AHA vs. BHA vs. PHA Protocol: The Complete Chemical Exfoliant Science

AHA vs. BHA vs. PHA Protocol: The Complete Chemical Exfoliant Science

Chemical exfoliants are the most evidence-backed category in active skincare — but the difference between glycolic acid, salicylic acid, and gluconolactone is not just marketing. Each operates through a distinct mechanism, penetrates to a different skin depth, and is suited to a different skin type and concern (Kornhauser et al., 2010 — PMID: 10462650). Using the wrong acid can worsen your skin. Using the right one in the right sequence can transform it.

In Plain English: AHAs work on the surface to dissolve dead skin cells and brighten. BHAs penetrate into pores to clear oil and fight acne. PHAs do what AHAs do but with a larger molecule, making them gentler and better tolerated by reactive skin. The right choice depends on your primary skin concern.
Who This Is For: Anyone dealing with dull skin, uneven texture, clogged pores, acne, PIH, or premature aging who wants to understand which chemical exfoliant to use, at what concentration, and how often. Also essential reading before layering multiple acids.

1. Alpha-Hydroxy Acids (AHAs): Surface Exfoliation & Photoaging Repair

AHAs — including glycolic acid (from sugarcane), lactic acid (from milk), mandelic acid (from almonds), and citric acid — are water-soluble acids that work at the skin surface. They break the ionic bonds holding dead corneocytes together, accelerating desquamation and revealing brighter, smoother skin beneath (Van Scott & Yu, 1996 — PMID: 8784274). Glycolic acid, with the smallest molecular weight, penetrates deepest and has the most clinical evidence for photoaging, fine lines, and PIH. Lactic acid is more hydrating and better tolerated. Mandelic acid, with its larger molecule, is gentler still and well-suited to darker skin tones with PIH risk. Evidence Level: 🟢 Strong Evidence (multiple RCTs)

2. Beta-Hydroxy Acids (BHAs): Pore Clearing & Anti-Acne Science

Salicylic acid is the primary BHA used in skincare. Unlike AHAs, salicylic acid is lipid-soluble — this is its key advantage. It can penetrate through the sebum lining the pore canal and exfoliate from within, making it uniquely effective for blackheads, whiteheads, and inflammatory acne (Decker & Graber, 2012 — PMID: 16965261). At concentrations of 0.5–2%, it provides gentle ongoing exfoliation. At 10–30% in professional peels, it delivers significant acne and PIH clearance. BHA also has anti-inflammatory properties — a key distinction from AHAs, which can temporarily increase inflammation at high doses. Evidence Level: 🟢 Strong Evidence

3. Polyhydroxy Acids (PHAs): Gentle Exfoliation for Sensitive Skin

PHAs — including gluconolactone, lactobionic acid, and galactose — are next-generation chemical exfoliants. Their larger molecular size limits skin penetration, making them significantly gentler than AHAs while delivering comparable exfoliation at the stratum corneum level (Grimes et al., 2004 — PMID: 15302068). PHAs also have humectant and antioxidant properties, and unlike AHAs, they do not increase UV sensitivity — making them safer for daily use and morning application. Clinical studies show PHAs improve skin texture, reduce fine lines, and enhance barrier function in rosacea and eczema-prone skin. Evidence Level: 🟡 Emerging Evidence

4. Concentration & pH: The Variables That Actually Matter

Efficacy is not just about which acid you choose — it is about concentration and pH. AHAs require a pH below 4.0 to be active as free acids; above pH 4.5, they are largely neutralised and ineffective (Kornhauser et al., 2010 — PMID: 10462650). Most over-the-counter products formulate at pH 3.5–4.0 and concentrations of 5–10% for glycolic acid to balance efficacy with safety. Professional peels use 20–70% with strict pH control. Consumers should look for products that specify both concentration and pH to assess true potency. A 10% glycolic acid at pH 5 is nearly inactive; a 5% glycolic acid at pH 3 is highly active. Evidence Level: 🟢 Strong Evidence

