No one factor causes acne. Acne happens when oil (sebaceous) glands are activated at puberty, stimulated by male hormones from the adrenal glands in both boys and girls. Oil is a natural substance which lubricates and protects the skin. Under certain circumstances, cells that are close to the surface block the openings of sebaceous glands and cause a buildup of oil underneath. This oil stimulates bacteria, which live in everyone's skin and generally cause no problems, to multiply and cause surrounding tissues to become inflamed.
If the inflammation is right near the surface, you get a pustule; if it's deeper, a papule (pimple); deeper still and it's a cyst. If the oil breaks though to the surface, the result is a "whitehead." If the oil becomes oxidized (that is, acted on by oxygen in the air), the oil changes from white to black, and the result is a "blackhead."
Heredity: With the exception of very severe acne, most people do not have the problem exactly as their parents did. Almost everyone has some acne at some point in life.
Food: All over the world, parents tell teens to avoid pizza, chocolate, greasy and fried foods, and junk food. While these foods may not be good for overall health, they don't cause acne or make it worse. Studies show dairy products and high glycemic foods, however, can trigger acne.
Dirt: Some individuals have more "oily" skin than others (as mentioned above, "blackheads" are oxidized oil, not dirt). Sweat does not cause acne; therefore, it is not necessary to shower instantly after exercise for fear that sweat will clog pores. On the other hand, excessive washing can dry and irritate the skin.
Hormones: Most women break out cyclically. Some oral contraceptive pills help relieve acne.
Cosmetics: Most cosmetic and skin care products are not pore-clogging (“comedogenic.”) Of the many available brands, those which are listed as “water-based” or “oil-free” (non-comedogenic) are a better choice.
In occasional patients, contributing factors may be:
Pressure: In some patients, pressure from helmets, chinstraps, collars, and the like can aggravate acne.
Drugs: Some medications may cause or worsen acne, such as those containing iodides, bromides, or oral or injected steroids (either the medically prescribed prednisone or the steroids bodybuilders or athletes take). Most cases of acne, however, are not drug-related.
Occupations: In some jobs, exposure to industrial products like cutting oils may produce acne.