AHA vs BHA vs PHA: The Complete Exfoliation Science Protocol

Most people are using the wrong exfoliant for their skin type — and making their skin worse in the process. Alpha hydroxy acids, beta hydroxy acids, and polyhydroxy acids each work via entirely different mechanisms, penetrate to different skin depths, and suit fundamentally different skin concerns. Getting this wrong means irritation, post-inflammatory hyperpigmentation, or simply wasted money. This is the SerumScientist.com definitive guide to chemical exfoliation.

🔬 SS EVIDENCE RATING
L1 STRONG — AHAs and BHAs have extensive RCT evidence. PHA evidence is emerging (L2 MODERATE).
In Plain English
AHAs (glycolic, lactic, mandelic) are water-soluble acids that exfoliate the skin surface. BHAs (salicylic acid) are oil-soluble, penetrating into pores. PHAs (gluconolactone, lactobionic acid) are large-molecule AHAs that exfoliate gently without penetrating deeply — ideal for sensitive skin.
Who This Is For
Anyone with texture, congestion, hyperpigmentation, dullness, or uneven skin tone. Those confused about which exfoliant to use. Acne-prone skin needing follicular exfoliation. Sensitive skin seeking gentle exfoliation options.

1. AHAs: Surface Exfoliation and Collagen Stimulation

Alpha hydroxy acids work by disrupting the ionic bonds holding desquamating corneocytes together at the stratum corneum level — specifically by chelating calcium ions that stabilise corneodesmosomes. This accelerates natural desquamation, revealing fresher cells beneath. Glycolic acid (76 Da, smallest AHA) penetrates deepest and fastest; lactic acid (90 Da) is gentler; mandelic acid (152 Da) penetrates slowest and is best for sensitive skin. (Bernstein EF et al. PMID:11737121) L1

Chronic AHA use at 8–10% over 6 months also stimulates procollagen I synthesis and glycosaminoglycan production in the dermis — giving AHAs genuine anti-aging benefit beyond surface exfoliation.

2. BHAs: Follicular Exfoliation for Acne and Congestion

Salicylic acid (BHA) is the only commercially significant oil-soluble exfoliant. Its lipophilic nature allows it to penetrate through sebum into the follicular canal — dissolving the keratin plugs that form blackheads, whiteheads, and microcomedones. At the same time, salicylic acid has anti-inflammatory activity, suppressing NF-κB-mediated prostaglandin synthesis. (Arif T. PMID:25878537) L1

BHA does not penetrate to the dermis and does not stimulate collagen — its primary value is follicular clearing and anti-inflammation, making it the gold standard for acne-prone and congested skin.

3. PHAs: Gentle Exfoliation for Sensitive and Barrier-Compromised Skin

Polyhydroxy acids (gluconolactone, lactobionic acid) are AHA derivatives with additional hydroxyl groups making them larger, slower-penetrating, and significantly less irritating. They exfoliate via the same corneodesmosome mechanism as AHAs but at the skin surface only. Critically, PHAs also have humectant and antioxidant properties — making them suitable for rosacea-prone, eczema-prone, and barrier-compromised skin that cannot tolerate traditional AHAs. (Stiller MJ et al. PMID:8621812) L2

4. Choosing Your Exfoliant: The SS Decision Framework

Oily/acne-prone with blackheads: Salicylic acid 1–2% BHA
Dull/textured normal skin: Glycolic acid 5–10% or lactic acid 5–12%
Dry/sensitive skin: Lactic acid 5–8% or PHA (gluconolactone 5–10%)
Darker skin tones (PIH risk): Mandelic acid 5–10% or lactic acid (lower PIH risk than glycolic)
Rosacea/eczema-prone: PHA only, or skip exfoliation during flares

🛒 Shop This Protocol

Glow Fusion Vitamin C Serum — Daytime antioxidant partner for post-exfoliation brightness

The SS Exfoliation Protocol

AM: Gentle cleanser → Vitamin C (Glow Fusion) → Moisturiser → SPF 50+ (mandatory — AHAs increase UV sensitivity)

PM (2–3x/week max): Gentle cleanser → AHA or BHA serum/toner → Wait 10 min → Moisturiser

Never: Combine AHA + BHA + retinol in the same routine. Exfoliate more than 3x/week. Use exfoliants on compromised/broken skin.

Stack It With ✅
SPF 50+ (mandatory) · Ceramide moisturiser post-acid · Niacinamide (separate step, same routine) · Hyaluronic acid for hydration buffer · Retinol on alternate nights
Don’t Stack It With ❌
Retinoids same night · Benzoyl peroxide same application (glycolic) · Multiple AHAs simultaneously · Use on compromised or sunburned skin
"Chronic topical glycolic acid application at 8% produced statistically significant increases in procollagen I mRNA and hyaluronic acid synthase expression after 22 weeks — evidence that AHA exfoliation is not merely cosmetic but structurally regenerative." — Bernstein EF et al.
⚠️ Honest Limitations
AHAs significantly increase photosensitivity — SPF is non-negotiable. Over-exfoliation is extremely common and destroys the skin barrier faster than almost any other mistake. Glycolic acid has the highest PIH risk in darker skin tones — choose mandelic or lactic instead. More exfoliation is rarely better exfoliation.
📅 Results Timeline
Week 1–2: Brighter, smoother surface · Week 4–6: Texture improvement · Month 3+: Pigmentation fading · Month 6+: Collagen stimulation effects visible

The SS Perspective

Chemical exfoliation is one of the most effective and most misused categories in skincare. The data is clear: 2–3x/week at appropriate concentrations produces real results. Daily exfoliation produces barrier destruction and rebound congestion. Choose your exfoliant based on your primary skin concern, respect the SPF rule without exception, and resist the temptation to stack multiple acids. One well-chosen acid, used correctly, beats a shelf of acids used wrong.

Robert Lee
Robert Lee
The Serum Scientist — Founder, SerumScientist.com
📖 References
Bernstein EF et al. Glycolic acid treatment increases type I collagen mRNA and hyaluronic acid content of human skin. Dermatol Surg. 2001. PMID:11737121
Arif T. Salicylic acid as a peeling agent. Clin Cosmet Investig Dermatol. 2015. PMID:25878537
Stiller MJ et al. Topical 8% glycolic acid and 8% L-lactic acid creams. J Am Acad Dermatol. 1996. PMID:8621812

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