Skin cancer is the most common form of cancer. Exposure to the ultraviolet radiation in the sun's rays is responsible for almost all cases of basal cell carcinoma and squamous cell carcinoma. Repeated sunburns are a probable important cause of malignant melanoma. Basal cell and squamous cell carcinomas account for more than 90 percent of all new cases. Of the three, however, malignant melanoma is the most serious and is responsible for approximately 68,000 new cases and 8,700 deaths each year in the United States. Over the past two decades, the incidence of melanoma in the white population has doubled.

Ultraviolet radiation is made up of wavelengths shorter than those found in the visible spectrum. This means that even when the sun's rays are not particularly bright (on a hazy day, for example), ultraviolet radiation still is reaching you. Two types of ultraviolet radiation bands, increase the risk of cancer: Ultraviolet A (UVA), which is not absorbed by ozone, and Ultraviolet B (UVB). UVB is especially damaging to the cells' DNA, and that damage can lead to cancer. UVB can be absorbed by the ozone, but as the ozone layer becomes depleted, the cancer risk posed by UVB rays will become greater.

• Limit sun exposure to less than 30 minutes per day, enough exposure to have your skin activate vitamin D.
• Avoid even minor sunburns.
• When you do go in the sun for limited periods of time, try to avoid doing so in the middle of the day (between 10 a.m. and 3 p.m.) when the rays are the strongest.
• Pay attention to the UV index, which often is included in the weather report on television or in newspapers. This index is a relative measure of how damaging exposure to the sun will be on any particular day. The index is a scale of 1 to 10+. An index value below 5 means that exposure to UV will be low, 5 to 6 means moderate exposure, and 7 or higher indicates a dangerously high level of exposure. When the index is 10 or higher, it usually is recommended that people stay indoors if possible.
• Cover up to minimize the UV rays that reach your skin. Wear hats that shade the face and the back of the neck, long-sleeve shirts, and long pants if possible. (The thicker the weave, the more protection you'll get.)
• Wear a sunscreen with a sun protection factor (SPF) of at least 30 that work against both UVA and UVB rays. Remember to reapply sunscreen after sweating or swimming, and periodically throughout the day. Follow directions on the package.
• Avoid tanning booths. They use UV rays, and recent studies have shown that they increase your risk of cancer.
• Wear UV light-filtering sunglasses to protect eyelids and lens.

Smoking is estimated to be a factor in approximately 30 percent of all cancer deaths in the United States, and it causes about 85 percent of lung-cancer deaths.

The risk of cancer from smoking is a dose-response relationship, meaning that the greater your exposure to cigarette smoke, the greater your risk of developing cancer, although there is no safe dose for tobacco exposure. The age at which smoking began, how long smoking has continued, and the number of cigarettes smoked per day all play a part in determining the specific risk for an individual. Risk drops for all smoking-related diseases after quitting, although that drop varies from disease to disease.

Cigarettes have been linked with cancer since 1950, and hundreds of studies have confirmed that cigarette smoking causes not only lung cancer, but cancer of the mouth, throat, esophagus, bladder, kidney and pancreas. It also has been associated with cancers of the cervix, liver, breast, and colon and with leukemias. Smokeless tobacco has been associated with cancers of the mouth, and the smoking of pipes or cigars has been shown to cause cancer of the mouth, throat and lungs. Tobacco use along with heavy alcohol consumption has been identified as an important risk factor for head, neck and esophageal cancer.

Aside from its direct cancer-causing effects, cigarette smoking also enhances the cancer-causing abilities of other factors. For example, people who work with asbestos are five times as likely to develop lung cancer as those who do not work with asbestos, but those who work with asbestos and smoke have a risk 90 times greater than those who are not exposed to asbestos and do not smoke.

The only preventive measure is not to smoke. A smoker's risk of developing cancer decreases after quitting, and continues to decrease every year thereafter. The preventive effect of smoking cessation is dependent on the intensity and duration of the prior smoking history. Data suggests that smoking cessation may halt the early stages of cancer, but may have little effect on the late stages of cancer. If you can't quit on your own, look for help from support groups. There are prescription and over-the-counter medications that can help you quit. Talk to your health-care provider to find out which one might be right for you.

Radon is a naturally occurring, invisible gas that is released from the decay of uranium in rocks and soil. Outside the home, radon is harmless because it diffuses into the air. Indoors, radon enters through cracks in a building's foundation and is concentrated in a much smaller area. Radon emits tiny radioactive particles when it decays, and, if the gas is inhaled, those particles can damage the lungs. Over a long period of time and with enough exposure, this can lead to lung cancer.

