Sebum is a fatty substance secreted by the sebaceous glands in the skin. It consists mainly of fatty acids, waxes, triglycerides and cholesterol. The substance reaches the surface of the skin via the small ducts that lead to the hair follicles.
Sebum is one of many oily substances associated with the skin's surface. More sebaceous glands can be found on the forehead and chin than in other areas of the face. For this reason, the face's sebum levels are more pronounced in these areas.
Under normal circumstances, sebum provides the skin with a thin film, which protects the skin from drying. It also has an antibacterial effect. Overproduction of sebum can, however, aggravate the skin and lead to the development of blackheads, whiteheads and full-blown acne.
It is interesting to note that sebum is linked to humans' odour profile and that its smell can be detected by certain canines. It is also believed that this profile might have played a role in primitive man's communication with his peers.