Sebaceous glands produce a substance called sebum, which is responsible for keeping the skin moisturised. During adolescence sebaceous glands enlarge and produce more sebum under the influence of hormones called androgens. After about age 20, sebum production begins to decrease.
Problems arise when the sebum becomes trapped in the skin and hair follicles. The sebum is produced but gets trapped on the way out, and the cells that are normally sloughed off become "sticky", plugging up the follicle. The process of obstructing follicles is called comedogenesis. It causes some follicles to form a type of acne called comedones, also known as blackheads and whiteheads.
Skin exfoliation treatment methods that buff off the dead skin cells like Orbital Microdermabrasion and an Ultrasonic Peel can prevent sebum from becoming trapped in the follicles. These exfoliation methods also help to unblock obstructed follicles by removing the comedones.