The association of radon to lung cancer first was made when scientists discovered that uranium miners exposed to radon were found to have much greater rates of lung cancer than miners who were not exposed to radon. Animal studies have confirmed the relationship. Cigarette smoke combined with radon creates a greater risk for lung cancer than either carcinogen alone. In fact, the majority of radon-related cancer deaths occur among smokers.

It is not clear that level of radon found in homes contributes to cancer in nonsmokers. This is an area of great controversy. Many epidemiological studies have been performed to try to settle this issue, but the results have been inconsistent. It is therefore an open question whether indoor radon contributes to lung cancer in nonsmokers.

Since radon is colorless and odorless, the only way to know if you have dangerously high levels in your home is to have the air tested. There are short-term tests that take about a week and long-term tests that measure the radon levels over a period of 90 days. Long-term tests are usually more accurate than the short-term tests, since radon levels can vary widely depending on barometric pressure, weather and other factors. Do-it-yourself tests are relatively inexpensive. They are available at most home improvement stores, or you can call a home inspector who can do the measurement for you. It is an open question whether universal radon abatement is a responsible public health recommendation.

The U.S. Environmental Protection Agency recommends taking action if the radon level in your home is 4 picocuries per liter or greater. Protective action may be as simple as installing a fan in your basement and sealing cracks in the foundation, or more extensive measures may be required. For more information, contact your state radon office, or call the National Safety Council's radon hotline (800-SOS-RADON).

The health effects of high-level exposure to ionizing radiation — such as those found at the site of a nuclear accident — are serious. For example, children who lived in the area of the Chernobyl nuclear accident, where very high levels of radiation were released into the air, have been developing thyroid cancer at a greater rate than normally would be expected.

But exposure to low-dose radiation appears to be safe, according to a 1991 National Cancer Institute (NCI) study to determine the risk of living near a nuclear facility (and potentially receiving low doses of radiation). After examining 107 counties that contained or were near 62 nuclear facilities, the NCI concluded that there was no evidence of increased risk of death from cancer among those living near a facility. Workers in nuclear facilities also are at minimal risk of developing cancer at rates in excess of those found in the general population.

Diagnostic X-rays and dental X-rays are set at very low-dose levels to minimize their risk, and studies have shown that the exposures experienced from medical diagnostic procedures are not harmful. High doses of X-rays, such as those used in radiation for cancer treatment, have been implicated in the development of secondary cancers; these higher doses have been estimated to cause about 3 percent of all secondary cancers. The secondary cancer risk from radiation therapy is higher in children and teens than in older adults.

Electromagnetic radiation has generated a lot of media attention as a potential cause of cancer, but there are few data to support this worry. Electric and magnetic fields (EMF) arise from the motion of electric charges. EMFs are emitted from devices that use, transmit or produce electric power. These fields are classified as non-ionizing radiation, because they lack sufficient power to remove electrons from atoms. Ionizing radiation, such as X-rays or gamma-rays, differ in being able to break atomic bonds and cause chromosomal change. Interest in EMFs was sparked when some epidemiological studies suggested that living near sources of electromagnetic radiation (such as power lines) was associated with increased risk of cancer. Many other epidemiological studies and some experimental data have not found an increased risk. There is no evidence, according to the latest scientific data, that electromagnetic radiation from power lines causes cancer.

In addition to the study of low-frequency (50 to 60 Hz) magnetic fields associated with electric power lines and household appliances, information is being gathered and analyzed on high frequency (800 to 900 MHz) fields emitted, for example, from handheld cellular telephones. To date, expert panels reviewing available data conclude that data is insufficient to establish magnetic fields as a cause of cancer.

Since the risk of cancer from outside radiation generally is small for most individuals, there are few preventive measures. Recommendations include:

• Do not stop having diagnostic X-rays because you fear the radiation will cause cancer. The health risk from an undetected illness is greater than the health risk from diagnostic X-rays.
• If you have cancer, do not avoid radiation therapy because you fear radiation. In many cases, radiation is the best option for potentially curing cancer. The health risk of avoiding radiation is much greater than the health risk from radiation treatment.
• If you work in a nuclear facility, follow all safety procedures.

This risk factor (also called secondhand smoke, involuntary smoking, or passive smoking) occurs with the breathing of tobacco-smoke-polluted air by nonsmokers and is responsible for an estimated 3,000 lung-cancer deaths each year. It also causes bronchitis, pneumonia, other lung diseases and cardiovascular diseases, and is an irritant to eyes, nose and throat. Children living in an environment where tobacco smoke is common suffer from increased respiratory infections, colds and asthma. Epidemiologic studies suggest about one-third of lung-cancer cases occur in nonsmokers who live with smokers, and approximately one-quarter of all lung-cancer cases can be linked to environmental tobacco smoke. Cigarette smoking produces three types of tobacco smoke: Inhaled mainstream smoke, mainstream smoke exhaled by the smoker, and sidestream smoke which is released into the air by the burning cigarette. Undiluted sidestream smoke carries many of the toxic compounds in far greater concentrations and accounts for 85 percent of the smoke present in an average room of smokers. Nonsmokers in the same room with smokers inhale tar, carbon monoxide, nicotine and other harmful compounds as smokers do, only in smaller doses. With enough exposure, and over a long enough period of time, the risk becomes significant.

