The most common risk factors for skin cancer are as follows.

• Ultraviolet light exposure, either from the sun or from tanning beds. Fair-skinned individuals with a history of repeated sunburns, those with hazel or blue eyes, and people with blond or red hair are particularly vulnerable. The problem is worse in areas of high altitude or near the equator where sunlight exposure is more intense.
• A chronically suppressed immune system (immunosuppression) from underlying diseases such as HIV/AIDS infection or cancer, or from some medications such as prednisone or chemotherapy
• Exposure to ionizing radiation (X-rays) or chemicals known to predispose to cancer such as arsenic
• Certain types of wart virus infections
• People who have a history of one skin cancer have a 20% chance of developing a second

Skin cancer is a common, usually low-grade cancerous (malignant) growth of the skin. It starts from cells that begin as normal skin cells and transform into those with the potential to reproduce in an out-of-control manner. Unlike other cancers, the vast majority of skin cancers have no potential to spread to other parts of the body (metastasize) and become life-threatening.

• There are two major types of skin cancer: basal cell carcinoma (the most common) and squamous cell carcinoma (the second most common). Melanoma is also a form of skin cancer but is far less common, though more dangerous, than the other two varieties.
• The exact cause of skin cancer is unknown. It appears basal cell skin cancers arise from basaloid cells in the upper layer of the skin. Uncontrolled growth of these cells is regulated by other factors in the skin. When that regulation is lost, skin cancer cells begin to grow into tumors.
• In squamous cell skin cancers, the tumors arise from a normal cell in the top layer of the skin, the epidermis. As with basal cell cancers, these cells are prevented from growing wildly by genetically controlled factors. When there is an alteration in the genes that regulate these cells, the control is lost and skin cancers start to grow. In many instances, the genes are altered by sunlight exposure.