What causes acne?
Understanding normal skin
Small sebaceous glands lie just under the skin surface. These glands make the 'oil' (sebum) that keeps the skin supple and smooth. Tiny pores (holes) on the skin allow the sebum to come onto the skin surface. Hairs also grow through these pores. During the teenage years, you make much more sebum than when you were a child. This is due to the hormone changes of puberty which stimulate the sebaceous glands. As a rule, the more sebum that you make, the more greasy your skin feels, and the worse acne is likely to be. Some people make more sebum than others.
Mild to moderate acne - blackheads, whiteheads, and small pimples
Some pores become blocked ('plugged'). This is due to the skin at the top of the pores becoming thicker, combined with dead skin cells that are shed into the pores. You can see the 'plugs' that block the top of the pores as tiny spots known as comedones (blackheads and whiteheads). Note: the black of the blackheads is due to skin pigment, and is not dirt as some people think. In many cases, acne does not progress beyond this mild stage.
Some sebum may collect under blocked pores. You can see this as small spots called pimples or papules. In some cases, acne does not progress beyond this mild-to-moderate stage when you can see a number of small pimples, blackheads, and whiteheads.
Moderate to severe acne - larger spots and inflammation
Trapped sebum is ideal for a bacterium (germ) called P.acnes to live and multiply. Small numbers of this bacterium normally live on the skin, and do no harm. However, if a large number develop in the trapped sebum, the immune system may react and cause inflammation. If inflammation develops, it causes the surrounding skin to become red, and the spots become larger and filled with pus (pustules). In some cases the pustules become even larger and form into small 'nodules' and cysts.
Each inflamed spot will heal eventually. In some cases the area of skin that was inflamed remains discoloured for several months after the inflammation has gone (post-inflammatory hyperpigmentation). This is often more noticeable in darker skinned people. Also, a small pitted scar is commonly left on the skin where there was an inflamed spot. These small scars often do not fade fully and are a marker in older people that they once had inflamed acne spots.
Rare causes of acne
The description above is the cause of almost all cases of acne. Rarely, certain diseases in girls and women may cause acne, or make acne worse. For example, polycystic ovary syndrome, and conditions that cause excess male hormone to be made in the ovary or adrenal gland. These conditions cause other symptoms in addition to acne such as thinning of scalp hair, excess growth of facial or body hair (hirsutes), and other problems. Another rare cause of acne is exposure to halogenated hydrocarbons (chemicals that occur in some work places).
