Classifying Peels: Acids, Combinations, Modified

Peels are best classified along two axes at once: by agent family (which acid or molecule, and therefore which mechanism) and by formulation (a single acid, a combination, or a modified/buffered preparation). The first axis tells you how the peel injures; the second tells you how controllable and layered the injury is. Neither axis alone sets the depth — that is still governed by concentration, free acid, contact time and layering — but together they give you a working taxonomy for selecting a peel.

Axis 1 — the agent families

• AHAs (alpha-hydroxy acids) — glycolic (smallest molecule, fastest penetration), lactic (larger, more hydrating, lower-irritation), mandelic (largest, slowest, lowest PIH risk). Water-soluble keratolytics; require neutralisation.
• BHA (salicylic acid) — lipophilic, so it concentrates in sebaceous units; comedolytic and anti-inflammatory, the BHA of choice for oily, acne-prone skin. Self-neutralising (pseudofrost).
• TCA — a coagulant whose depth is a direct function of concentration and coat count; superficial at low strength, medium at ~35%, deep above ~50%.
• Retinoids (retinoic acid / "yellow" peels) — act via receptor-mediated turnover rather than acute keratolysis, producing delayed, low-inflammation exfoliation that is comparatively pigment-friendly — attractive in darker skin.

Axis 2 — single, combination, modified

Single-acid peels

One active at a defined strength: glycolic 35%, salicylic 30%, TCA 15%. Advantages: simple, predictable, easy to titrate and to attribute a reaction to a single variable — the best starting point and the cleanest for building a depth ladder across a series.

Combination peels

Two or more actives in one protocol, deliberately at lower individual concentrations to hit several pathways (keratolysis, melanogenesis inhibition, comedolysis, collagen stimulation) while keeping any single acid's irritation down.

• Jessner's solution is the classic combination: resorcinol + salicylic acid + lactic acid in ethanol. It is keratolytic and is very commonly used to prime/breach the barrier before TCA, letting a lower TCA strength reach medium depth with more control.
• Modern multi-acid "yellow"/depigmenting peels combine, for example, retinoid with glycolic, lactic and kojic acid — multi-pathway depigmentation at modest single-agent strengths, which is why combinations are often favoured in Fitzpatrick IV–VI.

Modified / buffered peels

A single agent or combination that has been buffered, partially neutralised, gelled or vehicle-modified to slow penetration and raise the effective pH. This trades peak potency for control and tolerability: a buffered glycolic at the same label percentage is gentler than a fully free one. Modification is a deliberate safety lever — particularly valuable when you want metabolic benefit without driving inflammation in reactive, melanocompetent skin.

The families and formulations at a glance

Class	Representative agents	Mechanism	Typical depth	Best-fit indications	Phototype note
AHA (single)	Glycolic, lactic, mandelic	Keratolytic / metabolic; neutralised	Superficial	Dullness, tone, mild acne, priming	Mandelic safest in IV–VI
BHA (single)	Salicylic acid	Lipophilic, comedolytic; self-neutralising	Superficial	Oily/acne-prone skin, comedones	Pseudofrost ≠ depth
TCA (single)	Trichloroacetic acid	Coagulant (protein denaturation)	Superficial→deep by strength	Photoaging, scars, focal CROSS	Keep to lower strengths in IV–VI
Retinoid (single/combo)	Retinoic acid "yellow" peels	Receptor-mediated, delayed turnover	Superficial–medium	Pigment, photoaging	Pigment-friendly, low inflammation
Combination	Jessner's; multi-acid yellow peels	Multi-pathway at lower strengths	Superficial–medium	Acne, PIH, melasma, pre-TCA priming	Often preferred in IV–VI
Modified / buffered	Buffered/gelled AHA or combos	Slowed penetration, raised pH	Superficial	Sensitive/reactive skin, first peels	Tolerability lever for darker skin

Choosing a class in practice

A simple decision flow: start single and simple to learn a patient's reactivity and build a depth ladder; move to combinations when one concern needs several pathways at once (acne with PIH, melasma) or when you want medium depth with less peak insult (Jessner's-primed TCA); and reach for modified/buffered formulations when tolerability or phototype demands a gentler on-ramp. In Indian and other Fitzpatrick IV–VI practices, combination and modified peels carry much of the workload precisely because they deliver multi-pathway benefit while keeping single-acid irritation — and therefore PIH risk — low.

Key takeaway

Classify every peel twice: which family (mechanism) and which formulation (control). Single acids are your titratable baseline, combinations let you hit several pathways at lower strengths, and modified/buffered preparations are a deliberate safety lever. The category never sets the depth — but it tells you how the peel will behave and how forgiving it will be.