What Are Alpha Hydroxy Acids? The Complete AHA Guide

Alpha hydroxy acids are a family of water-soluble organic acids used in skincare for exfoliation, anti-aging, and hyperpigmentation treatment. They occur naturally in foods like sugarcane, milk, almonds, grapes, citrus fruits, and apples. The name "alpha hydroxy" refers to their shared chemical structure: a hydroxyl group (-OH) attached to the alpha carbon, the carbon atom directly adjacent to the carboxylic acid group. The simplest and smallest member of this family is glycolic acid, also known by its systematic chemistry name alpha hydroxy acetic acid, which is derived from sugarcane and has become the most widely studied AHA in dermatology [1].

What Are Alpha Hydroxy Acids?

Alpha hydroxy acids are organic compounds defined by two functional groups: a carboxylic acid (-COOH) and a hydroxyl group (-OH) bonded to the carbon immediately next to it (the alpha position). This structural motif is what gives AHAs their skin-active properties. The hydroxyl group makes the molecule water-soluble, while the acidic proton at low pH allows the molecule to interact with and disrupt protein structures in the outermost layer of skin [2].

The AHA family has deep roots in traditional skincare, even if the chemistry was not understood at the time. Cleopatra reportedly bathed in sour milk (lactic acid). French women in the 18th century applied old wine (tartaric acid) to their faces. But the modern scientific foundation for AHA skincare begins with dermatologists Eugene Van Scott and Ruey Yu, who published their landmark research in 1974 demonstrating that topical application of alpha hydroxy acids could normalize keratinization in patients with ichthyosis, a genetic skin-scaling disorder [3]. By the 1990s, AHAs had moved from the dermatologist's office into mainstream consumer skincare, and glycolic acid had emerged as the dominant member of the family.

Today, hydroxy acids are among the most common active ingredients in skincare products worldwide. They appear in cleansers, toners, serums, masks, and professional chemical peels. Understanding the differences between individual AHAs — their molecular weights, penetration profiles, and secondary properties — is essential for choosing the right acid for your skin.

Natural Sources of AHAs

Each alpha hydroxy acid is named for and historically associated with a natural source:

• Glycolic acid — sugarcane, beets, unripe grapes
• Lactic acid — fermented milk, yogurt
• Mandelic acid — bitter almonds
• Malic acid — apples, pears
• Tartaric acid — grapes, wine
• Citric acid — lemons, oranges, grapefruit

Most cosmetic-grade AHAs are now produced synthetically to ensure consistent purity, concentration, and pH. The natural source tells you where the acid was first isolated, not necessarily where your skincare product's acid comes from.

How Do Alpha Hydroxy Acids Work?

All alpha hydroxy acids work through the same fundamental mechanism: they disrupt corneodesmosomes, the protein structures that hold dead skin cells (corneocytes) together in the stratum corneum, the outermost layer of skin [4]. This process is called corneodesmolysis.

The Stratum Corneum and Desquamation

Your skin continuously produces new cells in the basal layer of the epidermis. These cells migrate upward over approximately 28 days (longer with age), flattening and dying as they go, until they reach the stratum corneum — a stack of about 15 to 20 layers of dead, protein-rich cells held together by desmosomal junctions. Under normal conditions, enzymes gradually break down these junctions from the surface, and dead cells shed naturally. This shedding process is called desquamation.

With age, sun damage, and certain skin conditions, desquamation slows. Dead cells accumulate, leading to dullness, rough texture, uneven tone, and clogged pores.

How AHA Chemical Exfoliation Works

When applied at low pH, an alpha hydroxy acid in its free (protonated, uncharged) form penetrates the intercellular spaces of the stratum corneum and weakens desmosomal bonds. This accelerates the shedding of dead cells — chemical exfoliation in the most literal sense. Fartasch, Teal, and Menon (1997) used electron microscopy to confirm that glycolic acid causes targeted desmosomal breakdown in the outermost dead cell layers while leaving the deeper, living layers intact and the skin barrier undamaged [5].

The depth and speed of this exfoliation depend on three factors:

1. Molecular weight — smaller AHAs penetrate deeper and faster
2. pH and free acid value — lower pH means more of the acid exists in its active, uncharged form (see our pH science guide for a full explanation)
3. Concentration and contact time — more acid for longer produces more exfoliation

Beyond Surface Exfoliation

At sufficient concentrations, some AHAs — particularly glycolic acid — penetrate deep enough to stimulate biological activity in the living epidermis and dermis. This includes increased keratinocyte proliferation (signaling the skin to produce new cells faster) and fibroblast stimulation, which leads to increased production of collagen and hyaluronic acid [6]. These deeper effects are what give AHAs their anti-aging properties, not just their exfoliating ones.

