Clinical Glossary — Chemical Peel Terms for Dermatologists

AHA: Alpha-hydroxy acids, a family of water-soluble acids (such as glycolic, lactic and mandelic) that loosen corneocyte adhesion in the upper epidermis. They produce superficial peels and require neutralisation to halt their action.
BHA: Beta-hydroxy acids, principally salicylic acid, which are lipophilic and penetrate sebaceous follicles. This makes them well suited to oily and acne-prone skin for superficial peeling.
Cosmelan: A proprietary depigmenting protocol combining an in-clinic mask with a home maintenance regimen, used chiefly for melasma. It works by inhibiting tyrosinase rather than by causing a classical exfoliating peel.
Epidermolysis: Separation or detachment of the epidermis from underlying layers. During a peel it appears as greyish-white skin sliding or wrinkling and signals that penetration has reached the dermo-epidermal junction.
Fitzpatrick scale: A six-point classification (I–VI) of constitutive skin colour and its response to ultraviolet exposure. Higher phototypes (IV–VI) carry greater risk of post-inflammatory hyperpigmentation and guide conservative peel selection.
Frosting: The whitening of the skin during a chemical peel as surface proteins coagulate. Frost level (I, II or III) is the principal visual endpoint used to gauge peel depth, particularly with TCA.
Glycolic acid: The smallest alpha-hydroxy acid, derived from sugar cane, used at concentrations up to about 70% for superficial peels. Its effect depends on concentration, pH and contact time, and it must be neutralised.
Jessner's solution: A combination peeling solution classically containing resorcinol, salicylic acid and lactic acid in ethanol. It is used as a superficial peel and as a primer before TCA to even penetration.
Kojic acid: A tyrosinase-inhibiting agent used topically to reduce melanin synthesis. It is commonly incorporated into priming and maintenance regimens for melasma and post-inflammatory hyperpigmentation rather than as a peeling acid.
Mandelic acid: A larger-molecule alpha-hydroxy acid derived from bitter almonds. Its slower, more even penetration makes it a gentler option often selected for sensitive skin and higher Fitzpatrick phototypes.
Medium-depth peel: A peel that reaches the papillary to upper reticular dermis, classically with TCA around 35% often combined with a primer such as Jessner's solution. It addresses dyschromia and fine wrinkling but carries greater downtime and pigmentary risk.
Melasma: A chronic, acquired disorder of symmetrical facial hyperpigmentation driven by hormonal and ultraviolet triggers. It is prone to recurrence and rebound, so peels are used cautiously and combined with priming and strict photoprotection.
Neutralization: Stopping a peel's activity, typically by applying a buffering or alkaline agent or copious water. Alpha-hydroxy acids require active neutralisation, whereas TCA is self-neutralising and is instead diluted with water once the endpoint is reached.
Post-inflammatory hyperpigmentation (PIH): Acquired darkening of the skin following inflammation or injury, caused by excess melanin deposition in the epidermis or dermis. It is more common and more persistent in higher Fitzpatrick phototypes.
Priming: Pre-treating the skin for two to four weeks before a peel, often with a retinoid plus a tyrosinase inhibitor. Priming thins the stratum corneum, promotes even penetration and reduces the risk of post-inflammatory hyperpigmentation.
Salicylic acid: A lipophilic beta-hydroxy acid, typically used at 20–30% for superficial peels. It concentrates in sebaceous units, has comedolytic and anti-inflammatory effects, and produces a characteristic white pseudo-frost as it precipitates.
Superficial peel: A peel whose injury is confined to the epidermis down to the basal layer. It has minimal downtime and a strong safety profile across phototypes, usually requiring a course of repeated sessions for cumulative benefit.
TCA: Trichloroacetic acid, a self-neutralising peeling agent used at concentrations from roughly 10% to 35% for superficial-to-medium-depth peels and at higher strengths for focal CROSS scar treatment. Depth of injury rises with concentration and number of coats.