Chemical Exfoliation Protocol: AHA, BHA & PHA — The Complete Evidence Guide

Q: What is the difference between AHA and BHA?

AHAs are water-soluble and work on the skin surface — best for dry skin, dull skin, fine lines, and PIH. BHAs are oil-soluble and penetrate the follicle to clear sebum and keratin plugs — best for oily, acne-prone skin and blackheads.

Q: How often should I use chemical exfoliants?

Beginners: 1x per week, building to 2–3x over 4–6 weeks. Established users: AHAs 2–3x per week, BHAs daily to alternate days. Never use AHA and BHA on the same night. Stop immediately if stinging, redness, or waxy skin texture appears.

Q: Can I use AHA and BHA together?

Not on the same night. Alternate them: AHA one night, BHA the next, recovery night with barrier moisturizer. Combined same-night use significantly increases barrier damage risk.

Q: What are signs of over-exfoliation?

Key signs: skin stings when applying water or gentle products, persistent redness, shiny or waxy texture, breakouts in new areas, hypersensitivity to temperature. Stop all exfoliation immediately and follow the Barrier Repair Protocol for 2–4 weeks.

Chemical Exfoliation Protocol

AHA, BHA, and PHA — the evidence-based guide to choosing the right acid, the right concentration, and the right frequency for your skin type.

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Chemical exfoliation is one of the highest-impact categories in skincare — and one of the most commonly misused. Over-exfoliation is the leading cause of iatrogenic (self-inflicted) barrier damage in skincare enthusiasts. This protocol establishes the evidence-based framework for using acids correctly: right type, right concentration, right frequency, right skin type.

AHA vs BHA vs PHA: The Core Distinction

AHAs (Alpha-Hydroxy Acids) — Water-Soluble

AHAs work on the skin surface, dissolving the bonds between dead skin cells to accelerate desquamation. They improve texture, brightness, and fine lines. They increase photosensitivity — mandatory SPF. Best for: dry skin, dull skin, fine lines, uneven texture, PIH.

Glycolic acid — smallest molecule, deepest penetration, most studied. Strongest and most irritating.
Lactic acid — larger molecule, gentler, also a humectant. Best entry-point AHA for sensitive skin.
Mandelic acid — largest AHA molecule, gentlest, antibacterial properties. Ideal for sensitive and acne-prone skin. See Mandelic Acid Protocol.

BHAs (Beta-Hydroxy Acids) — Oil-Soluble

BHAs are oil-soluble, allowing them to penetrate the follicle and dissolve the sebum and keratin plug causing comedones. Best for: oily skin, acne-prone skin, blackheads, congestion.

Salicylic acid (0.5–2%) — the primary BHA in skincare. Anti-inflammatory in addition to keratolytic. Zander & Weisman (1992), JAAD, PMID 1569789.

PHAs (Polyhydroxy Acids) — Gentlest Option

PHAs have larger molecules that penetrate more slowly, making them the gentlest exfoliants. They also have humectant properties. Best for: sensitive skin, rosacea-prone skin, beginners, post-procedure skin. Evidence tier: TIER 2 — less studied than AHAs/BHAs but well-tolerated with consistent results in sensitive populations.

Evidence Review

🟢 TIER 1 — AHAs: Ditre et al. (1996), Journal of the American Academy of Dermatology, PMID 8609256, demonstrated that 25% glycolic acid and 25% lactic acid significantly increased epidermal and dermal thickness and improved skin texture after 6 months. Stiller et al. (1996), Cutis, PMID 8654128, confirmed lactic acid's efficacy for photoaged skin.

🟢 TIER 1 — BHAs: Salicylic acid has multiple RCTs confirming efficacy for comedonal and inflammatory acne.

