Understanding Ultraviolet Rays

The ultraviolet portion of sunlight is an invisible form of radiation that can penetrate and change the structure of skin cells. Exposure to UV rays has been associated with the development of serious diseases, including cancer. In fact, UV exposure appears to be the single most important environmental factor in the development of skin cancer, and the primary factor in the development of lip cancer. Eye damage, such as cataracts, has also been linked to UV rays.

There are three types of UV rays: ultraviolet A (UVA), ultraviolet B (UVB), and ultraviolet C (UVC).

UVA is the most abundant source of solar radiation at the earth's surface and penetrates beyond the top layer of human skin. It is UVA that is believed to cause damage to connective tissue and increase your risk of developing skin cancer.
UVB rays are less abundant on the earth's surface because a significant portion of UVB is absorbed by the ozone layer. UVB rays do not penetrate as deeply as UVA rays, but they are still capable of damaging skin tissue.
UVC is extremely hazardous to you skin, but it is completely absorbed by the ozone and does not reach the earth.

The National Weather Service and the Environmental Protection Agency provides a forecast of the expected risk of overexposure to UV from the sun and indicates the degree of caution you should take when outdoors. This is called the UV Index. The UV Index predicts exposure levels on a scale of 1 to 10, with 0 being the lowest risk and 10 the highest. Calculated on a next-day basis for dozens of cities across the U.S., the UV Index takes into account clouds and other local conditions that affect the amount of UV radiation reaching the ground.