Acne is an inflammatory skin disorder of the skin's sebaceous glands and hair follicles. During puberty high levels of hormones are produced in both girls and boys. This leads to the production of large quantities of sebum. Sebum is an irritant that can clog the pores and form a pimple which may become infected and form a pustule. Hormones don't go away after adolescence.
Many women still get premenstrual acne from of the release of progesterone after ovulation.
The sebaceous glands located in each hair follicle or tiny pit of skin, produce oil that lubricate the skin and keep it soft. Sebaceous glands are found in large numbers on the face, back, chest and shoulders. If this oil becomes trapped, bacteria multiply in the follicle and the skin becomes inflamed. Acne is NOT caused by dirty pores but most likely by over active oil glands. The excess oil makes the pores sticky allowing bacteria to become trapped inside.