A comprehensive guide to alpha-hydroxy acids, beta-hydroxy acids, polyhydroxy acids, and azelaic acid — covering mechanisms, skin type and condition relevance, safe use concentrations, and combining principles.
Chemical exfoliation uses acids or chemical agents to accelerate the natural desquamation process — the shedding of corneocytes from the skin surface. Done correctly, this results in improved skin texture, reduced comedonal congestion, more even pigmentation, and — with regular use — stimulation of epidermal renewal and dermal remodelling.
In aesthetic medicine, chemical exfoliants exist on a spectrum from gentle daily-use retail products to professional chemical peels administered in clinic. The same ingredients appear at both ends of this spectrum; it is primarily concentration, pH, and application technique that determines clinical depth and associated risk.
Normal skin desquamation (the natural shedding of corneocytes) is regulated by serine proteases — enzymes that cleave the proteins holding corneocytes together in the stratum corneum. These enzymes, particularly kallikrein-5 and kallikrein-7, are pH-sensitive: they are most active at a slightly acidic pH (5.5–6) and become less active at higher or lower pH values.
Acids accelerate this process through two primary mechanisms, depending on the acid class:
AHAs are a family of water-soluble organic acids characterised by a hydroxyl group (-OH) at the alpha (first) carbon adjacent to the carboxylic acid group. They are derived from natural sources — fruit, milk, sugar cane — though most cosmetic-grade AHAs are synthetically produced. All AHAs share a common mechanism but differ significantly in molecular size, penetration depth, and secondary properties.
Glycolic acid is the most studied and clinically validated AHA. Its small molecular size gives it the deepest epidermal penetration of any AHA — reaching the stratum spinosum and papillary dermis with higher concentrations. This makes it the most potent but also the most potentially irritating member of the AHA family.
Beyond exfoliation, glycolic acid has demonstrated ability to stimulate collagen I and III synthesis by fibroblasts, normalise aberrant corneocyte turnover, reduce comedonal congestion, and improve post-acne hyperpigmentation with regular use.
Lactic acid's larger molecular size compared to glycolic acid translates to slower, gentler penetration. This makes it more appropriate for sensitive skin while retaining meaningful exfoliating and collagen-stimulating efficacy. A unique additional property of lactic acid is its humectant effect — it attracts and retains water in the stratum corneum, making it uniquely beneficial for dry and dehydrated skin types.
Lactic acid is also a natural component of the skin's natural moisturising factor (NMF) — it contributes to the skin's own hydration and acid mantle. This biological familiarity means it is generally very well tolerated.
Mandelic acid's large molecular size means it penetrates the skin more slowly than glycolic or lactic acid — making it the gentlest and most suitable AHA for sensitive and darker skin types. Beyond exfoliation, mandelic acid has demonstrated direct antibacterial activity against Cutibacterium acnes (formerly Propionibacterium acnes) and Staphylococcus aureus — a meaningful secondary benefit in acne management that distinguishes it from other AHAs.
Its anti-inflammatory properties and lower irritation potential have made mandelic acid one of the preferred acids for Fitzpatrick IV–VI skin — an important consideration given that glycolic acid peels in darker skin types carry a documented risk of PIH.
Citric acid (from citrus fruit), malic acid (from apples), and tartaric acid (from grapes) are less frequently used as standalone exfoliants. They appear most often in combination AHA formulations and as buffering agents to adjust pH. Citric acid also has chelating and antioxidant properties. At typical use concentrations in combination products, their individual exfoliating contribution is modest.
| AHA | MW (Da) | Penetration | Best skin type | Special property |
|---|---|---|---|---|
| Glycolic | 76 | Deepest | Normal / oily / photoaged | Strongest collagen stimulation; most studied |
| Lactic | 90 | Moderate | Dry / sensitive / mature | Also a humectant; NMF component |
| Mandelic | 152 | Slowest | Darker phototypes / sensitive / acne | Antibacterial; anti-inflammatory; safest for Fitzpatrick IV–VI |
| Citric / Malic / Tartaric | Variable | Moderate | Combination formulas | Primarily pH buffering and antioxidant in combination products |
BHAs have a hydroxyl group at the beta (second) carbon position. The key difference from AHAs is lipophilicity: BHAs are oil-soluble, which allows them to penetrate the lipid-rich environment of the sebaceous follicle — where AHAs cannot reach effectively. This makes BHAs the most targeted chemical exfoliant for acne, comedones, and oily skin.