The best preventive measure is not to share breathing space with a smoker.

• If you live with a smoker, try to get that person to quit smoking, or suggest that the smoking be done only outside the house.

• Nearly all states have some form of legislation to protect nonsmokers, including regulations regarding smoke-free workplaces. If your workplace is not smoke-free, ask your employer to set aside smoke-free zones or contact the National Institute for Occupational Safety and Health (1-800-35-NIOSH) for information.

• If you can't get away from the smoke, put a fan or two around your workspace to keep the air circulating.

The National Institute for Environmental Health Sciences has indicated that approximately 5 percent of the 75,000 chemicals in commercial use "might be reasonably expected to be carcinogenic to humans." Many chemicals used every day in building materials, cleaning agents, paint solvents and other substances have been proven to cause cancer in humans or are probable human carcinogens, based on animal studies. These include:

• Benzene, a gasoline additive found in paints, paint thinners, adhesives and plastics. You can be exposed to benzene fumes while pumping gas.
• Asbestos, the name given to a variety of mineral filters, was used widely in building materials. It is estimated that nearly 30 million tons have been used in the United States since the turn of the century. Although phased out in U.S. construction, older buildings might have asbestos — especially as insulation around furnaces and hot-water pipes. It also is found in acoustic ceiling tiles, floor tiles, textured paint, exterior siding and appliances. Asbestos is only dangerous when its fibers become loose or when the material crumbles, which causes small particles to be set free and inhaled. Once inhaled, the microscopic fibers remain in the body forever. Asbestos can cause lung and bowel cancer, mesothelioma, asbestosis and other lung diseases. Smoking substantially increases the risk of lung cancer in people exposed to asbestos.

Concern about increased cancer risk has been raised but not proven for:

• Vinyl chloride and its derivative polyvinyl chloride (PVC) in the environment. These plastic resins found in many products, including pipes, hoses, flooring, windows and credit cards. There is greater risk to workers in these industries who manufacture and handle these products daily. High levels also are found in the materials used in the interiors of new cars, making that "new-car smell" potentially dangerous.
• Pesticides. While pesticide levels in foods are relatively harmless, pesticides used in your home or on your lawn may be dangerous because of the quantities used. In some studies, farmers with high exposure to pesticides were found to have a higher risk of several different cancers, including leukemia and non-Hodgkin's lymphoma. Pesticide residues on fruits and vegetables are considered negligible and pose almost no cancer danger, according to an expert panel which reviewed 50 published studies through 1996.
• Formaldehyde. This can be a component of many products including cosmetics, paper, textiles and drugs. The most prominent use in the home is in the glue in wood paneling or in urea-formaldehyde foam insulation, which might be present in homes built before 1982. Formaldehyde initially was found to cause nasal cancer in rats. Since then, there has been considerable controversy as to the role of formaldehyde in causing cancer in humans. In 1989 the EPA classified formaldehyde as a "probable human carcinogen" under conditions of unusually high or prolonged exposure.
• Other chemicals to watch out for include chloroform, trichloroethylene, tetrachlorethylene and dichlorobenzene. These may be found in solvents, cleaning products or deodorizers.

To protect against exposure to these products, read labels and learn what substances are in the products you use. According to the EPA, you can reduce your exposure to cancer-causing agents in the home by minimizing your use of solvents, cleaning products or deodorizers containing any of the chemicals listed above.

If you are not sure if your home contains asbestos, call a licensed asbestos contractor for advice, especially if you suspect asbestos is crumbling in your home. Do not try to remove this yourself. Call a licensed asbestos contractor. The EPA can provide a list of laboratories and approved contractors if you call them at 1-800-368-5888.

Studying the effects of air pollution on cancer risk is very difficult because, while everyone breathes air, they also smoke, eat fatty foods, and come in contact with other contaminants. It is nearly impossible to control for all those factors to get a clear picture of the effects of air pollution alone. In Western industrialized countries, which have pollution regulations, air pollution poses only a very small risk for developing cancer. In heavily polluted areas — environments that are reported to be "smoky" — there is a slight increased risk of developing lung cancer, and the risk is likely to be greater in developing countries where pollution remains relatively uncontrolled.