason to be careful.
In practical terms:
- Use daily sunscreen during the day, every day, while using AHAs. Our sunscreen basics article covers how to choose one.
- Apply enough of it. Many people under-apply, which reduces protection.
- Many people prefer to use leave-on AHAs at night.
- Hats, shade, and protective clothing add to your sunscreen, especially on strong-sun days.
Note that an AHA does not make you burn instantly, and sensitivity is not the same for everyone. It is simply sensible to treat the skin as more vulnerable.
Starting out
If you are new to AHAs, a few cautious habits help:
- Patch test on a small area first.
- Start with one product and one or two nights a week.
- Choose a gentler acid such as lactic or mandelic if your skin is dry or easily irritated.
- Avoid combining it the same night with other strong actives like retinol products until you know how your skin responds.
- Stop if you get lasting burning, swelling, blistering, or a rash.
If irritation does appear, back off and focus on moisture and protection until the skin settles. Our article on skin over-exfoliation describes signs that you have done too much.
Who should be cautious
People with very sensitive skin, eczema, rosacea or broken skin should check with a health professional before starting AHAs. Pregnant or breastfeeding people may also want to ask a doctor or pharmacist about specific ingredients and concentrations, since advice varies. Any persistent, painful, or changing skin problem is a reason to see a qualified professional rather than adding more products.