Salicylic acid is the primary and most clinically validated BHA. Its lipophilic nature allows it to dissolve within and penetrate sebaceous follicles — dislodging the sebum plugs and desquamating follicular cells that form comedones. It is keratolytic (breaks down keratin plugs), anti-inflammatory (inhibits arachidonic acid and prostaglandin synthesis), and has mild antibacterial activity against C. acnes.
Salicylic acid is structurally related to aspirin — its anti-inflammatory mechanism shares the prostaglandin inhibition pathway. This is clinically significant: patients with aspirin hypersensitivity may also react to topical salicylate-based products.
Betaine salicylate is a gentler derivative in which salicylic acid is bound to betaine (a naturally occurring amino acid derivative found in sugar beets). The betaine portion imparts humectant and anti-inflammatory properties, while the salicylate portion provides BHA exfoliation. The result is a gentler exfoliating BHA with less irritation potential than free salicylic acid — suitable for sensitive or combination skin types that still require BHA activity.
PHAs are a third generation of hydroxy acids. They share a similar mechanism to AHAs but have multiple hydroxyl groups, increasing their molecular size and reducing their penetration speed dramatically. This slower penetration makes PHAs the most tolerable exfoliating acids — suitable for skin types that do not tolerate AHAs or BHAs. PHAs also function as humectants (the multiple -OH groups attract water) and antioxidants.
Gluconolactone is the most commonly used PHA. Its large molecular size means it sits primarily at the stratum corneum surface — providing gentle exfoliation without penetrating deep enough to cause the erythema, stinging, or barrier disruption seen with AHAs in sensitive skin. It also chelates metal ions (copper, iron) that catalyse free radical production, providing a mild antioxidant benefit.
Clinical studies have shown gluconolactone to be effective as an exfoliant in rosacea patients — a skin condition where AHAs may be too stimulating. It has also been shown to be appropriate for use post-laser resurfacing and post-chemical peel as a maintenance exfoliant during recovery.
Lactobionic acid is a disaccharide (galactose + gluconate) — the largest commonly used hydroxy acid, which gives it the gentlest penetration profile of all. Its numerous hydroxyl groups make it an exceptionally effective humectant — attracting moisture even more powerfully than glycerin. It also inhibits matrix metalloproteinases (the enzymes that degrade collagen) — a mechanism directly relevant to anti-ageing.
Azelaic acid is one of the most clinically versatile topical actives in aesthetic medicine — yet it is often underutilised, particularly outside dermatology. It is a dicarboxylic acid (not technically an AHA or BHA) with a unique combination of mechanisms that make it particularly valuable for three commonly co-occurring conditions: hyperpigmentation, rosacea, and acne.
Tyrosinase inhibition: Azelaic acid selectively inhibits tyrosinase in hyperactive melanocytes — reducing melanin synthesis specifically in dyspigmented cells without affecting normally pigmented skin. This selectivity is a key advantage over some other tyrosinase inhibitors.
Anti-inflammatory: Inhibits reactive oxygen species generation by neutrophils, reducing the inflammatory component of acne and rosacea.
Antibacterial: Inhibits protein synthesis in C. acnes at therapeutic concentrations — comparable to topical antibiotics in some studies, without the antibiotic resistance risk.
Keratolytic: Normalises the abnormal keratinisation of follicular epithelium — addressing comedone formation similarly to (but less potently than) retinoids.
