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.

The main symptoms are spots that range from tiny, painless whiteheads to large, angry red cysts filled with sebum and pus. Scarring may leave discoloured pitting of the skin.

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.

Healthy skin needs a good intake of vitamins A, C and E

Healthy skin needs a good intake of vitamins A, C and E, zinc and at least eight glasses of water a day (all of us should do this whether or not we have spots).

If someone only has a few spots occasionally, a medicated lotion or face wash that removes excess grease and cleanses the skin is all that's needed. Gels, creams and lotions containing benzoyl peroxide are also helpful because the antibacterial and skin peeling actions help to reduce inflammation and unblock skin pores.

Many complementary treatments that claim to treat acne work for some people but not others. Some of the more reliable ones are a facial gel mask of colloidal silicic acid (which should be used twice a day to remove excess grease and dead skin cells) and tea-tree oil gel, which has antiseptic and antibacterial effects.

Avoid squeezing spots, which can push infection and inflammatory chemicals deeper into the skin, making cysts and permanent scars more likely.

Where spots are numerous and resistant to simple measures, a course of antibiotics to reduce the number of bacteria and the inflammation is used. It sometimes takes up to eight weeks for any noticeable improvement and the treatment is usually needed for at least six months.

Some people with severe scarring acne need powerful medicines supplied by hospital skin specialists to control acne, and laser therapy to reduce the scars.

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