AHAs, or alpha hydroxy acids (Alpha Hydroxy Acids), are organic acids widely used in dermatology and cosmetics to promote skin exfoliation, improve skin texture, and reduce certain complexion irregularities. Present in various fruits, milk, or sugar cane, they are also produced or reproduced synthetically to meet the needs of cosmetic formulations.
Their value lies primarily in their ability to facilitate the removal of dead cells accumulated on the surface of the epidermis. Depending on the molecule used, its concentration, the product's pH, and the frequency of use, AHAs can also help improve the appearance of fine lines, dark spots, and certain marks left by acne.
But not all AHAs have the same properties, and their efficacy depends as much on their formulation as on the sensitivity of the skin. Understanding how they work makes it easier to distinguish their real benefits, their limitations, and the precautions necessary for their use.
1. What are AHAs and how do they work?
AHAs are a family of molecules characterized by the presence of a hydroxyl group located on the carbon adjacent to their carboxyl group. This chemical structure gives them specific properties, particularly their ability to act on the cohesion of the skin's superficial cells.
In cosmetics, they are mainly used as water-soluble exfoliating agents intended to improve the appearance and quality of the skin surface.
Some AHAs are naturally present in plants or fermented products, while others are manufactured industrially. Their origin alone does not determine their efficacy or tolerance: it is notably the molecule, the concentration, the pH, and the final formulation that influence their action.
The mechanism of action on the epidermis
The outermost layer of the epidermis, called the stratum corneum, is made up of dead cells—corneocytes—held together by a matrix rich in lipids and proteins, including structures involved in their cohesion (desmosomes).
During skin renewal, these cells gradually detach. When their elimination becomes less uniform, the skin may appear dull, rough, or uneven.
AHAs promote desquamation by modifying the interactions that ensure cohesion between cells in the stratum corneum. This action facilitates the elimination of surface cells and makes the skin texture more regular.
At appropriate concentrations and in suitable formulations, some AHAs can also influence biological mechanisms within the epidermis and dermis, particularly those involved in skin renewal and the synthesis of extracellular matrix components (such as collagen). These effects depend, however, on the molecule and the conditions of use; they should not be equated with deep or systematic skin regeneration.
2. What are the properties and benefits of AHAs?
Alpha hydroxy acids have several interesting properties in dermatology and cosmetics. Their primary effect is exfoliating, but their applications extend to various skin concerns.
Exfoliating the skin and refining its texture
Chemical exfoliation is the most well-established effect of AHAs. By facilitating the detachment of dead cells, they help make the skin surface smoother and more uniform. Appropriate use can improve the appearance of rough skin, reduce the look of an uneven skin texture, and promote a visually more luminous complexion.
Unlike mechanical exfoliation, which relies on the physical action of scrubbing, exfoliation via AHAs relies on their chemical interaction with the structures of the stratum corneum.
Improving complexion radiance
The accumulation of dead cells can accentuate a dull complexion. By facilitating their removal, AHAs promote a more homogeneous skin surface, which improves light reflection and gives the skin a more luminous appearance. This effect primarily concerns the appearance of the skin's surface. It does not necessarily mean that AHAs increase overall skin hydration or correct all causes of a dull complexion.
Reducing certain dark spots
AHAs can help reduce the appearance of certain superficial pigment irregularities, including post-inflammatory marks left by breakouts and certain spots related to photo-aging. Their exfoliating action facilitates the elimination of pigmented cells present in the superficial layers of the epidermis.
Their efficacy remains variable, however. Deep spots, melasma, and persistent hyperpigmentation require appropriate treatment, as excessive irritation can itself aggravate certain pigmentations, particularly on darker skin (higher phototypes).
Smoothing the appearance of fine lines
With age and sun exposure, the skin can develop fine lines, a less regular texture, and a gradual decrease in certain structural components of the dermis.
Some AHAs, notably glycolic acid, have been studied for their effects on the epidermis and on markers associated with extracellular matrix renewal. Appropriate use can improve the appearance of superficial fine lines and certain signs of photo-aging. These effects are generally gradual and depend on concentration, duration of use, and individual tolerance. AHAs do not replace dermatological treatments for deep wrinkles.
