Causes
Like melanoma, basal and squamous cell cancers are linked to sun exposure
early in life. Most people get the bulk of their 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 UV (ultraviolet light) skin damage.
In the United States, skin cancer is becoming more common in people under
40. The sunbathing responsible for today's skin tumors mostly happened 20,
30, or 40 years ago. In Australia, where people are exposed to a great deal of ultraviolet
light from an early age, skin cancer is a disease that strikes people in their
20s and 30s.
In the southern hemisphere, the changes in the ozone layer are far more dramatic
and are already affecting skin cancer rates. In Australia, where beach culture
has grown at the same time as the ozone has been depleted, skin cancer rates
have increased greatly.
People with fair skin, blonde or red hair, freckles, blue or green eyes, or
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 can cause a rare subtype of squamous
cell carcinoma.