Skin cancer is the uncontrolled growth of abnormal skin cells. If left unchecked, these cancer cells can spread from the skin into other tissues and organs.

There are different types of skin cancer. Basal cell carcinoma is the most common. Melanoma is less common, but more dangerous.

The outer layer of skin, the epidermis, is made up of different types of cells. Skin cancers are classified by the types of epidermal cells involved:

• Basal cell carcinoma develops from abnormal growth of the cells in the lowest layer of the epidermis and is the most common type of skin cancer.
• Squamous cell carcinoma involves changes in the squamous cells, found in the middle layer of the epidermis.
• Melanoma occurs in the melanocytes (cells that produce pigment) and is less common than squamous or basal cell carcinoma, but more dangerous. It is the leading cause of death from skin disease.

Skin cancers are sometimes classified as either melanoma or nonmelanoma. Basal cell carcinoma and squamous cell carcinoma are the most common nonmelanoma skin cancers. Other nonmelanoma skin cancers are Kaposi's sarcoma, Merkel cell carcinoma, and cutaneous lymphoma.

Skin cancer is the most common form of cancer in the Unites States. Known risk factors for skin cancer include the following:

• Complexion: Skin cancers are more common in people with light-colored skin, hair, and eyes.
• Genetics: Having a family history of melanoma increases the risk of developing this cancer.
• Age: Nonmelanoma skin cancers are more common after age 40.
• Sun exposure and sunburn: Most skin cancers occur on areas of the skin that are regularly exposed to sunlight or other ultraviolet radiation. This is considered the primary cause of all skin cancers.

Skin cancer can develop in anyone, not only people with these risk factors. Young, healthy people -- even those with with dark skin, hair, and eyes -- can develop skin cancer.

Skin cancers may have many different appearances. They can be small, shiny, waxy, scaly and rough, firm and red, crusty or bleeding, or have other features. Therefore, anything suspicious should be looked at by a physician. See the articles on specific skin cancers for more information.

Here are some features to look for:

• Asymmetry: one half of the abnormal skin area is different than the other half
• Borders: irregular borders
• Color: varies from one area to another with shades of tan, brown, or black (sometimes white, red, blue)
• Diameter: usually (but not always) larger than 6 mm in size (diameter of a pencil eraser)
• Any skin growth that bleeds or will not heal

Use a mirror or have someone help you look on your back, shoulders, and other hard-to-see areas.

Different types of skin cancer require different treatment approaches. Surgical removal of the cancer is very common.

See the specific type of skin cancer for information:

• Basal cell carcinoma
• Squamous cell carcinoma
• Melanoma

For additional resources, see cancer support group.

The outlook depends on a number of factors, including the type of cancer and how quickly it was diagnosed. Basal cell carcinoma and squamous cell carcinoma rarely spread to other parts of the body. However, melanoma is more likely to spread. See the specific skin cancer articles for additional information.

Any suspicious mole, sore, or skin growth should be looked at by a physician immediately. You should take seriously any changes in a mole or any sudden growth on the skin.

Minimizing sun exposure is the best way to prevent skin damage, including many types of skin cancer:

• Protect your skin from the sun when you can -- wear protective clothing such as hats, long-sleeved shirts, long skirts, or pants.
• Try to avoid exposure during midday, when the sun is most intense.
• Use sunscreen with an SPF of at least 15. Apply sunscreen at least one-half hour before sun exposure, and reapply frequently.
• Apply sunscreen during winter months as well.