Sun Spots in the Shade

Computer model predicts ultraviolet protection offered by trees

MONDAY, April 29, 2002 (HealthDayNews) -- It's a summer ritual: You grab a good book and amble over to the lawn chair tucked under a big, shady tree. You feel you can spend hours there without worrying about sunscreen or wearing a hat.

Well, you may have to turn the page on that notion. It seems your leafy refuge may not offer you all that much protection against skin-cancer causing ultraviolet(UVB) radiation. It really depends on a number of factors.

Purdue University researchers have developed a computer model to predict how much UVB radiation people receive under different types and amounts of tree cover. The model shows that while trees do provide some shielding, they aren't always the ultimate oasis.

"The attitude is that tree shade produces a lot of protection because it's blocking the sun. But in reality, tree shade is not protecting you as much as you think it is," says research team leader Richard H. Grant, a professor of applied meteorology at the university.

The research model uses several elements to determine UVB exposure, including a particular location's altitude, latitude, time of day, and tree cover. The research appears in the current issue of Photochemistry and Photobiology.

Depending on the tree species, the number of trees around your reading spot, reflection from buildings and other factors, the shade beneath the trees may offer you the equivalent of sunscreen protection anywhere between 2.5 and 15 sun protection factor (SPF).

For comparison, the American Cancer Society recommends you use a sunscreen with a minimum of SPF 15.

Sure, that big tree shadow means you're not subjected to the direct rays of the sun. However, it may not stop all the UVB radiation.

For example, if you stand in what most people consider a shaded area, you're still being hit with 40 percent to 60 percent of the UVB exposure you'd receive in full sunlight.

"Therein lies the risk. You have a perception that you're safe because that's what your eyes see. But you're not as safe as you think you are," Grant says.

Grant can't offer exact UVB protection values for different species of trees. But he says trees with dense foliage, such as maple or sweet gum, afford better sun sanctuary than more open trees such as honey locust.

"If you can see significant (amounts of) sky through the tree, it's relatively porous to the (UV) radiation, and so you're going to have a higher radiation environment under that tree," Grant says.

And the more trees the better. For example, if you have a cluster of trees in your backyard and the grass beneath them doesn't grow -- that indicates a fairly decent haven from UVB.

While he stresses that you still need sunscreen and hat when you're beneath any kind of tree, Grant says trees certainly do help reduce UVB exposure and the risk of skin cancer.

He and his colleagues say their model could help city planners understand how to make the best use of trees. For example, the model could predict UVB exposure in different areas of a city park or the playground of a day-care center.

"From distribution of trees and relative sizes of trees in those locations, we can predict what the radiation is for people who are walking, playing and doing things in that environment," Grant says.

He also says that the greatest benefit of using this type of computer modeling would be at higher latitudes. American cities in that range would include Detroit and Juneau, Alaska, and their exposure to a lower angle of the sun results in more UVB radiation in the sky.

Skin cancer is the most common cancer in the United States, and one of the deadliest. Melanoma is the most serious skin cancer, causing 75 percent of skin cancer deaths. About 53,600 new cases of melanoma will be diagnosed in the United States in 2002, a 4 per cent increase over last year, says the American Cancer Society. About 7,400 Americans will die from melanoma this year.

About 1.3 million cases of nonmelanoma skin cancers will be diagnosed in the United States this year, and 2,200 people will die from them.

What To Do

Go to the American Cancer Society to learn more about melanoma and nonmelanoma skin cancers.

Skin cancer isn't the only danger from too much UVB exposure. Cataracts and a suppressed immune system are other effects. Learn more here.

UVB radiation is the most dangerous form of UV. To find out more about it and other types of UV radiation, go to Long Island University.

SOURCES: Richard H. Grant, Ph.D., professor, applied meteorology, Purdue University, West Lafayette, Ind.; April 2002 Photochemistry and Photobiology
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