The AHA Family: A Complete List

Not all alpha hydroxy acids are created equal. The single most important variable that distinguishes one AHA from another is molecular weight — the physical size of the molecule, measured in daltons (Da). Smaller molecules slip more easily through the tightly packed dead cell layers of the stratum corneum, penetrating deeper and faster. Van Scott and Yu's foundational research established that molecular size directly correlates with the depth and speed of keratolytic effects [3].

Here is the complete list of alpha hydroxy acids used in skincare, ranked from smallest to largest:

| Acid | Molecular Weight | Natural Source | Penetration | Primary Skincare Use | |------|-----------------|----------------|-------------|---------------------| | Glycolic acid | 76.05 Da | Sugarcane | Deepest / fastest | Texture, anti-aging, hyperpigmentation | | Lactic acid | 90.08 Da | Milk | Deep / moderate | Dry skin, hydration, gentle exfoliation | | Malic acid | 134.09 Da | Apples | Moderate | Multi-acid blends, pH adjustment | | Tartaric acid | 150.09 Da | Grapes | Moderate / shallow | pH adjustment, antioxidant support | | Mandelic acid | 152.15 Da | Bitter almonds | Shallow / gentle | Sensitive skin, acne, melanin-rich skin | | Citric acid | 192.12 Da | Citrus fruits | Shallow | pH adjustment, antioxidant |

Glycolic Acid (76.05 Da)

Glycolic acid is the smallest and most potent alpha hydroxy acid. Its low molecular weight gives it the deepest penetration of any AHA, making it the most effective for resurfacing, collagen stimulation, and hyperpigmentation treatment. It is also the most extensively researched, with decades of clinical trials supporting its use. The trade-off is a higher risk of irritation compared to gentler alternatives. Glycolic acid — formally named alpha hydroxy acetic acid in chemistry — remains the gold standard AHA for anti-aging and texture correction. Read our complete guide to glycolic acid for an in-depth look.

Lactic Acid (90.08 Da)

Lactic acid is the second most popular AHA, derived historically from fermented milk. It is about 18% larger than glycolic acid, which results in slightly shallower penetration and lower irritation potential. Its defining feature is its humectant properties — lactic acid draws moisture into the skin, making it the AHA of choice for people with dry or dehydrated skin. Clinical studies have shown it is effective for photodamage, though glycolic acid holds a slight edge in head-to-head comparisons [7]. See our glycolic acid vs lactic acid comparison for details.

Malic Acid (134.09 Da)

Malic acid, found in apples, sits in the middle of the AHA molecular weight spectrum. It has significantly less clinical evidence than glycolic or lactic acid for standalone skincare use. Most products that include malic acid use it as part of multi-acid blends, where it contributes to the overall exfoliating effect alongside more established AHAs. Some researchers have noted mild moisturizing properties, though this has not been established in controlled clinical studies.

Tartaric Acid (150.09 Da)

Tartaric acid is derived from grapes and is a common component of wine-based skincare formulations. It is rarely used as a standalone exfoliant. Its primary role in cosmetic products is as a pH adjuster and buffering agent — helping maintain the low pH that other AHAs need to remain active. While it does have mild exfoliating properties, its larger molecular size makes it far less potent than glycolic or lactic acid for resurfacing.

Mandelic Acid (152.15 Da)

Mandelic acid, derived from bitter almonds, is roughly twice the molecular weight of glycolic acid, making it one of the gentlest AHAs. It has a unique property that sets it apart: partial lipophilicity. Most AHAs are purely water-soluble, but mandelic acid has a phenyl group that gives it partial oil solubility, allowing a degree of pore penetration that other AHAs lack [8]. This makes mandelic acid particularly useful for acne-prone skin. It is also widely recommended for Fitzpatrick skin types IV through VI, because its gentler penetration carries less risk of triggering post-inflammatory hyperpigmentation [9]. For a full comparison, see glycolic acid vs mandelic acid.

Citric Acid (192.12 Da)

Citric acid, from citrus fruits, is the largest commonly used AHA. At 192 Da, it penetrates the stratum corneum slowly and shallowly. Its main contribution to skincare formulations is not exfoliation but antioxidant activity — citric acid chelates (binds) metal ions that catalyze free radical formation [8]. Like tartaric acid, it appears most often as a pH adjuster and formulation ingredient rather than as a primary active exfoliant.