Concentration Guide

Acid	OTC Range	Frequency	Best For
Glycolic Acid	5–10%	2–3x/week	Texture, fine lines, dull skin
Lactic Acid	5–10%	2–3x/week	Dry skin, gentle brightening
Mandelic Acid	5–10%	Daily–3x/week	Sensitive, acne-prone skin
Salicylic Acid	0.5–2%	Daily–3x/week	Oily, acne-prone, blackheads
PHA	3–8%	Daily	Sensitive, rosacea-prone

The Chemical Exfoliation Protocol

For Beginners

Start with the gentlest option for your skin type: lactic acid (dry), mandelic acid (sensitive), or salicylic acid (oily/acne)
Use 1x per week for 2 weeks; if no irritation, increase to 2x per week
Apply in the evening only — acids increase photosensitivity
Apply to clean, dry skin; wait 10–15 minutes before next step
Follow with ceramide moisturizer and occlusive
Mandatory SPF 50+ every morning

For Established Routines

AHA (lactic or glycolic) — 2–3x/week PM — for texture, brightness, fine lines
BHA (salicylic) — daily or alternate days PM — Cosrx Original Clear Pad for daily BHA use
Do not layer AHA + BHA on the same night — alternate or use a combined product
Barrier support after every acid night — Fragrance Free Tallow + Honey Cream + SKIN1004 Centella Ampoule

The Over-Exfoliation Warning Signs

Stop all exfoliation immediately if you experience: stinging from water or gentle products, persistent redness, shiny or waxy skin texture, increased sensitivity to products that previously caused no reaction. Switch to the Barrier Repair Protocol for 2–4 weeks before reintroducing acids.

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Cosrx Original Clear Pad

BHA salicylic acid pads for daily or alternate-day exfoliation — the most convenient format for consistent BHA use.

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COSRX BHA Blackhead Power Liquid

4% betaine salicylate BHA treatment for blackheads and congested pores — gentler than standard salicylic acid.

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SKIN1004 Madagascar Centella Ampoule

Apply after every acid night to calm post-exfoliation inflammation and support barrier recovery.

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Fragrance Free Tallow + Honey Cream

Barrier moisturizer for post-exfoliation nights — seals in CICA and supports barrier recovery after acid use.

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SKIN1004 Centella Hyalu-Cica Sun Serum SPF50+

Mandatory SPF after every acid use — AHAs and BHAs increase photosensitivity; unprotected skin after exfoliation worsens PIH.

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Cross-reference: Mandelic Acid Protocol — Barrier Repair Protocol — Skin Cycling Protocol — Acne Protocol — Skin pH Protocol — Complete Veracil System

Frequently Asked Questions

What is the difference between AHA and BHA?

AHAs (glycolic, lactic, mandelic) are water-soluble and work on the skin surface to dissolve dead cell bonds — best for dry skin, dull skin, fine lines, and PIH. BHAs (salicylic acid) are oil-soluble and penetrate the follicle to clear sebum and keratin plugs — best for oily, acne-prone skin and blackheads. If you have combination skin, you can alternate both.

How often should I use chemical exfoliants?

Beginners should start at 1x per week and build up over 4–6 weeks. Established users with tolerant skin can use AHAs 2–3x per week and BHAs daily to alternate days. Never use AHA and BHA on the same night — alternate them. If you experience stinging, redness, or shiny/waxy skin texture, stop immediately and follow the Barrier Repair Protocol.

Can I use AHA and BHA together?

Not on the same night. The combined acidity and exfoliation rate increases barrier damage risk significantly. Alternate: AHA one night, BHA the next, recovery night with barrier moisturizer. Some combination products formulate both at lower concentrations for simultaneous use — these are lower-risk but also lower-efficacy.

What are signs of over-exfoliation?

Key warning signs: skin stings when you apply water or gentle products that previously caused no reaction, persistent redness, shiny or waxy texture, breakouts in new locations, and hypersensitivity to temperature. These indicate barrier damage. Stop all exfoliation immediately and follow the Barrier Repair Protocol for 2–4 weeks before reintroducing acids.

Do I need to use SPF with chemical exfoliants?

Yes, without exception. AHAs increase photosensitivity by removing the stratum corneum cells that provide some UV protection. Using AHAs without daily SPF accelerates UV damage and worsens the PIH and hyperpigmentation you're trying to treat. SPF 50+ every morning is non-negotiable during any AHA or BHA regimen.