Center for Disease Control
Published June 20, 2007
Q: When do I need to protect myself from sun exposure?
A: Protection from sun exposure is important all year round, not just during the summer or at the beach. UV rays can cause skin damage during any season or temperature.
Relatively speaking, the hours between 10 a.m. and 4 p.m. during daylight savings time (9 a.m. - 3 p.m. during standard time) are the most hazardous for UV exposure in the continental United States. UV radiation is the greatest during the late spring and early summer in North America.
UV rays reach you on cloudy and hazy days, as well as bright and sunny days. They will also reflect off any surface like water, cement, sand, and snow.
Q: What exactly are "ultraviolet rays"?
A: Ultraviolet (UV) rays are a part of sunlight that is an invisible form of radiation. UV rays can penetrate and change the structure of skin cells.
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. Scientists believe that UVA radiation can cause damage to connective tissue and increase a person's risk for developing skin cancer.
UVB rays are less abundant at the earth's surface than UVA because a significant portion of UVB rays is absorbed by the ozone layer. UVB rays penetrate less deeply into the skin than do UVA rays, but also can be damaging.
UVC radiation is extremely hazardous to skin, but it is completely absorbed by the stratospheric ozone layer and does not reach the surface of the earth.
Q: What can excessive exposure to UV rays do to my health?
A: UV exposure appears to be the most important environmental factor in the development of skin cancer and a primary factor in the development of lip cancer.
Although getting some sun exposure can yield a few positive benefits, excessive and unprotected exposure to the sun can result in premature aging and undesirable changes in skin texture. Such exposure has been associated with various types of skin cancer, including melanoma, one of the most serious and deadly forms. UV rays also have been found to be associated with various eye conditions, such as cataracts.
Q: What is the UV Index?
A: The UV Index was developed by the National Weather Service and the Environmental Protection Agency. It provides a forecast of the expected risk of overexposure to UV rays and indicates the degree of caution you should take when working, playing, or exercising outdoors.
The UV Index predicts exposure levels on a 0-10+ scale, where 0 indicates a low risk of overexposure and 10+ means a very high risk of overexposure. 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.
Q: What does a suntan indicate? Why does the skin tan when exposed to the sun?
A: The penetration of UV rays to the skin's inner layer results in the production of more melanin. That melanin eventually moves toward the outer layers of the skin and becomes visible as a tan. Some physicians consider the skin's tanning a response to injury because it appears after the sun's UV rays have killed some cells and damaged others.
Q: Not everyone burns or tans in the same manner. Are there ways to classify different skin types?
A: Whether individuals burn or tan depends on a number of factors, including their skin type, the time of year, and the amount of sun exposure they have received recently. Though everyone is at risk for damage as a result of excessive sun exposure, people with skin types that easily burn are at the highest risk.
Q: Does it matter what kind of sunscreen I use?
A: Sunscreens come in a variety of forms such as lotions, gels, and sprays, so there are plenty of different options. Regardless of the type of sunscreen you choose, be sure that you use one that blocks both UVA and UVB rays and that it offers at least SPF 15. Also, check the sunscreen's expiration date. Sunscreen without an expiration date has a shelf life of no more than three years. Exposure to extreme temperatures can shorten the expiration date or shelf life of sunscreen.
Q: What does a sunscreen's SPF rating mean?
A: Sunscreens are assigned a Sun Protection Factor (SPF) number according to their effectiveness in offering protection from UV rays. Higher numbers indicate more protection. As a rule of thumb, you should always use a sunscreen with at least SPF 15.
Q: Do sunscreens need to be reapplied during the course of a day?
A: You should follow the manufacturer's directions regarding reapplication or you risk not getting the protection that you might think you are getting. Though recently developed sunscreens are more resistant to loss through sweating and getting wet than previous sunscreens were, you should still reapply frequently, especially during peak sun hours or after swimming or sweating.