Sebum
Sebum is the oily substance produced by sebaceous glands and released into hair follicles, from where it reaches the skin surface. It contributes to the film covering the skin and to the environment that organisms living on skin encounter.
The oil on skin has a name and a source. Sebum is produced by sebaceous glands, released into the hair follicle each gland is attached to, and carried up the follicle channel to the surface.
It is a mixture of fatty substances rather than a single compound, and its composition differs from the lipids that sit between cells in the outermost layer. The two are often confused: the mortar of the skin barrier is not sebum, and an oily surface does not indicate an intact barrier.
On the surface it forms part of a thin film, contributing to how skin feels and to the conditions organisms living there encounter. Whether it is genuinely protective is discussed more confidently in marketing than in dermatology.
Production is not constant. It responds to hormonal signals, which is why output changes markedly through life and why the conditions associated with it follow the same pattern. Where sebaceous glands are densest, sebum is most abundant, and that distribution explains where those conditions appear: face, scalp, chest and upper back.
Two conditions on this site turn on it. Acne begins when the follicle carrying sebum becomes blocked. Seborrhoeic dermatitis reflects a reaction to a yeast that thrives in the oily environment sebum creates, which is a quite different problem arising from the same anatomy.
Last updated .