AHA vs BHA: What's the Difference?

Alpha hydroxy acids are not the only chemical exfoliants. Beta hydroxy acids (BHAs) — most notably salicylic acid — are the other major class. The distinction matters because AHAs and BHAs are suited for different skin concerns.

Water-soluble vs oil-soluble. AHAs are water-soluble, which means they work on the surface of the skin where water is present. BHAs (salicylic acid) are oil-soluble, which allows them to dissolve into the sebum inside pores and exfoliate from within [8].

Surface exfoliation vs pore penetration. Because AHAs stay on the surface, they are better for treating texture, dullness, fine lines, and superficial hyperpigmentation. BHAs penetrate into the pore lining, making them more effective for blackheads, whiteheads, and inflammatory acne.

When to use which. Use an AHA when your primary concerns are texture improvement, anti-aging, sun damage, or dry-skin exfoliation. Use a BHA (salicylic acid) when your primary concern is acne, oily skin, or clogged pores. Many people benefit from both — used on alternating days or in different areas of the face. Check our interaction checker to confirm your specific combination is safe.

For the full comparison, see glycolic acid vs salicylic acid.

Do AHAs Stimulate Collagen?

Yes — and glycolic acid has the strongest evidence for this effect among all AHAs. In a vehicle-controlled human study, Bernstein et al. (2001) applied 20% glycolic acid lotion to forearm skin twice daily for three months and found significant increases in type I collagen mRNA and hyaluronic acid content in both the epidermis and dermis [6]. Type I collagen is the primary structural protein responsible for skin firmness, and its degradation is a hallmark of aging.

Separate in vitro studies confirmed that glycolic acid directly stimulates fibroblast proliferation and collagen production in a dose-dependent manner [10] [11]. AHA collagen stimulation appears to work through two pathways: directly via fibroblasts in the dermis, and indirectly through cytokines released by keratinocytes in the epidermis [12].

This is a meaningful anti-aging benefit, but it requires consistent use over weeks to months. Short-contact products (cleansers, rinse-off masks) and low-concentration formulations are unlikely to deliver enough active acid deep enough to meaningfully affect collagen production. For the science behind this mechanism, see how glycolic acid works on skin.

Alpha Hydroxy Acid Side Effects

Alpha hydroxy acids are generally well-tolerated, but they are biologically active compounds that interact with the skin barrier. Side effects exist on a spectrum from expected and manageable to genuinely concerning.

Sun Sensitivity

The most important side effect of AHA use is increased photosensitivity. Research by Kaidbey et al. (2003) demonstrated that topical glycolic acid increases the skin's sensitivity to ultraviolet radiation by approximately 18%, measured as a reduction in the minimum erythemal dose (the UV dose needed to cause visible redness) [13]. This effect is reversible — it resolves within about one week of discontinuing AHA use — but it makes daily broad-spectrum sunscreen (SPF 30 or higher) mandatory for anyone using alpha hydroxy acids.

Irritation and Adjustment

Mild redness, stinging, tingling, and dryness during the first 2 to 4 weeks of AHA use are normal. These occur because the acid is disrupting desmosomal bonds and activating pH-sensitive ion channels (TRPV1) on keratinocytes [14]. For most people, these symptoms subside as the skin acclimates. If redness persists beyond 4 weeks, or if you experience peeling, cracking, or a raw sensation, reduce the frequency or switch to a gentler AHA.

Purging

AHAs can cause a temporary increase in breakouts during the first 2 to 6 weeks of use. This "purge" happens because accelerated cell turnover pushes pre-existing clogged pores (microcomedones) to the surface faster than they would have emerged on their own. Purging resolves on its own. If new breakouts continue past 6 to 8 weeks, the product may not be right for your skin.

Regulatory Safety Limits

Both US and EU regulatory bodies have set concentration and pH limits for alpha hydroxy acids in consumer products:

| Standard | Max AHA Concentration | Minimum pH | |----------|----------------------|------------| | CIR (US) | 10% | 3.5 | | SCCS (EU) | 4% | 3.8 |

Products exceeding these limits are restricted to professional use [1] [15]. These limits apply to leave-on consumer products — rinse-off products and professional peels operate under different guidelines.

For a detailed breakdown of glycolic acid side effects specifically, including management strategies, see our side effects and safety guide.

