Alpha hydroxy acids, abbreviated as AHA, are naturally derived exfoliating acids that loosen the bonds between dead cells in the stratum corneum and promote their turnover. The abbreviation does not refer to a single ingredient: it represents a family of molecules that behave very differently from one another, and choosing glycolic acid instead of mandelic acid is not a minor detail for formulators.
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If you're looking for the difference between AHA, BHA and PHA — that is, between the three major exfoliant families — you'll find it in the guide to exfoliating acids. This page focuses on AHAs: what they are, which ones exist and which is best for each skin type.
What are alpha hydroxy acids?
Alpha hydroxy acids are organic acids characterized by a hydroxyl group in the alpha position relative to the carboxyl group: hence the name. They have traditionally been obtained from food sources — sugar cane, fermented milk, fruit, almonds — although in cosmetics they are now almost always used in purified or synthetic form, which is much more stable and safer.
On the skin, they work by weakening desmosomes, the adhesion points between corneocytes in the outermost layer. The result is faster cell turnover: skin texture becomes smoother, the complexion appears more even, and creams applied afterward encounter fewer obstacles. With continued use, studies also document an indirect stimulation of epidermal renewal.
In the ingredient list, AHAs appear under their INCI names: Glycolic Acid, Lactic Acid, Mandelic Acid, Malic Acid, Tartaric Acid, Citric Acid. Whether written as alpha hydroxy acids or alphahydroxyacids, as one word, they refer to the same family.
Which AHA to choose: molecular weight determines everything
Alpha hydroxy acids share the same chemical structure and basic action. What distinguishes them is the molecule size, and everything else follows from that.
A small molecule crosses the stratum corneum quickly and reaches deeper layers: it acts more strongly and causes more irritation. A large molecule remains closer to the surface and works gradually: slower results, but much better tolerated.
| Acid | Molecular weight | Penetration | Who it is for |
|---|---|---|---|
| Glycolic | 76 Da | High | Thick, resilient skin with uneven texture |
| Lactic | 90 Da | Medium | Dry or dehydrated skin |
| Malic | 134 Da | Medium-low | Mixed formulas, rarely used alone |
| Tartaric | 150 Da | Low | Stabilizer for other acids |
| Mandelic | 152 Da | Low | Reactive skin, darker phototypes, first-time users |
The scale is not a ranking of quality. Glycolic acid is not better than mandelic acid: it is more aggressive, which is an advantage for resilient skin and a problem for reactive skin.
Glycolic acid: the most penetrating
It is derived from sugar cane and is the smallest of the AHAs. It is also the most extensively studied and has the best evidence for photoaging: fine lines, uneven texture, and dull complexion.
The trade-off is tolerability. On sensitive skin, or for those who have never used chemical exfoliants, glycolic acid can easily cause redness and stinging. And on darker phototypes, it should be used with particular caution: poorly managed inflammation can leave post-inflammatory hyperpigmentation, exactly the opposite of the desired result.
Percentages, pH, and how to use it are explained in the guide dedicated to glycolic acid.
Lactic acid: the one that hydrates while exfoliating
It is derived from fermentation and is the only AHA that is part of the skin's natural moisturizing factor: the molecule is already present in the stratum corneum, where it retains water.
This makes it the most sensible choice when the skin is dry or dehydrated but still needs renewal. Glycolic acid can worsen the skin barrier on dry skin; lactic acid exfoliates while also replenishing moisture. If you are unsure which of the two conditions applies to you, the distinction is explained in the in-depth guide on dry and dehydrated skin.
Mandelic acid: the most tolerable
At 152 Daltons, it is the largest molecule in the family and is derived from bitter almonds. It penetrates slowly and evenly, which is why it is the AHA recommended for beginners, people with reactive skin, and those with darker phototypes.
It also has a characteristic that the other AHAs do not: being partially lipophilic, it partly resembles the behavior of BHAs and has an effect on blemish-prone skin that glycolic acid does not replicate. The complete in-depth article is dedicated to mandelic acid, along with its ingredient profile.
Citric, tartaric and malic acids: the AHAs that often do not exfoliate
This is the part that almost no one explains, and it is essential for reading a label without being misled.
Citric acid and tartaric acid appear in many cosmetics, but almost always at very low concentrations and with a technical function: adjusting the formula’s pH. Seeing them at the end of an INCI list does not mean that the product is exfoliating. Malic acid is found mainly in blends, where it accompanies the others rather than acting on its own.
Rule of thumb: if an AHA appears after the preservatives in the ingredient list, its concentration is almost certainly technical rather than functional.
Concentration and pH: why percentage alone says nothing
In cosmetics for home use, AHAs are used up to approximately 10%, with a pH no lower than 3.5. Higher concentrations are intended for professional treatments and must be performed by qualified personnel.
