Understanding Acids, AHA, BHA, PHA
The three families of exfoliating acids, where they work, and when each is the right choice.
AHA, alpha hydroxy acids
AHAs are water-soluble and work primarily on the surface of skin, loosening the bonds between dead corneocytes to accelerate surface turnover. The family includes glycolic, lactic, mandelic, tartaric, and citric acids.
- Glycolic acid, smallest molecule, deepest penetration, strongest evidence base. Effective at 4–10% leave-on, 30–70% in professional peels.
- Lactic acid, larger molecule, gentler, additional humectant properties. Effective at 5–12%.
- Mandelic acid, largest of the common AHAs, gentlest, well-tolerated in skin of color. Effective at 5–10%.
- Tartaric and citric acids, usually formulation-adjusting rather than primary actives.
BHA, beta hydroxy acid (salicylic acid)
Salicylic acid is lipid-soluble, it dissolves into sebum and penetrates the follicle. That makes it uniquely useful for acne and closed comedones. Effective at 0.5–2% leave-on; higher concentrations in professional peels. Research suggests anti-inflammatory effects in addition to keratolytic action.
PHA, poly hydroxy acids
PHAs are larger molecules that penetrate less, work more on the surface, and produce substantially less irritation. The family includes gluconolactone, lactobionic acid, and maltobionic acid. Research suggests they are appropriate for sensitive, reactive, or rosacea-prone skin where AHAs and BHAs are too aggressive.
How to choose
- Surface dullness, fine lines, mild pigmentation: AHA (glycolic or lactic).
- Sensitive skin or skin of color: mandelic or PHA.
- Acne, congestion, oily skin: BHA (salicylic).
- Combination presentation: alternating AHA and BHA across the week, never same night.
Pairing rules
- Daily SPF is non-negotiable while using any exfoliating acid.
- Do not stack acids with retinoids on the same night.
- Do not use acids the same night as benzoyl peroxide.
- Vitamin C in the morning, acids at night, on separate days for sensitive skin.
Timeline
Surface texture and brightness changes: 4 to 8 weeks. Pigmentation changes: 8 to 12 weeks. Sustained use is required for sustained results; stopping returns turnover to baseline within weeks.
Sources
- Kornhauser A et al (2010). Applications of hydroxy acids: classification, mechanisms, and photoactivity.
- Kessler E et al (2008). Comparison of alpha- and beta-hydroxy acid chemical peels.
- Sharad J (2013). Glycolic acid peel therapy, a current review.