The human eye is sensitive to wavelengths of light from about 400 nanometers, or nm (violet light) to about 700 nm (red light). Light with a wavelength shorter than 400 nm is called ultraviolet light or ultraviolet radiation. A nanometer is 1/1,000,000,000 of a meter or 10^-9 meters.

Ultraviolet light is often called ultraviolet radiation. The word “radiation” does not refer to anything related to radioactivity; it is the scientific term applied to all forms of electromagnetic energy, including visible light, infrared light, or ultraviolet light. These forms could equally be referred to as visible radiation, infrared radiation, and ultraviolet radiation.

Ultraviolet light is produced by the Sun and penetrates into the atmosphere. Ozone in the upper atmosphere completely absorbs the shortest wavelength type UV light, called UVC. The UV range is:

UVA 315-400 nm
UVB 290-315 nm
UVC 220-290 nm

UVB is partially absorbed by the atmosphere, but is very damaging to cells. The longer wavelength UV A 360-400 nm is emitted by so-called “black lights." This light is not thought to be particularly harmful, though wavelengths shorter than black light emissions may be dangerous.

The amount of UVB light that reaches the ground depends on the elevation of the Sun (the higher it is in the sky, the more UVB), the amount of ozone in the atmosphere (more is better), and the degree of cloudiness of the sky. Clouds must be relatively thick to appreciably block UVB.

Because of the great potential for sunburn from ultraviolet light (especially UVB) an ultraviolet index has been devised to label UV energy exposure from the minimal to very high. The UV Index is categorized by the Environmental Protection Agency as follows:

The UV index can be calculated for local noon (1 p.m. Daylight Savings Time)

For example, on a summer day in late August, the UV index was 12, very high, for Honolulu, Hawaii and San Juan, Puerto Rico. It was low, at a value of 1, for only Anchorage, Alaska.

Daily values for major cities are given every day at the United States Environmental Protection Agency's Ultraviolet Index Web site.

Some basic strategies for avoiding Sun damage and aging to skin, as well as reducing the risk of skin cancer are:

• Limit your time in the sun between 10 a.m. and 4 p.m. These are the brightest times of the day.
• Wear a wide-brimmed hat and if possible, tightly woven, full-length clothing.
• Use a broad-spectrum sunscreen with an SPF of at least 15.
• Seek shade whenever possible.
• Wear UV-protective sunglasses (check the tag). Polycarbonate lenses block UV.
• Avoid sunlamps and tanning salons.
• Check the UV Index for your location daily.

Sun damage to skin is not hard to assess. Parts of your body that are not exposed to the Sun have a very different appearance and texture than parts that are exposed regularly or tanned.