But percentage alone is an incomplete measure. An acid works in its free form, and the proportion of molecules in free form depends on the formula’s pH: the same 8% glycolic acid at pH 3.5 and at pH 5 are two different products. This is why one serum can perform better than another with a higher stated percentage. We have a dedicated in-depth article on skin pH.
Serum, cream or peel: the forms in which AHAs are found
The same acid performs differently depending on the vehicle in which it is formulated, which explains why two products with the same percentage on the label can deliver very different results.
| Form | Contact time | What it is used for |
|---|---|---|
| Serum | Prolonged, remains on the skin | Gradual action, daily or every other day |
| Cream | Prolonged, with a richer vehicle | Moderate concentrations, good tolerability |
| Rinse-off peel | Short, from 1 to 10 minutes | Higher concentrations with controlled exposure |
| Toner or solution | Short, no rinse | Skin preparation, gentle effect |
One more thing should be said about pure active ingredients: powdered or concentrated-solution alpha hydroxy acids sold loose are laboratory raw materials. Diluting them at home without controlling the final pH can easily result either in an ineffective product or a chemical burn. Professional formulation exists precisely to manage that margin.
Alpha hydroxy acid peels: at home and in the clinic
The term peeling covers two worlds that are best kept distinct.
At-home peels use up to 10% acid at a pH no lower than 3.5: they act on the stratum corneum, are used once or twice a week, and results become apparent over several weeks. This is the realm of cosmetics, where you can proceed independently.
Professional peels use much higher concentrations and lower pH levels, reach deeper layers, and require a preliminary skin assessment. They must be performed by a doctor or qualified professional, who also manages the recovery process. If you have an ongoing skin condition - active acne, rosacea, extensive melasma - choosing between the two approaches is a decision for a dermatologist.
There is also a third option, the enzyme peel, which exfoliates using proteolytic enzymes rather than acidity. It is gentler and, because it does not increase photosensitivity, remains suitable even during months of intense sun exposure.
AHA in LeLang formulas
We chose mandelic acid as our reference AHA precisely for the reason described above: its large molecule makes it suitable for use on its own, without the margin for error associated with glycolic acid.
In the enzyme peel, it works together with papain and bromelain: two different mechanisms - acidic and enzymatic - that achieve the result with lower concentrations of each.
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Whichever AHA you choose, daily sun protection should always accompany the treatment: alpha hydroxy acids increase sun sensitivity, and freshly exfoliated skin is more likely to develop dark spots.
Frequently asked questions about AHAs
What does the abbreviation AHA mean?
AHA stands for alpha hydroxy acids, called alfa idrossiacidi in Italian. It refers to a family of water-soluble exfoliating acids - glycolic, lactic, mandelic, malic, tartaric, and citric - that act on the skin's most superficial layer. The parallel acronym BHA stands for beta hydroxy acids; the best-known is salicylic acid, which, being oil-soluble, also penetrates the pore.
Is glycolic acid or mandelic acid better?
It depends on the skin, not the acid. Glycolic acid delivers more on thick skin accustomed to exfoliants; mandelic acid is the right choice for reactive skin, darker skin tones, and beginners. An irritating glycolic acid is less effective than a well-tolerated mandelic acid, because you stop using the former.
Can you use two different AHAs together?
Professional formulas often combine them specifically to take advantage of different molecular weights. Layering two different products at home is another matter: it adds up the irritation without any control and is the quickest way to compromise the skin barrier.
How often should you use an AHA?
Start once or twice a week, in the evening, and increase frequency only if your skin responds well. Brief tingling upon application is normal; redness that persists the next day, tight-feeling skin, or burning when you apply moisturizer means you are overdoing it, either in frequency or concentration.
Are AHAs suitable for summer?
Yes, but only with sunscreen applied every single day and reapplied. If you know you will not do that, in summer it is better to switch to enzymatic exfoliation, which does not increase photosensitivity.
Can AHAs be used during pregnancy?
Topical alpha hydroxy acids at cosmetic concentrations are among the exfoliants considered compatible with pregnancy, along with PHAs: systemic absorption is minimal. Professional high-concentration peels and retinoids, on the other hand, are treatments best postponed. It is still worth confirming this with your gynecologist or dermatologist.
Can people allergic to almonds use mandelic acid?
Mandelic acid used in cosmetics is a purified molecule that is generally synthetic, not an almond extract, so it does not contain the proteins that trigger an allergic reaction. Anyone with a confirmed allergy should still discuss it with their doctor.
Do AHAs thin the skin?
It is a widespread misconception. Proper use thins the stratum corneum - the layer made up of already-dead cells - while studies suggest that the living epidermis actually tends to become thicker. True thinning results from overuse, which is an entirely different matter.
