Using The Ordinary 7% Glycolic Acid on Underarms & Scalp: pH Chemistry vs. Intertriginous Burn Risk

Human sweat secreted by eccrine and apocrine glands is inherently odorless. Body odor originates when resident cutaneous bacteria — predominantly *Corynebacterium* species and *Staphylococcus hominis* — metabolize odorless sweat amino acids and steroids into volatile foul-smelling thioalcohols and branched volatile fatty acids.

These bacterial enzymes function optimally at a neutral to slightly alkaline pH (6.0 to 7.5). When a low-pH acid like Glycolic Acid (pH ~3.6) is applied, it immediately acidifies the microenvironment.

This acidification denatures bacterial enzymes and inhibits bacterial proliferation without requiring aluminum chlorohydrate pore plugs.

The underarm is an intertriginous zone: an anatomical fold where skin rubs against skin. The combination of natural occlusion, increased body heat, and mechanical friction elevates stratum corneum permeability by 300% to 500% compared to facial skin.

Applying a 7% unbuffered glycolic solution under these occlusive conditions causes rapid free-acid penetration, triggering acute contact dermatitis and intertrigo.

Crucially, in darker skin types (Fitzpatrick IV–VI), this chemical irritation causes intense post-inflammatory hyperpigmentation (PIH), making underarms appear dramatically darker over time.

On the scalp, glycolic acid breaks down cross-linked keratin plaques and stubborn styling polymer residues. However, leave-on overnight application will inflame hair follicle ostia and disrupt the scalp microbiome.

If using on the scalp: apply as a 10-minute pre-shampoo clarifying treatment no more than once every two weeks. Massage gently onto damp roots, wait 10 minutes, and shampoo out thoroughly with a sulfate-free syndet wash.