Chemical Exfoliation Protocol: AHA, BHA & PHA — The Complete Evidence Guide
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.
Shop Exfoliants →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 Guide.
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 (gluconolactone, lactobionic acid) 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. See Acne Protocol.
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 Here)
- 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 — for oily/acne-prone skin. 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. These are signs of barrier damage. Switch to the Barrier Repair Protocol for 2–4 weeks before reintroducing acids.
Skin Cycling: Structuring Acid Use
Skin cycling is a structured approach to alternating exfoliation and recovery nights. See Skin Cycling Protocol for the complete framework.
Related Protocols
- Acne Protocol
- Barrier Repair Protocol
- Oily Skin Protocol
- Skin Cycling Protocol
- The Complete Veracil Skincare System
Commercial disclosure: Veracil sells skincare products including some referenced in this protocol. All recommendations are based on ingredient evidence and protocol compatibility.