Acne is a disorder of the skin's oil glands (sebaceous glands) that results in plugged pores and outbreaks of lesions commonly called pimples or zits. Acne lesions usually occur on the face, neck, back, chest, and shoulders. Nearly 17 million people in the United States have acne, making it the most common skin disease. Although acne is not a serious health threat, severe acne can lead to disfiguring, permanent scarring, which can be upsetting for people who suffer from the disorder.

Doctors describe acne as a disease of the pilosebaceous units. Found over most of the body, pilosebaceous units consist of a sebaceous (oil) gland connected to a hair-containing canal called a follicle (Graphic Omitted.) These units are largest and most numerous on the face, upper back, and chest—areas where acne tends to occur. The sebaceous glands make an oily substance called sebum that normally empties onto the skin surface through the opening of the follicle.

Acne is believed to result from a change in the inner lining of the follicle that prevents the sebum from passing through. For reasons not understood, cells from the lining of the follicle are shed too fast and clump together. The clumped cells plug up the follicle's opening so sebum cannot reach the surface of the skin. The mixture of oil and cells causes bacteria that normally live on the skin, called Propionibacterium acnes (P. acnes), to grow in the plugged follicles. These bacteria produce chemicals and enzymes that can cause inflammation. (Inflammation is a characteristic reaction of tissues to disease or injury and is marked by four signs: swelling, redness, heat, and pain.) When the plugged follicle can no longer hold its contents, it bursts and spills everything onto the nearby skin—sebum, shed skin cells, and bacteria. Lesions or pimples develop as a result of the skin's being irritated.

People with acne frequently have a variety of lesions, some of which are shown in figures 2, 3, and 4 (graphic omitted.) The basic acne lesion, called the comedo (kom´-e-do) or comedone, is simply an enlarged hair follicle plugged with oil and bacteria. This lesion is often referred to as a microcomedo because it cannot be seen by the naked eye. If the plugged follicle, or comedo, stays beneath the skin, it is called a closed comedo or whitehead. Whiteheads usually appear on the skin surface as small, whitish bumps. A comedo that reaches the surface of the skin and opens up is called a blackhead because it looks black on the skin's surface. This black discoloration is not due to dirt. Both whiteheads and blackheads may stay in the skin for a long time.

Other troublesome acne lesions can develop, including the following:

• Papules—inflamed lesions that usually appear as small, pink bumps on the skin and can be tender to the touch.
• Pustules (pimples)—inflamed, pus-filled lesions that can be red at the base.
• Nodules—large, painful, solid lesions that are lodged deep within the skin.
• Cysts—deep, inflamed, pus-filled lesions that can cause pain and scarring.

The exact cause of acne is unknown, but doctors believe it results from several related factors. One important factor is rising hormone levels. These hormones, called androgens (male sex hormones), increase in both boys and girls during puberty and can cause the sebaceous glands to enlarge and make more sebum. Another factor is heredity or genetics. Researchers believe that the tendency to develop acne can be inherited from parents. For example, studies have shown that many school-age boys with acne have a family history of the disorder.

Several factors can contribute to the cause of acne or make it worse. Changing hormone levels in girls and women may cause a flare in their acne 2 to 7 days before their menstrual period starts. Hormonal changes related to pregnancy or starting or stopping birth control pills can also cause acne. Stress, particularly severe or prolonged emotional tension, may aggravate the disorder.

In addition, certain drugs, including androgens, lithium, and barbiturates, are known to cause acne. Greasy cosmetics may alter the cells of the follicles and make them stick together. Friction caused by leaning on or rubbing the skin or the pressure from bike helmets, backpacks, or tight collars can contribute to or worsen acne. Also, environmental irritants (such as pollution and high humidity), squeezing or picking at blemishes, and hard scrubbing of the skin can make acne worse.

There are many myths about what causes acne. Chocolate and greasy foods are often blamed, but research has shown that foods seem to have little effect on the development and course of acne in most people. Another common myth is that dirty skin causes acne; however, blackheads and other acne lesions are not caused by dirt.

People of all ages get acne, but it is most common in adolescents. Nearly 85 percent of adolescents and young adults between the ages of 12 and 24 develop the disorder. People of all races can have acne, but it is more common among Caucasians. For most people, acne tends to go away by the time they reach their thirties; however, some people in their forties and fifties continue to have the disorder.