Ultraviolet Radiation

Absorption, Distribution, Metabolism

Absorption

Two main types of UV radiation reach the earth, UVA and UVB. Scientists now believe that both UVA and UVB rays contribute to skin damage, including skin cancer. UVB radiation is known to cause damage to the DNA of skin cells. Skin cancers develop when this damage affects the DNA of genes that control growth and division of skin cells. Recent research has found that UVA also contributes to skin cancer formation.

The sun radiates energy over a broad spectrum of wavelengths. Ultraviolet (UV) radiation, which has a shorter wavelength than either visible blue or violet light, is responsible for sunburn and other adverse health effects. Fortunately for life on Earth, our atmosphere's stratospheric ozone layer shields us from most UV radiation. What gets through the ozone layer, however, can cause the following problems, particularly for people who spend substantial time outdoors:

• Skin cancer
• Suppression of the immune system
• Cataracts
• Premature aging of the skin
  

Because of these serious health effects, you should limit your exposure to UV radiation and protect yourself when outdoors.

UVB is typically the most destructive form of UV radiation because it has enough energy to cause photochemical damage to cellular DNA, yet not enough to be completely absorbed by the atmosphere. UVB effects can include erythema (sunburn), cataracts, and development of skin cancer. Individuals working outdoors are at the greatest risk of UVB effects. Most solar UVB is blocked by ozone in the atmosphere, and there is concern that reductions in atmospheric ozone could increase the prevalence of skin cancer.

The photochemical effects of UV radiation can be exacerbated by chemical agents including birth control pills, tetracycline, sulphathizole, cyclamates, antidepressants, coal tar distillates found in antidandruff shampoos, lime oil, and some cosmetics. Protection from UV is provided by clothing, polycarbonate, glass, acrylics, and plastic diffusers used in office lighting. Sun-blocking lotions offer limited protection against UV exposure.

Distribution

The incidence of skin cancer in the United States has reached epidemic proportions. One in five Americans will develop skin cancer in their lifetime, and one American dies every hour from this devastating disease. Medical research is helping us understand the causes and effects of skin cancer. Many health and education groups are working to reduce the incidence of this disease, of which 1.3 million cases had been predicted for 2000 alone, according to The American Cancer Society.

Melanoma

Melanoma, the most serious form of skin cancer, is also one of the fastest growing types of cancer in the United States. Many dermatologists believe there may be a link between childhood sunburns and melanoma later in life. Melanoma cases in this country have more than doubled in the past 2 decades, and the rise is expect-ed to continue.

Nonmelanoma Skin Cancers

Nonmelanoma skin cancers are less deadly than melanomas. Nevertheless, left untreated, they can spread, causing disfigurement and more serious health problems. More than 1.2 million Americans will develop nonmelanoma skin cancer in 2000 while more than 1,900 will die from the disease. There are two primary types of nonmelanoma skin cancers, basal cell carcinomas, and squamous cell carcinomas. These two cancers have a cure rate as high as 95 percent if detected and treated early. The key is to watch for signs and seek medical treatment.

Basal cell carcinomas are the most common type of skin cancer tumors. They usually appear as small, fleshy bumps or nodules on the head and neck, but can occur on other skin areas. Basal cell carcinoma grows slowly, and rarely spreads to other parts of the body. It can, however, penetrate to the bone and cause considerable damage.

Squamous cell carcinomas are tumors that may appear as nodules or as red, scaly patches. This cancer can develop into large masses, and unlike basal cell carcinoma, it can spread to other parts of the body.

Skin Damage

Other UV-related skin disorders include actinic keratoses and premature aging of the skin. Actinic keratoses are skin growths that occur on body areas exposed to the sun. The face, hands, forearms, and the "V" of the neck are especially susceptible to this type of lesion. Although premalignant, actinic keratoses are a risk factor for squamous cell carcinoma. Look for raised, reddish, rough-textured growths and seek prompt medical attention if you discover them. Chronic exposure to the sun also causes premature aging, which over time can make the skin become thick, wrinkled, and leathery. Since it occurs gradually, often manifesting itself many years after the majority of a person's sun exposure, premature aging is often regarded as an unavoidable, normal part of growing older. With proper protection from UV radiation, however, most premature aging of the skin can be avoided.

Cataracts and Other Eye Damage

Cataracts are a form of eye damage in which a loss of transparency in the lens of the eye clouds vision. If left untreated, cataracts can lead to blindness. Research has shown that UV radiation increases the likelihood of certain cataracts. Although curable with modern eye surgery, cataracts diminish the eyesight of millions of Americans and cost billions of dollars in medical care each year. Other kinds of eye damage include pterygium (i.e., tissue growth that can block vision), skin cancer around the eyes, and degeneration of the macula (i.e., the part of the retina where visual perception is most acute). All of these problems can be lessened with proper eye protection from UV radiation.

Immune Suppression

Scientists have found that overexposure to UV radiation may suppress proper functioning of the body's immune system and the skin's natural defenses. All people, regardless of skin color, might be vulnerable to effects including impaired response to immunizations, increased sensitivity to sunlight, and reactions to certain medications. http://www.epa.gov/sunwise/uvandhealth.html

Metabolism

DNA is the genetic material in our cells. It passes along genetic information to the next generation that makes children resemble their parents. In addition to information about our outward appearance, DNA also contains information that tells the cells of our body how to grow and how to perform all the activities needed for life.

Damage to DNA can be caused by UV radiation (from sunlight or tanning lamps), by ionizing radiation (a form of radiation that includes x-rays), and by certain chemicals. This damage makes the DNA less able to control how and when cells grow and divide. In some situations, this results in the start of a cancer.

Researchers have found that many skin cancers contain changes in one of two genes. When one of these, called ptc, is damaged it allows for the stimulation of cell growth. The second, called p53, normally causes damaged cells to die. When this gene is damaged, these abnormal cells will live and perhaps go on to become cancerous.

This theory explains at least in part the connection between sunlight and skin cancer. It also explains why people with xeroderma pigmentosum (XP) have such a high risk for skin cancer. XP is a rare, inherited condition resulting from a defect in an enzyme that repairs damage to DNA. Because people with XP are less able to repair DNA damage caused by sunlight, they develop huge numbers of cancers on sun-exposed areas of their skin.

Scientists are studying some other connections between DNA and skin cancer. They have found the DNA of certain genes in skin cancer cells is often damaged, usually as a result of overexposure to sunlight. In the future, better understanding of how damaged DNA leads to skin cancer might be used in designing treatments to overcome or repair that damage.

The association between nonmelanoma skin cancer and human papillomavirus infection also involves DNA and genes. These viruses contain genes that instruct infected cells to produce certain proteins that interact with the growth-regulating proteins of normal skin cells. These viral proteins can cause skin cells to grow too much and can also interfere with processes that normally destroy cells with irreparable DNA damage.

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