Improving the texture of blemish-prone skin
By promoting the elimination of dead cells, AHAs can help reduce the rough appearance of skin prone to imperfections. They are useful for improving the appearance of surface marks and comedones, but their action on acne is less directly targeted than that of other active ingredients, notably salicylic acid, a lipophilic beta-hydroxy acid (BHA) that penetrates more easily into sebum-clogged pores.
Do AHAs hydrate the skin?
Some AHAs, notably lactic acid, exhibit humectant properties: they can help attract and retain water in the stratum corneum, depending on their concentration and the formulation used.
However, this effect must be distinguished from their exfoliating action. A high concentration, a very acidic pH, or too frequent use can alter the skin barrier, cause irritation, and lead to a feeling of dryness. AHAs are therefore not systematically hydrating under all conditions of use.
3. What are the main types of AHAs?
All alpha hydroxy acids belong to the same chemical family, but they differ in their molecular structure, size, properties, and use in cosmetics.
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Glycolic acid (the gold standard exfoliant): Its very small molecular size facilitates its penetration into the stratum corneum, which contributes to its high exfoliating efficacy on texture, radiance, and fine lines. This penetration capacity is, however, accompanied by a higher risk of irritation if the pH is very acidic or the concentration is high.
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Lactic acid (exfoliation and hydration): Naturally present in certain biological processes and obtained through fermentation, it promotes exfoliation while possessing marked humectant properties. It is ideal for improving skin softness and treating surface dryness.
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Mandelic acid (large molecule): Derived from bitter almonds, its bulkier molecular structure slows its skin penetration. It is used to work on skin texture and surface imperfections more gradually.
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Malic acid (fruit-derived): Naturally present in apples, it has acidifying and exfoliating properties. It is often integrated into complexes combining several AHAs to potentiate the formula.
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Tartaric acid (grape-derived): Used as a mild exfoliating agent or pH regulator, its role depends heavily on the overall formulation.
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Citric acid (exfoliation, antioxidant, and pH regulator): Found in citrus fruits, it is very frequently used to adjust the pH of cosmetics. At appropriate concentrations, it offers an exfoliating action while acting as an antioxidant and chelator (neutralizing heavy metals in the formula). Its presence alone in the INCI does not, however, guarantee an active exfoliating effect if it is used only as an acidity adjuster.
4. Comparison table of main AHAs
| AHA | Common properties and uses | Caution |
| Glycolic acid | Intense exfoliation, skin texture, radiance, superficial fine lines | Deep penetration, higher risk of irritation |
| Lactic acid | Gentle exfoliation, softness, humectant properties | Tolerance depends on pH and concentration |
| Mandelic acid | Gradual exfoliation, imperfections, surface spots | Heavier molecule, less abundant clinical data |
| Malic acid | Exfoliation in combination with other acids | Individual effect difficult to isolate |
| Tartaric acid | Gentle exfoliation and formula pH adjustment | Efficacy varies according to overall concentration |
| Citric acid | pH regulation, antioxidant action, and exfoliation | Often present only to stabilize pH |
5. AHAs, BHAs, and PHAs: what are the differences?
To choose the right exfoliating acid, it is important to distinguish the three main families of hydroxy acids used in cosmetics:
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AHAs (Alpha Hydroxy Acids): Water-soluble, they act primarily on the surface of the stratum corneum. They are preferred for treating a dull complexion, surface dehydration, fine lines, and dark spots.
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BHAs (Beta Hydroxy Acids): Represented mainly by salicylic acid, BHAs are lipophilic (fat-soluble). They penetrate deep into the pores to dissolve sebum and are particularly suited to oily skin, prone to blackheads and acne.
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PHAs (Poly Hydroxy Acids): Such as gluconolactone or lactobionic acid, PHAs have a very large molecular structure that limits deep penetration. They offer gentle surface exfoliation, ideal for very sensitive skin or skin prone to rosacea.
6. How to use AHAs safely?
The efficacy and safety of an AHA treatment rely on a strict balance between acid concentration, product pH, contact time, and frequency of application. A higher percentage does not guarantee better results and increases the risk of weakening the skin barrier.
