Chemical Peels: Depth Selection for Clinic Protocols

The depth of a peel matters more than the brand on the label; pH, acid family and contact time decide the outcome.

Selecting the correct chemical peel depth is one of the most consequential decisions in an aesthetic clinic protocol. Get it right and you achieve controlled keratolysis, collagen remodelling and pigment correction with predictable recovery. Get it wrong and you risk post-inflammatory hyperpigmentation (PIH), scarring, or an outcome that undershoots the clinical objective entirely. This reference maps every major variable, acid type, concentration, pH and depth, to the indications most likely to present in a UK practice.

Peel Depth Classification

Chemical peels are classified into three depth categories based on the histological layer reached during treatment.

Superficial peels penetrate to the stratum granulosum or the upper papillary dermis. They are suitable for mild photo-damage, comedonal acne, uneven texture and early dyschromia. Recovery is minimal, typically 24 to 72 hours of erythema and light flaking.

Medium-depth peels reach the mid-papillary to upper reticular dermis. They address moderate photo-damage, actinic keratoses, moderate acne scarring and more established pigmentation disorders. Expect five to seven days of significant peeling and erythema. According to the British Association of Dermatologists (BAD), medium-depth procedures require thorough pre-treatment assessment and written consent.

Deep peels penetrate to the mid-reticular dermis. They produce the most pronounced remodelling but carry the highest risk profile, including prolonged recovery of two to four weeks, risk of permanent hypopigmentation, and contraindication in darker Fitzpatrick phototypes. Deep peels are rarely appropriate outside a clinical setting with full resuscitation capability and experienced oversight.

How Acid Family, Concentration and pH Interact

Depth is not a fixed property of any single acid. It is determined by the interplay of three variables: acid type, concentration, and formulation pH.

Acid family determines the mechanism. Alpha-hydroxy acids (AHAs) such as glycolic acid and lactic acid are water-soluble and work primarily by dissolving desmosomes in the stratum corneum. Beta-hydroxy acids (BHAs) such as salicylic acid are lipid-soluble, making them particularly effective in sebaceous follicles. Trichloroacetic acid (TCA) works via protein precipitation, producing a visible frost that is used as an endpoint marker during application. Pyruvic acid is an alpha-keto acid that bridges AHA and BHA mechanisms, offering keratolytic and antimicrobial effects. Mandelic acid is a large-molecule AHA with slower epidermal penetration, which gives it a more favourable PIH risk profile in darker skin types. Kojic acid and azelaic acid are tyrosinase inhibitors used primarily as adjuncts in pigment correction protocols rather than as primary peeling agents.

Concentration alone does not define depth. A 70% glycolic acid at pH 3.8 is significantly more aggressive than the same acid at pH 4.5 because free-acid availability (the truly active fraction) increases sharply as pH falls. Buffered formulations reduce free-acid availability even at high percentage concentrations, which is why some commercial superficial-peel products use 30% glycolic acid effectively in a gel vehicle at near-neutral pH.

Contact time is the practitioner-controlled variable that modulates depth in real time. Shorter contact time and prompt neutralisation limit penetration. This is particularly relevant with AHA peels, which require active neutralisation. TCA and BHA peels self-neutralise and should not be washed off prematurely once frosting has occurred, as this can cause uneven penetration.

Peel Depth Reference Table

The following table summarises commonly used peel types, their key parameters and primary indications. Concentration ranges reflect formulations typically available through UK aesthetic wholesale channels, including those stocked in our professional peel range.

Matching Depth to Indication

Indication drives depth selection; depth selection then governs acid choice. The following framework reflects established practice guidance from the British Association of Dermatologists and is consistent with the principles outlined by the British College of Aesthetic Medicine (BCAM) for practitioner competency in aesthetic procedures.

Acne vulgaris and seborrhoea: Salicylic acid is the first-line choice due to its lipophilicity and comedolytic activity. A series of 20% to 30% salicylic peels at two- to four-week intervals typically produces visible reduction in active comedones within four sessions. For inflammatory acne with PIH tendency, mandelic acid is a lower-risk alternative.

Melasma and hormonally driven pigmentation: Superficial peels used in combination with a depigmentation home protocol produce the most durable outcomes. Cosmelan from Mesoestetic is the most extensively documented combination depigmentation system available to practitioners, using a home-continuation phase to suppress tyrosinase activity after the in-clinic mask. Standalone TCA is not recommended for melasma because the inflammatory response can worsen pigmentation, particularly in Fitzpatrick III to VI.

Photo-damage and fine lines: Glycolic acid series at 30% to 50% is a well-established protocol for mild-to-moderate solar elastosis. For greater effect without a series commitment, a single TCA 25% to 35% peel with appropriate pre-treatment produces measurable collagen remodelling. BioRePeel offers an option where practitioners want the depth of a medium-superficial TCA with the added biostimulatory component and reduced visible peeling, making it suitable for patients with limited social downtime.

Acne scarring: Depth must match the scar morphology. Ice-pick and boxcar scars with fibrotic floors generally require medium-depth stimulation; rolling scars respond better to a series of superficial-to-papillary interventions combined with a skin booster or microneedling programme. For severe atrophic scarring, combination approaches using fractional energy devices alongside a peel series achieve better outcomes than peels alone.

