Ultraviolet Index: What You Need to Know

Updated February 21, 2017 | Factmonster Staff
Source: American Academy of Dermatology

Did you know that overexposure to the sun can cause skin and eye injury? While some sunlight is necessary, too much is dangerous, causing sunburn, premature aging of the skin, skin cancer, cataracts, allergies, and damage to the immune system. Though the average person gets 50% of his or her lifetime sun by the age of 18, everyone needs to be aware of the dangers of exposure to sunlight.

The ill effects of sunlight are caused by ultraviolet (UV) radiation. These invisible rays from the sun come in two types, called UVA and UVB, both of which cause sun-related skin damage.

What Is the UV Index?

The Ultraviolet (UV) Index, developed in 1994 by the National Weather Service (NWS) and the US Environmental Protection Agency (EPA), helps Americans plan outdoor activities to avoid overexposure to UV radiation and thereby lower their risk of adverse health effects. EPA and NWS report the Index as a prediction of the UV intensity at noon, though the actual UV level rises and falls as the day progresses.

Previously the UV Index was reported on a scale of 0 to 10+, with 0 representing “Minimal” and 10+ representing “Very High.” The new global scale (see below) now uses a scale of 1 (representing “Low”) to 11 and higher (representing “Extreme”), a new color scheme, revised exposure categories, and different breakpoints between exposure categories. (A UV Index of “0” is still possible, but there is no corresponding health message because there either is no UV at that level or the amount is trivially small.)

Index Number Exposure Level
< 2 Low
3–5 Moderate
6–7 High
8–10 Very High
11+ Extreme

Always take precautions against overexposure, and take special care whenever the UV Index is 5 and above.

How Much Sun Am I Getting?

Though the amount of UV radiation to which you are exposed varies with the time of day, season, latitude, and altitude, it can also be increased depending on your immediate environment. Clouds actually provide little protection from UV rays, and clouds, water, white sand, concrete, and snow all reflect UV rays and increase exposure. Of course, exposure to the midday sun or for long periods of time is most damaging.

What Role Does Ozone-Layer Depletion Play?

The ozone layer shields the earth from the sun's harmful UV rays. Over the past ten years, scientists worldwide have recorded decreasing levels of ozone in the atmosphere. Less ozone means that more UV radiation reaches earth, increasing the danger of sun damage. The cause of the ozone depletion is under debate, but scientists agree that future levels of ozone will depend upon a combination of natural and man-made factors, including the phase-out of chlorofluorocarbons and other ozone-depleting chemicals.

Effects of Sun

Sunburn. Overexposure to the sun can happen in just a few hours. A bad reaction includes tenderness, pain, swelling, and blistering, and may include fever, chills, and nausea. While there is no cure for sunburn, wet compresses, cool tub baths, and soothing lotions may help. If you have a bad burn, see your dermatologist.

Tanning. Some people think that a tan means good health and looks. Dermatologists know that a tan does not prevent sun damage, it is sun damage. Tanning occurs when the UV rays penetrate the skin and injure the pigment cells. The effects are cumulative, and with every burn, the skin becomes more damaged.

Premature wrinkling. People who work or lay in the sun without sufficient protection get sagging cheeks and deep wrinkles that may make them look much older. The sun can also cause unsightly red, yellow, gray, or brown spots and scaly growths that may develop into skin cancer.

Skin cancer. Skin cancer is caused by too much sun, both long-term exposure and bad sunburns. More than 90 percent of all skin cancers occur on parts of the body exposed to the sun. The face, neck, ears, forearms, and hands are the most common places for skin cancer to develop.

The three main types of skin cancer are basal cell, squamous cell, and melanoma.

Basal cell carcinoma usually appears as a small, shiny, fleshy nodule on the exposed parts of the body. It grows slowly and rarely spreads to other parts of the body; but it can severely damage skin around and below it. When diagnosed and treated early, it has a high cure rate.

Squamous cell carcinoma typically develops on the face, ears, lips, and mouth, beginning as a red scaly patch. It also has a high cure rate when detected and treated early but left untreated, squamous cell carcinoma can spread to other areas of the body and can be fatal.

Melanoma is the most dangerous form of skin cancer and usually appears as a dark brown or black lump with irregular edges. Sometimes, it is multicolored with shades of red, blue, or white. If ignored, melanoma can spread or metastasize to other areas of the body, which can be fatal.

Eye damage. The sun can cause cataracts and other eye damage. Cataracts are one of the leading causes of blindness.

Immune system suppression and disease. Short periods of sun exposure can damage the human immune system and make the body more susceptible to infections and cancers. Also, some diseases can become worse with sun exposure. These include herpes simplex (cold sores), chicken pox, lupus, and certain genetic problems.

What Are Proper Precautions?

Preventing skin cancer and eye damage. Skin cancer is increasing faster than any other form of cancer, with over 1 million new cases predicted to occur in the U.S. this year.

  • Listen to the UV Index reports.
  • Minimize sun exposure at midday (10:00 a.m. to 4:00 p.m.)
  • Avoid sunlamps, tanning beds and tanning parlors.
  • Apply a broad-spectrum sunscreen with Sun Protection Factor-15 or higher and reapply every 2 hours.
  • Wear protective, tightly-woven clothing, a broad-brimmed hat and sunglasses. Children who will not wear sunglasses should wear a hat with a wide brim.
  • Protect children by keeping them indoors between 10:00 a.m. and 4:00 p.m., and by applying sunscreen to children older than 6 months. Children under the age of 6 months should be kept out of the sun.

Need More Information?

For more information on the UV Index, please call the EPA Stratospheric Ozone Hotline, 800-296-1996, or the National Weather Service, 301-713-0622.

Sources +