In Australia, the Therapeutic Goods Administration (TGA) regulates the concentrations at which acid-based products can be sold over the counter (OTC) versus dispensed through professional channels or prescribed. Understanding these thresholds is clinically important — they inform the advice you can give and the products you can recommend.
| Acid | OTC limit (AU) | Professional / clinic use | Prescription | Key safety consideration |
|---|---|---|---|---|
| Glycolic acid | ≤10% (pH ≥3.5) | 10–70% | Not applicable | High irritation at >10%; professional peels require training and neutralisation protocols |
| Lactic acid | ≤10% | 10–50% | Not applicable | Better tolerated than glycolic at equivalent %; neutraliser required for professional strengths |
| Mandelic acid | ≤10% | 10–40% | Not applicable | Self-neutralising — lower risk profile for professional use; broad dark-skin safety profile |
| Salicylic acid | ≤2% | 10–30% | Not applicable | Self-neutralising; aspirin hypersensitivity cross-reactivity; avoid in pregnancy at pro strengths |
| Gluconolactone / Lactobionic | No specific limit; widely OTC | Up to 14–15% | Not applicable | Lowest risk profile of all exfoliating acids; pH-controlled products self-limiting |
| Azelaic acid | ≤10% | Not typically a peel agent | 15–20% (Skinoren, Finacea) | Well tolerated; no neutralisation required; prescription products require appropriate scope of practice |
| Tretinoin (retinoic acid) | Not OTC in AU | Not applicable | 0.025–0.1% prescription (S4) | Prescription only; requires clinical assessment; refer to Module 03.04 for full retinoid guidance |
Acid combinations can enhance efficacy — but they also multiply the risk of barrier disruption, over-exfoliation, and irritation if not managed carefully. The following principles guide safe acid combination in home-use and clinical contexts.
| Acid | Photoageing / texture | Acne / oily / comedonal | Hyperpigmentation / PIH | Sensitive / rosacea | Fitzpatrick IV–VI | Post-procedure | Dry / dehydrated |
|---|---|---|---|---|---|---|---|
| Glycolic acid | ✓✓✓ | ✓ (adjunct) | ✓✓ | ✗ | Low % only | Phase 3+ only | ✓ (hydrate concurrently) |
| Lactic acid | ✓✓ | ✓ (adjunct) | ✓✓ | Low % | ✓✓ | Phase 3 | ✓✓✓ |
| Mandelic acid | ✓✓ | ✓✓ | ✓✓ | ✓✓ | ✓✓✓ | Phase 3 | ✓ |
| Salicylic acid | ✓ (surface only) | ✓✓✓ | ✓✓ | Low % | ✓✓ | Phase 3+ | Hydrate concurrently |
| Gluconolactone | ✓✓ | ✓ | ✓ | ✓✓✓ | ✓✓✓ | ✓✓✓ | ✓✓ |
| Lactobionic acid | ✓ | ✓ | ✓ | ✓✓✓ | ✓✓✓ | ✓✓✓ | ✓✓✓ |
| Azelaic acid | ✓ | ✓✓✓ | ✓✓✓ | ✓✓✓ | ✓✓✓ | Phase 2–3 | ✓✓ |
✓✓✓ = strong match / first-line · ✓✓ = good match · ✓ = suitable / adjunct · Caution = conditional use · ✗ = generally contraindicated or ineffective · Post-procedure phases: 1 = re-epithelialisation (days 1–7); 2 = proliferation (days 4–21); 3 = remodelling (week 3+)
All clinical statements, concentration data, and mechanism descriptions in this module are drawn from or consistent with the following peer-reviewed and regulatory sources.
Test your understanding before moving on. Select the best answer for each question, then click Check Answers.
1. The key property that makes salicylic acid uniquely effective for comedonal and acne-prone skin is:
2. Glycolic acid is the most potent common AHA because:
3. For professional peels in Fitzpatrick IV–VI skin, which acid offers the best balance of efficacy and safety?
4. PHAs are recommended for sensitive and post-procedure skin primarily because:
5. Azelaic acid's tyrosinase inhibition is described as 'selective' because:
6. Free acid value (FAV) is determined by:
This module is in draft. Your clinical input — particularly around accuracy, relevance to your scope of practice, and any content gaps — directly shapes the final version.