Are Alpha Hydroxy Acids Safe?

Yes — alpha hydroxy acids have a strong safety record when used within established guidelines. The Cosmetic Ingredient Review (CIR) panel, which evaluates ingredient safety for the US market, concluded that glycolic acid and lactic acid are safe for consumer use at concentrations up to 10% with a final product pH of 3.5 or above [1]. The European Scientific Committee on Consumer Safety (SCCS) applies stricter limits: 4% maximum concentration at pH 3.8 or above for leave-on products [15].

Key Safety Guidelines

• Concentration limits. Stick to products at or below 10% AHA for unsupervised home use. Professional peels (20% to 70%) should only be administered by licensed practitioners.
• pH requirements. Effective AHA products must be formulated at low pH (typically 3.0 to 4.0), but extremely low pH (below 3.0) increases irritation risk without proportional benefit for most people. Our concentration guide covers how to evaluate product strength.
• Sunscreen is non-negotiable. AHAs increase UV sensitivity. Daily broad-spectrum SPF 30+ is required — every day, including cloudy days and days spent mostly indoors near windows.
• Patch test first. Apply a small amount to your inner forearm or behind the ear and wait 24 to 48 hours before applying to the face.

Pregnancy Considerations

Topical AHAs at consumer concentrations (under 10%) are generally considered low-risk during pregnancy because systemic absorption is minimal. However, no controlled studies have been conducted in pregnant populations, and most dermatologists recommend discussing AHA use with your OB-GYN. High-concentration professional peels should be avoided during pregnancy and breastfeeding.

The FDA Stance

The US FDA does not classify AHAs as drugs when used in cosmetic products at consumer concentrations. However, the FDA has issued guidance noting that AHA-containing products should include sunscreen use recommendations on their labeling, based on the documented increase in UV sensitivity [13].

How to Choose the Right AHA for Your Skin

With six or more alpha hydroxy acids to choose from, the decision can feel overwhelming. In practice, most people will choose between three: glycolic acid, lactic acid, or mandelic acid. Here is a simplified decision framework.

For texture, anti-aging, and hyperpigmentation — glycolic acid is the first choice. It has the deepest penetration, strongest clinical evidence, and most product options. Start with 5% to 7% and build tolerance gradually.

For dry or dehydrated skin — lactic acid is the better starting point. Its humectant properties add moisture while exfoliating, and its slightly larger molecular size makes it gentler than glycolic acid.

For acne-prone or melanin-rich skin — mandelic acid offers gentler exfoliation with partial pore-penetrating ability (thanks to its lipophilicity). It carries less risk of post-inflammatory hyperpigmentation in darker skin tones [9].

For very sensitive skin or rosacea — polyhydroxy acids (PHAs) like gluconolactone or lactobionic acid are the safest option. They exfoliate at the surface with minimal irritation and no significant UV sensitization. See our AHA comparison guide for a detailed PHA breakdown.

Not sure where to start? Our skin types guide offers tailored AHA recommendations based on your specific skin type and concerns.

Frequently Asked Questions

Is glycolic acid an alpha hydroxy acid?

Yes. Glycolic acid is the smallest member of the alpha hydroxy acid family, with a molecular weight of 76.05 daltons. It is derived from sugarcane and is the most widely used and most extensively studied AHA in dermatology. Its small size gives it the deepest penetration of any AHA, which makes it the most potent for exfoliation, collagen stimulation, and hyperpigmentation treatment — but also the most likely to cause irritation in sensitive skin. See our full guide to glycolic acid.

How do alpha hydroxy acids exfoliate the skin?

Alpha hydroxy acids exfoliate by disrupting corneodesmosomes — the protein structures that hold dead skin cells together in the stratum corneum. When applied at low pH, the free (protonated) form of the acid penetrates between dead cells and weakens these bonds, accelerating the natural shedding process (desquamation). The result is smoother skin texture, more even tone, and reduced pore congestion. Smaller AHAs like glycolic acid penetrate deeper and exfoliate more aggressively; larger AHAs like mandelic and citric acid work more superficially and gently. For the full mechanism, see how glycolic acid works on skin.

What is the strongest AHA?

Glycolic acid is the strongest alpha hydroxy acid by penetration depth and clinical potency. Its 76.05 Da molecular weight is the smallest in the AHA family, allowing it to reach deeper into the stratum corneum than any other AHA at equivalent concentrations. However, "strongest" does not always mean "best" — the right AHA depends on your skin type, concerns, and tolerance. For sensitive skin, a gentler AHA like lactic or mandelic acid may produce better outcomes with fewer side effects.