Application recommendations
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Gradual approach: Start with one or two applications per week, then increase gradually if the skin tolerates it well.
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Listen to your skin: Persistent redness, intense burning sensations, or excessive peeling require stopping the product.
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Mandatory photoprotection (SPF): AHAs thin the stratum corneum and increase skin sensitivity to UV rays. Daily application of a broad-spectrum sunscreen (SPF 30 to 50) is essential throughout the duration of the treatment and the following days.
Can AHAs be combined with other active ingredients?
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AHAs and Retinol / BHAs: Avoid layering these powerful active ingredients during the same application to avoid chemical burns or inflammation. Alternate their use (e.g., AHA one evening, Retinol the following evening).
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AHAs and Vitamin C: Pure L-ascorbic acid at a low pH can become irritating if combined directly with AHAs. Ideally, use Vitamin C in the morning and AHAs in the evening.
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AHAs and Hyaluronic Acid (The ideal combination): Hyaluronic acid has no exfoliating or acidifying action; it is a pure humectant. Combining it with AHAs helps immediately rehydrate the epidermis, soothe tension, and support the skin barrier. Combine the Polaire Night serum with AHAs with a Prunelle serum or Jardin d'hiver serum containing hyaluronic acid.
7. Are AHAs suitable for all skin types?
AHAs address a wide variety of skin needs, but are not mandatory for maintaining healthy skin:
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Normal to dry skin: Prefer lactic acid for its dual exfoliating and hydrating effect.
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Combination to mature skin: Glycolic acid is ideal for smoothing fine lines and boosting radiance.
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Reactive or sensitive skin: Mandelic acid or a transition to PHAs will be more suitable to avoid heating sensations.
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Dark skin tones (Phototypes IV to VI): Particular caution is required. Overly aggressive exfoliation can trigger post-inflammatory hyperpigmentation (the appearance of new dark spots).
8. What does scientific research say about their effectiveness?
AHAs are among the cosmetic active ingredients with the best-documented clinical efficacy. Scientific literature confirms their ability to accelerate cell renewal and correct uneven skin tone.
However, studies highlight that actual effectiveness depends on the amount of free acid available, which is directly governed by the formula's pH. A product stating 10% glycolic acid but formulated at a pH above 4.5 will have a significantly lower exfoliating action than a 5% concentrated formula with a pH of 3.5.
9. Buying guide: how to choose your AHA treatment?
To choose a suitable treatment, first identify your main need rather than focusing solely on a percentage:
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To revive dull skin: Opt for a glycolic acid lotion (3% to 7%).
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To smooth dry and rough skin: Choose a lactic acid serum (5% to 10%).
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To correct the texture of sensitive skin: Prioritize mandelic acid or a gentle complex.
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To act on clogged pores: Turn instead to a BHA (salicylic acid).
The Niuk Nuit Polaire serum contains glycolic acid, lactic acid, mandelic acid, and citric acid.
Frequently Asked Questions
What is the best time to apply AHAs?
It is best to apply AHAs in the evening on clean, dry skin, followed by a moisturizer (such as a hyaluronic acid serum). Using them at night limits the risks of reactions linked to immediate sun exposure.
Should a product containing AHAs be rinsed off?
Daily lotions, toners, and serums do not need to be rinsed. On the other hand, highly concentrated peel masks (more than 15% acids) must be rinsed off after the contact time indicated by the manufacturer.
How long does it take to see results from AHAs?
The effect on skin radiance and smoothness is often visible from the first applications. For action on fading dark spots and smoothing fine lines, regular use for 4 to 8 weeks is generally necessary.
Medical reference sources:
- Food and Drug Administration (FDA) Acids: mechanism of exfoliation, safety, and UV sensitivity.
- DermNet - Alpha hydroxy acid facial treatments: types of AHAs, dermatological uses, and precautions.
- Journal der Deutschen Dermatologischen Gesellschaft - Cosmetic and dermatologic use of alpha hydroxy: summary of cosmetic and dermatological applications of AHAs.