Layering and Neutralisation

Layering, applying two or three passes of the same acid to achieve a deeper or more even result, is an advanced technique that should be approached with caution. With TCA, each additional pass after the first frost is difficult to predict because the precipitation effect reduces further acid penetration. The Obagi Blue Peel protocol incorporates a proprietary blue base to allow the practitioner to control layers visually, which reduces the risk of inadvertent over-penetration.

AHA peels (glycolic, lactic, mandelic) require active neutralisation with a sodium bicarbonate solution or a proprietary neutraliser. Neutralise as soon as the clinical endpoint is reached, typically mild erythema and light tingling, not as a fixed time interval. BHA and TCA peels self-neutralise via protein precipitation and do not benefit from post-application washing within the procedure.

Combination peeling (for example, a Jessner solution pre-treatment followed by TCA 35%) increases depth of penetration beyond either agent used alone and requires particular care in patient selection, pre-treatment preparation and post-procedure monitoring. This combination is outside the scope of superficial-peel training and should only be performed by practitioners with formal medium-depth peel credentials.

Patient Selection and Fitzpatrick Considerations

Fitzpatrick phototype is the single most important safety variable in peel depth selection. PIH risk increases substantially from Fitzpatrick III upward and is the primary reason why practitioner protocols for these patients typically use lower-strength AHAs, mandelic acid, or the phytic-salicylic-mandelic combination products such as SkinTech Easy Phytic, rather than TCA or high-percentage glycolic.

Pre-treatment skin preparation for four to six weeks with a retinoid and a tyrosinase inhibitor (kojic acid, azelaic acid, or a hydroquinone-based product where clinically appropriate) is standard practice before any medium-depth peel in Fitzpatrick III and above. This pre-treatment thins the stratum corneum for more even acid penetration and reduces the PIH risk by downregulating melanocyte activity before the inflammatory stimulus of peeling.

Additional contraindications include: active herpes simplex (reactivation risk is significant with any medium or deep peel; prophylactic antivirals are indicated), isotretinoin use within the last six to twelve months (impaired wound healing, unpredictable depth), and recent ablative or non-ablative laser treatment to the same area within four to six weeks.

Patients with a history of hypertrophic scarring or keloid formation are not candidates for medium or deep peeling. Superficial peels are not absolutely contraindicated but require close monitoring and a conservative approach.

Aftercare and Complications

Aftercare instructions should be given in writing at every appointment. The core principles do not change with peel depth, but the intensity and duration of each element scale with the depth of treatment.

Barrier support: Fragrance-free, non-comedogenic emollients twice daily. For the first 48 to 72 hours after a medium-depth peel, an occlusive petrolatum-based product may be more appropriate to maintain a moist wound environment and reduce crusting.

Sun avoidance: SPF 50+ from day one, applied before any UV exposure. Patients should understand that any peel-induced inflammation increases melanocyte sensitivity. Failure to use adequate sun protection within the first two to four weeks is the most common cause of post-peel PIH.

No manipulation of peeling skin: Premature picking or removal of peeling tissue is a direct cause of scarring and PIH. Patients must be counselled clearly on this before the procedure.

The most common complications in clinical practice are PIH, contact dermatitis (more likely with salicylic acid in salicylate-sensitive patients), and prolonged erythema. True scarring from peels performed within licensed parameters is rare. Persistent hyperpigmentation that has not resolved by eight to twelve weeks post-treatment warrants reassessment; combination treatment with a tyrosinase inhibitor and low-strength retinoid is usually effective. For patients requiring adjunct topicals, our full clinic product range includes professional-grade options used alongside peel protocols.

Key Takeaways

• Depth is determined by acid family, concentration, pH and contact time, not by brand alone.
• Fitzpatrick phototype must be assessed before every peel appointment; it directly governs acid choice and the need for pre-treatment preparation.
• TCA self-neutralises via protein precipitation; frost is the clinical endpoint, not a fixed time interval.
• AHA peels require active neutralisation; neutralise on endpoint, not on the clock.
• Pre-treatment with a retinoid and tyrosinase inhibitor for four to six weeks reduces PIH risk in Fitzpatrick III and above.
• Salicylic acid is first-line for active acne due to lipophilicity and comedolytic activity; mandelic acid is the lower-risk alternative for darker phototypes.
• Combination peeling (Jessner plus TCA) increases depth unpredictably and requires medium-depth peel competency.
• SPF 50+ from day one post-peel is non-negotiable; UV exposure is the primary driver of post-peel PIH.

References

• British Association of Dermatologists (BAD). Chemical peels and skin resurfacing: general clinical guidance.
• British College of Aesthetic Medicine (BCAM). Practitioner competency guidance for aesthetic procedures.
• Mesoestetic. Cosmelan depigmentation system: product datasheet and protocol guide. Manufacturer: Mesoestetic Pharma Group, Barcelona.
• B&T Medical Science. BioRePeel Cl3 FND: technical data sheet and application protocol. Manufacturer: B&T Medical Science, Italy.
• Obagi Medical. Obagi Blue Peel Radiance: prescriber instructions for use. Manufacturer: Obagi Medical.
• SkinTech. Easy Phytic Solution: professional application protocol. Manufacturer: SkinTech, France.
• The Perfect Peel. Product formulation and protocol guide. Manufacturer: Perfect Image.

Reviewed for clinical accuracy under the supervision of our Superintendent Pharmacist, Alicia Barker (GPhC 2241860). Longeva Pharmacy is a GPhC-registered pharmacy (registration 9012378) operating under MHRA WDA(H) licence 59619. Information is intended for licensed UK practitioners and does not replace individual clinical judgement.