Causes
Like melanoma, basal and squamous cell cancers are linked to the sun. Most people get the majority of their lifetime sun exposure during childhood, and it's been shown that even one childhood sunburn increases the risk of developing skin cancer later in life. But a sunburn is not needed to damage the skin. A tan is also clear evidence of ultraviolet (UV) skin damage.
People with fair skin, blonde or red hair, freckles, blue or green eyes, or have difficulty tanning are at higher risk of skin cancer because they have less skin pigmentation and thus less protection from the sun.
Tumors generally grow when a normal cell suffers a mutation in its DNA, causing it to multiply without the usual restrictions. It's now believed that in basal cell carcinoma, a gene called PTC is damaged by UV radiation. This gene normally causes the cell to produce a protein that prevents runaway growth. A similar scenario may occur in squamous cell carcinoma.
Other causes of skin cancer include X-rays, skin contact with arsenic or radium, and possibly simple bad luck in that an error can occur spontaneously in a dividing cell despite low sun exposure. A sexually transmitted cancer-causing virus called the human papillomavirus (HPV) can cause a rare subtype of squamous cell carcinoma.