Tree Shade is Cool, but How Well Does it Protect You From UV Rays?
(HealthDay News) – With Spring here, there's little doubt that activity for many will soon switch from the house to the outdoors.
And when you start to feel warm and sticky, there's also a very good chance that you'll head for the nearest shade tree to cool off.
Nothing wrong with that. Except that, while the shade from the branches of your favorite tree will help cool you off, they may not protect you all that well from the sun's ultraviolet (UVB) rays, the ones that cause the most skin damage.
Purdue University researchers 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 used several elements to determine UVB exposure, including a particular location's altitude, latitude, time of day, and tree cover. The research first appeared in the journal 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.
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 62,480 new cases of melanoma will be diagnosed in the United States in 2008, says the American Cancer Society (ACS). About 8,110 melanoma deaths are predicted in 2008, according to the ACS, almost 4 times more than the other types of skin cancer.
On the Web
The American Cancer Society Web site has more about all types of skin cancers.
SOURCES:
Richard H. Grant, Ph.D., professor, applied meteorology, Purdue University, West Lafayette, Ind.; April 2002 Photochemistry and Photobiology
Author:
Robert Preidt, HealthDay Reporter
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