Ultraviolet (UV) light exposure, most commonly from sunlight, is overwhelmingly the most frequent cause of skin cancer.

Other important causes of skin cancer include the following:

• Use of tanning booths

• Immunosuppression-impairment of the immune system, which protects the body from foreign entities, such as germs or substances that cause an allergic reaction. This may occur as a consequence of some diseases or can be due to medications prescribed to combat autoimmune diseases or prevent organ transplant rejection.

• Exposure to unusually high levels of x-rays

• Contact with certain chemicals-arsenic (miners, sheep shearers, and farmers), hydrocarbons in tar, oils, and soot (may cause squamous cell carcinoma)

The following people are at the greatest risk:

• People with fair skin, especially types that freckle, sunburn easily, or become painful in the sun

• People with light (blond or red) hair and blue or green eyes

• Those with certain genetic disorders that deplete skin pigment such as albinism, xeroderma pigmentosum

• People who have already been treated for skin cancer

• People with numerous moles, unusual moles, or large moles that were present at birth

• People with close family members who have developed skin cancer

• People who had at least one severe sunburn early in life

Basal cell carcinomas and squamous cell carcinomas are more common in older people. Melanomas are more common in younger people. For example, melanoma is the most common cancer in people 25-29 years of age.