Looking at SKIN CANCER in a Different Light - causes of skin cancer

USA Today (Society for the Advancement of Education), May, 2000 by Darrell E. Ward

Think twice before you set out to get a suntan this summer.

IT IS DIFFICULT to imagine today, but for hundreds of years in Europe and the U.S., a suntan branded one as an inferior person. A bronzed face and hands were a sign of the lower classes--those who labored under the sun.

The Caucasian ideal of male and female beauty during that time was of milky white, alabaster skin. Like the individuals who populate Georges Seurat's famous 1884 painting, "A Sunday on La Grande Jatte," people hugged the shade and avoided the sun.

Then came Coco Chanel, the 1920s French fashion designer who gave women's apparel a freer, more comfortable style. Everything Chanel did set a standard. So, when she returned one day from a Palm Beach vacation with a suntan the bronzed look suddenly went from shunned to chic.

The passion for suntans further was nurtured by improvements in transportation that allowed seaside and mountain vacations and by the popularity of motion pictures. More leisure time, a burgeoning interest in outdoor activities, and the growth of suburbs with their lawns, gardens, and swimming pools all contributed to what has become an annual ritual to achieve the golden-brown tint of success, sexiness, beauty, and health.

Well, the latter is not necessarily true. For five decades, physicians have seen growing numbers of the three common skin cancers: basal cell carcinoma, squamous cell carcinoma, and malignant melanoma. In 1978, doctors diagnosed between 400,000 and 500,000 new cases of skin cancer; this year, over 1,000,000 new cases are expected. More than 90% of these cancers are thought to be caused by overexposure of the skin to ultraviolet (UV) radiation in sunlight. (Burn scars and radiation therapy for acne are also linked to non-melanoma skin cancers.)

Those most vulnerable to skin cancer are people with red or blond hair, light-color eyes, and fair skin that tends to freckle and burn, rather than tan. Knowing why this is so and how UV light causes cancer is to understand why people of all races--particularly those with light or pale skin--should protect themselves and their children from prolonged sun exposure.

Humans need a certain amount of sunlight for emotional and physical health. It is necessary for the production of vitamin D in the skin, for example. However, experiments in the 1920s and 1930s revealed that the UV radiation in sunlight leads to cancer.

Invisible to the human eye, UV radiation follows the violet band of the rainbow. Ultraviolet light ranges in wavelength from 200 to 400 nanometers (nm). Scientists divide this range into three groups: UVA (320-400 nm); UVB (290-320 nm), and UVC (200-290 nm).

When a beam of sunlight strikes the Earth, it first penetrates the atmosphere. About 30 miles above the planet's surface, it enters the ozone layer. When it comes out the other side, some 12 miles up, about half the UV radiation in the beam has been stripped away, absorbed by ozone molecules.

As a result, nearly no UVC and very little UVB--the most energy-packed UV wavelengths --reach the Earth's surface. What does reach the surface--and the people working and playing on it--is some UVB, the rays that cause sunburn and are most responsible for cancer, and most of the UVA, the longest of the ultraviolet wavelengths. UVA--the predominant radiation emitted by tanning lamps--is linked to premature wrinkling of the skin and to skin cancer.

Humans are protected from UV rays by their skin, which is composed of two main layers--the epidermis and dermis. The epidermis is the topmost and thinnest. It consists of multiple layers of cells that are in a state of constant turnover. Cells at the bottom of the stack divide, pushing the cells above them toward the surface. As these cells move outward, they flatten and gradually replace their internal organelles with large amounts of the protein keratin. In fact, most of the cells in the epidermis are known as keratinocytes. By the time these cells reach the skin's surface, they have become mere scales of keratin that eventually flake off.

Scattered among the innermost keratinocytes are the melanocytes. These cells make melanin pigment and do the actual work of protecting the body from UV radiation. They also give the skin color. Moles are islands of melanocytes.

The number of melanocytes in a person's skin helps determine his or her skin type, which is a measure of tanning ability. It is usually represented as a six-point scale and reflects the relative number of melanocytes in the skin. A dark African-American who never sunburns from normal exposure has a high proportion of melanocytes and has skin type six; someone who rarely develops sunburn and tans dark brown has skin type three; and someone who never tans and always burns has skin type one and a low proportion of melanocytes.

Beneath the epidermis lies the dermis, a relatively thick layer that consists largely of collagen and elastic fibers. These fibers give the skin its pliability and resilience. The dermis contains hair follicles, sweat and oil glands, blood and lymph vessels, and some melanocytes. When UVA penetrates the dermis, it can damage the collagen and elastic fibers, leading to wrinkling and premature aging. Most tanning equipment utilized today predominately emits UVA radiation, causing growing concern among dermatologists.