Seborrheic Dermatitis

Seborrheic dermatitis appears where sebaceous glands are densest: the sides of the nose, the brows and between them, the hairline, the ears, the scalp. It presents as redness with a greasy yellowish scale, often itchy, and it flares and settles in cycles that track stress, cold weather and illness.

The mechanism is not oiliness itself. Malassezia is a yeast that lives on everyone's skin and feeds on sebum; in seborrheic dermatitis the immune system reacts to its byproducts, and the visible rash is that reaction. This is why washing more aggressively makes it worse — you strip the barrier without touching the organism — and why it is neither contagious nor a sign of being unclean.

What works is antifungal rather than anti-oil. Ketoconazole, ciclopirox, zinc pyrithione and selenium sulfide are the evidence-backed options, usually as a wash left on for a few minutes; on the face, a short course of a mild anti-inflammatory may be added for a flare. Between flares the aim is a bland barrier-supporting routine.

The practical trap is skincare choice: Malassezia metabolises fatty acids in a specific chain-length range, so many oils and fatty-acid esters feed it. Squalane and mineral oil do not, which is why they turn up so often in routines built for this condition.