The skin has thousands of tiny hair follicles. Each has a sebaceous gland that produces an oily substance, sebum. An unusual reaction of the sebaceous glands in the skin to the male hormone testosterone causes them to make more sebum.
The characteristic 'blackhead' appearance is caused by the presence of skin pigment, not dirt
This overproduction of sebum - together with dead skin cells - blocks hair follicles. The characteristic 'blackhead' appearance is caused by the presence of skin pigment, not dirt.
Whiteheads are more common and are likely to become inflamed because the grease and bacteria that collects can't escape.
If the blocked hair follicle ruptures, more inflammation occurs. This can lead to nodules and cysts forming, which leave scars as they disappear.
There's no scientific basis to the idea that diet or particular foods (for example, chocolate, fatty or fried foods) make acne worse, but some people do identify specific foods they believe make their acne worse.
Acne appears most frequently on the face, but can also appear on the neck, behind the ears, on the chest and in the groin. Usually it's a mild condition, but for 15 per cent of people it's severe.
Acne affects young adults the most, but can also be a problem for older people. It usually starts in puberty, between ages 12 and 14, when increased levels of the male-type sex hormones androgens (girls have these as well as boys) stimulate the sebaceous glands to increase production of sebum.
More than 90 per cent of teenagers have acne.
Women are generally affected at a younger age, the peak severity being between ages 17 and 18, while men peak between 19 and 20 years old. Men tend to be worst affected, because they produce more testosterone.
The goal with treatment is to prevent skin scarring and psychological distress, and to shorten the time someone has to put up with acne. The treatments used depend on the severity of the acne.