Can you use AHAs every day?

At lower concentrations (2% to 7%), daily AHA use is safe for most people. Many toners and serums are designed for nightly application. However, daily use is not necessary for everyone, and it is better to start at 2 to 3 times per week, then increase frequency only after your skin has adjusted over 2 to 4 weeks. Signs of over-exfoliation — persistent redness, tightness, stinging, or a shiny "raw" appearance — mean you should reduce frequency. See our concentration guide for help calibrating your routine.

What is alpha hydroxy acetic acid?

Alpha hydroxy acetic acid is the systematic chemical name for glycolic acid (C2H4O3). The name describes its structure: an acetic acid molecule (like vinegar) with a hydroxyl group on the alpha carbon. In skincare and dermatology, it is almost always called glycolic acid. It is the simplest and smallest alpha hydroxy acid, and the most widely used AHA in both consumer skincare products and professional chemical peels.

References

1. 1. Andersen FA (1998). Final Report on the Safety Assessment of Glycolic Acid, Ammonium, Calcium, Potassium, and Sodium Glycolates, Methyl, Ethyl, Propyl, and Butyl Glycolates, and Lactic Acid, Ammonium, Calcium, Potassium, Sodium, and TEA-Lactates, Methyl, Ethyl, Isopropyl, and Butyl Lactates, and Lauryl, Myristyl, and Cetyl Lactates. Int J Toxicolsafety assessment
2. 2. Tang SC, Yang JH (2018). Dual Effects of Alpha-Hydroxy Acids on the Skin. Moleculesreview
3. 3. Van Scott EJ, Yu RJ (1974). Control of keratinization with alpha-hydroxy acids and related compounds. I. Topical treatment of ichthyotic disorders. Arch Dermatolclinical trial
4. 4. Sharad J (2013). Glycolic acid peel therapy — a current review. Clin Cosmet Investig Dermatolreview
5. 5. Fartasch M, Teal J, Menon GK (1997). Mode of action of glycolic acid on human stratum corneum: ultrastructural and functional evaluation of the epidermal barrier. Arch Dermatol Resclinical trial
6. 6. Bernstein EF, Lee J, Brown DB, et al. (2001). Glycolic acid treatment increases type I collagen mRNA and hyaluronic acid content of human skin. Dermatol Surgclinical trial
7. 7. Stiller MJ, Bartolone J, Stern R, et al. (1996). Topical 8% glycolic acid and 8% L-lactic acid creams for the treatment of photodamaged skin. A double-blind vehicle-controlled clinical trial. Arch DermatolRCT
8. 8. Green BA, Yu RJ, Van Scott EJ (2009). Clinical and cosmeceutical uses of hydroxyacids. Clin Dermatolreview
9. 9. Sarkar R, Garg S, Bansal S, Sethi S (2012). Chemical peels for melasma in dark-skinned patients. J Cutan Aesthet Surgreview
10. 10. Kim SJ, Park JH, Kim DH, Won YH, Maibach HI (1998). Increased in vivo collagen synthesis and in vitro cell proliferative effect of glycolic acid. Dermatol Surgclinical trial
11. 11. Moy LS, Howe K, Moy RL (1996). Glycolic acid modulation of collagen production in human skin fibroblast cultures in vitro. Dermatol Surgin vitro study
12. 12. Okano Y, Abe Y, Masaki H, et al. (2003). Biological effects of glycolic acid on dermal matrix metabolism mediated by dermal fibroblasts and epidermal keratinocytes. Exp Dermatolin vitro study
13. 13. Kaidbey K, Sutherland B, Bennett P, et al. (2003). Topical glycolic acid enhances photodamage by ultraviolet light. Photodermatol Photoimmunol Photomedclinical trial
14. 14. Kim SJ, Won YH (2010). Glycolic acid induces keratinocyte proliferation in a skin equivalent model via TRPV1 activation. J Dermatol Sciin vitro study
15. 15. Scientific Committee on Consumer Safety (SCCS) (2000). Opinion on Alpha-Hydroxy Acids. SCCNFP/0370/00regulatory opinion
16. 16. Ditre CM, Griffin TD, Murphy GF, et al. (1996). Effects of alpha-hydroxy acids on photoaged skin: a pilot clinical, histologic, and ultrastructural study. J Am Acad Dermatolclinical trial

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