Chemical Exfoliation and Peels
What this domain tests
Chemical exfoliation questions test acid classes, pH and free-acid concepts, neutralization, client selection, and complication recognition. You need to know when a peel is contraindicated and how concentration, pH, and contact time change penetration.
Core rules
- AHAs such as glycolic, lactic, and mandelic acids are water-soluble; BHA salicylic acid is lipid-soluble and more follicular. TCA peels coagulate protein and show frosting; Jessner solution combines resorcinol, salicylic acid, and lactic acid.
- Lower pH and greater pKa separation increase free acid. At pH equal to pKa, about 50% is un-ionized; each unit below pKa raises the un-ionized fraction roughly tenfold.
- Timing starts when the full face is covered. Neutralize AHAs with sodium bicarbonate or a formulated neutralizer; TCA is usually self-neutralizing.
- Contraindications include isotretinoin within 6 months in many protocols, pregnancy for salicylic acid, active herpes, recent waxing or dermaplaning, uncontrolled eczema, sunburn, and keloid history for deeper peels.
Worked example
Glycolic acid has a pKa of 3.83. A 30% peel buffered to pH 3.0 has an un-ionized fraction of 1 / (1 + 10^(3.0−3.83)) = 1 / (1 + 0.148), about 87%. At pH 4.0, the fraction is 1 / (1 + 10^0.17), about 40%. The lower-pH peel delivers more free acid and more penetration at the same concentration, so it behaves stronger.
Traps
Water on the skin can reactivate an AHA; count contact time accurately. Do not assume all peels need neutralizer. Do not treat recent waxing or active cold sores. Frosting is not a goal for every peel.