Sun in the shade?

The answer is C. Shade blocks 50% of the sun's UVB rays.

Standing in the cool shade of a tree may be a relief on a hot sunny day, but be aware: It's likely blocking only half, or less, of the sun's UVB rays that you would get standing in direct sunlight, says meteorologist Richard Grant, a professor of agronomy at Purdue University.

More than 600,000 new cases of skin cancer are reported every year, most of them attributed to the sun's ultraviolet radiation, especially UVB.

Dr. Grant recently published research in which he created a method for predicting how much radiation someone will receive depending on the amount of shade cast by a tree, along with other factors such as altitude, distance from the equator, and the time of day.

Even when a tree blocks 90 percent of the visible sky, it's equivalent to a sunscreen with only an SPF 10 protection factor, said Dr. Grant. That's less than the SPF 15 rating that experts recommend for sunscreens that they say people should use regularly. A sunscreen with an SPF rating of 15 means your skin is protected 15 times longer than if you didn't use any sunscreen.

Children, by the way, are especially vulnerable -- experts say kids who get serious sunburns may be much more susceptible to skin cancer later.

Resources

• Read more about Dr. Grant's study estimating the amount of protection trees provide against ultraviolet-B radiation. Also read Grant's explanation of ultraviolet-B light waves.
• The National UV Index tells Americans what the current UV level is in their part of the country. The service was launched by the National Weather Service, the Environmental Protection Agency, and the Centers for Disease Control.
• The Melanoma.com Web site provides recommended protective actions we can take to shield ourselves from harmful ultraviolet light, depending on the UV index.

May 18, 2002