The pores in your skin contain oil glands. At puberty, there is an increase in sex hormones, called androgens. The excess hormones cause the oil glands to become overactive, enlarge, and produce too much oil, or sebum. When there is too much sebum, the pores or hair follicles become blocked with skin cells. The increase in oil also results in an overgrowth of bacteria called Propionibacterium acnes.
If blocked pores become infected or inflamed, a pimple — a raised red spot with a white center — forms. If the pore clogs, closes, and then bulges, you have a whitehead. A blackhead occurs when the pore clogs, stays open, and the top has a blackish appearance due to oxidation or exposure to air. (This has nothing to do with skin being “dirty”).
When bacteria grow in the blocked pore, a pustule may appear, meaning the pimple becomes red and inflamed. Cysts form when the blockage and inflammation deep inside pores produce large, painful lumps beneath the skin’s surface.
Hormonal changes related to birth control pills, menstrual periods, and pregnancy can trigger acne. Other external acne triggers include heavy face creams and cosmetics, hair dyes, and greasy hair ointment — all of which can increase blockage of pores.
Clothing that rubs the skin may also worsen acne, especially on the back and chest. So can heavy sweating during exercise, and hot, humid climates. Stress is known to trigger increased oil production, which is why many teens have a new crop of pimples on the first day of school or just before that big date.