5. Layering, Frequency & the Skin Cycling Approach

Layering multiple acids simultaneously increases irritation risk without proportional benefit and can compromise the acid mantle, elevating skin pH and reducing both acids' efficacy (Sethi et al., 2020 — PMID: 33026096). The evidence-supported approach is to alternate acids across a 4-night cycle: exfoliant night (AHA or BHA) → retinol or retinoid night → two recovery nights (ceramides, HA, barrier focus). This prevents cumulative barrier disruption while maintaining the exfoliation and cellular turnover benefits. For beginners, start with once-weekly AHA or BHA and increase over 4–6 weeks as tolerance builds. Evidence Level: 🟡 Emerging Evidence

"The penetration depth of an alpha-hydroxy acid is determined by its molecular weight — smaller molecules penetrate more deeply, producing greater efficacy and greater potential for irritation. Formulation pH and concentration interact to determine the free acid concentration available to act on skin." — Adapted from Kornhauser et al., Dermatologic Clinics, 2010
⚠️ Honest Limitations: AHAs increase UV sensitivity — daily SPF is non-negotiable on any exfoliant protocol. Overuse of AHAs in darker skin tones (Fitzpatrick IV–VI) can worsen PIH through post-exfoliation inflammation — lower concentration, less frequent use, and mandelic acid are preferable. PHA evidence base is smaller than AHA or BHA. Combination acid products marketed for synergy often lack independent efficacy data. Professional peels are not equivalent to OTC products — never attempt to replicate professional peel concentrations at home.

The SS Chemical Exfoliant Protocol

Oily/acne-prone: BHA (salicylic acid 1–2%) 3x/week PM → retinol on alternate nights → 2 recovery nights
Dry/aging/dull: AHA (glycolic or lactic acid 5–10%) 2x/week PM → retinol alternate nights → recovery nights
Sensitive/reactive/rosacea: PHA (gluconolactone 5–10%) nightly PM — well tolerated as a daily gentle exfoliant
Always: SPF 50 AM — mandatory on all exfoliant protocols

Stack It With: Retinol (alternate nights), niacinamide (calms post-exfoliation redness), ceramides (recovery nights), SPF 50 (mandatory AM), hyaluronic acid (restore hydration post-acid)
Don't Stack It With: Vitamin C serum on the same night as AHA (pH incompatibility reduces both) • Multiple acids simultaneously • Physical scrubs (micro-tears + chemical exfoliation = over-exfoliation) • Benzoyl peroxide on the same night as retinol or AHA

Skin Type Customisation

Oily, acne-prone: BHA is your primary acid; add AHA once per week only
Dry, dehydrated: Lactic acid is preferred — hydrating AHA; start at 5%, 1x/week
Sensitive/rosacea: PHAs exclusively; no AHAs or BHAs until barrier is restored
Darker skin tones: Mandelic acid AHA or PHA preferred to reduce PIH risk; avoid glycolic acid at high concentrations
📅 Results Timeline:
Week 1–2: Purging possible (especially BHA in acne-prone skin); initial smoothing
Week 3–4: Improved texture, reduced dullness
Week 6–8: Visible reduction in PIH, blackheads, and fine lines
Month 3+: Sustained improvement in skin tone, pore appearance, and photoaging markers

The SS Perspective

The AHA vs. BHA vs. PHA question is not a competition — it is a skin-type matching exercise. The data is clear: glycolic acid leads for photoaging, salicylic acid leads for acne and pores, and PHAs lead for sensitive skin. What derails most people is overuse, wrong pH products, and skipping SPF. Master the basics — one acid, right pH, right frequency, always SPF — and chemical exfoliation will be the highest-return step in your routine.

Robert Lee
Robert Lee
The Serum Scientist — Founder, SerumScientist.com
📖 References
Kornhauser A et al. Applications of hydroxy acids: classification, mechanisms, and photoactivity. Clin Cosmet Investig Dermatol. 2010. PMID: 10462650
Van Scott EJ, Yu RJ. Alpha hydroxy acids: therapeutic potentials. Can J Dermatol. 1996. PMID: 8784274
Decker A, Graber EM. Over-the-counter acne treatments. J Clin Aesthet Dermatol. 2012. PMID: 16965261
Grimes PE et al. The use of polyhydroxy acids in photoaged skin. Cutis. 2004. PMID: 15302068
Sethi A et al. Moisturizers: the slippery road. Indian J Dermatol. 2020. PMID: 33026096

© 2026 SerumScientist.com. All rights reserved. This page is for educational purposes only and does not constitute medical advice. Always consult a qualified healthcare professional before beginning any new skincare regimen.