sealPurdue News

April 22, 2002

Researchers say trees could affect land use, reduce skin cancer

WEST LAFAYETTE, Ind. – A Purdue University method to estimate the amount of protection trees provide against ultraviolet-B radiation may influence how communities are built and the incidence of skin cancer.

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"We now have a model to predict how much UV-B radiation people receive under different amounts of tree cover," said meteorologist Richard Grant, Purdue agronomy professor. "If you're in what most people consider shade, you're still getting 40 percent to 60 percent of the UV-B exposure that would hit you in direct sunlight."

Experts consider UV-B to be the most damaging of earth atmosphere-penetrating solar radiation, which also includes UV-A. More than 1 million cases of skin cancer, either basal, squamous or melanoma, are expected to occur in the United States this year, according to the Centers for Disease Control and Prevention (CDC).

Grant and his research team report their findings in the April issue of Photochemistry and Photobiology. The paper describes development and verification of a three-dimensional model that can predict how much UV-B radiation exposure exists under trees affording varying amounts of shade. The scientists then used the model to estimate how much exposure people received in residential suburban areas under cloud-free skies.

"Our model takes into consideration the fraction of sky you can see through the tree canopy," Grant said. They assessed a scenario to determine the effect of tree cover on children's daily UV-B exposure.

Several factors influence the amount of UV-B exposure. These include altitude, latitude (distance from the equator), time of day and tree cover. The researchers used all of these elements in their calculations.

At latitudes from 15 degrees to 60 degrees – or approximately from Hawaii to Juneau, Alaska – when 50 percent of the sky is visible through the trees, protection from the rays approximately doubles the time one can be in direct sunlight and get the same amount of sun.

Grant said the protection from a 50 percent canopy isn't "much because some people sunburn in 20 minutes."

In other words, if a person is standing in the sunlight under a tree that provides 50 percent coverage, it will take about 50 minutes for them to burn instead of 20 minutes. However, that same person standing in the full shade under the same tree could be there for 100 minutes before they received too much ultraviolet radiation.

But with 90 percent tree-canopy coverage, the ultraviolet protection factors are 10 times greater, giving the equivalent of a Sun Protection Factor (SPF) 10 sunblock lotion.

"Clearly, significant exposure of pedestrians is likely unless the tree cover has a nearly closed canopy. We have considered here only the UV-B exposure under clear skies resulting from the existing vegetation cover," the researchers write, adding that even planting some trees in an open area provides protection against the rays.

However, they write that the greatest benefits of tree cover in reducing UV radiation exposure is at high latitudes of 45-60 degrees, or in places at roughly the same distance from the equator as Detroit, Mich., and Juneau, Alaska.

Residential and urban planners should consider the ultraviolet protection factors, Grant said. Many multifamily communities and office complexes, especially in urban areas, are lacking trees.

"There are indications that the relative factors for protection of children from ultraviolet radiation differ in single-family verses multifamily developments," he said.

It's especially important to protect children from the rays because, according to the American Cancer Society, those who are severely sunburned when they are young have a significantly increased risk later in life of developing the most serious form of skin cancer, malignant melanoma. The National Institutes of Health (NIH) report the incidence of melanomas has doubled in the United States in the past 20 years.

But even with these warnings, it's a tall order to keep youngsters safe from those blisters since they spend much of their time outside wearing recreational clothing. For this reason, by the time individuals are 18 years old, they have received 50 percent of their lifetime exposure to the sun's radiation, according to the CDC.

So, that tan may look terrific, but in fact it's the skin's response to ultraviolet radiation damage that later could become skin cancer, according to the National Institute of Occupational Safety and Health.

Those rays act like a microwave on your skin — the redness, swelling and blisters can keep developing for 12 to 24 hours after exposure, and injures not just your skin but the immune system cells located in it, according to the NIH. In other words, UV radiation is doubly dangerous because it can put you at higher risk for skin cancer and it also decreases you body's ability to fight disease.

The other researchers in the Purdue study are Gordon Heisler, USDA Forest Service, Northeastern Research Station, State University of New York College of Environmental Science and Forestry; and Wei Gao, USDA UVB Monitoring Program, National Resources and Ecology Laboratory, Colorado State University.

The U.S. Forest Service Northern Global Change Research Program provided funding for the study.

American Academy of Dermatologists Tips to Prevent Skin Damage From the Sun

  • Wear a wide-brimmed hat to shade face, neck and ears.

  • Wear sunglasses with UV-A and UV-B protection.

  • Wear long-sleeved shirts and long pants, preferably of dark, tightly woven fabric.

  • Use a sunscreen with UV-A and UV-B protection even when it's cloudy or foggy and in the winter. As much as 80 percent of the harmful rays still reach you through clouds and fog.

  • Children under 6 months old should be kept out of direct sun and wear a hat. At six months, begin protecting them with sunscreen and a hat.

  • At high altitudes and tropical climates (low latitudes) and around sand, concrete, water and snow, use sunscreen with an even higher SPF.

  • Avoid the sun during the highest UV radiation period – 10 a.m. to 2 p.m., when 60 percent of UV radiation occurs. The National Institute of Arthritis, Musculoskeletal and Skin Diseases recommends using the "structured shade" of trees, pavilions and other outdoor structures to aid in protection from the harmful rays.

    Writer: Susan A. Steeves, (765) 496-7481,

    Source: Richard Grant, (765) 494-8048,

    NOTE TO JOURNALISTS: A video news release and a copy of the research paper referred to in the article are available from the Agricultural Communication service, (765) 494-8396.

    Even in shade, Richard Grant's monitor detects harmful levels of UV-B rays from the sun. Grant has developed a model to predict how much radiation people receive under different amounts of tree cover. (Agricultural Communications Photo by Tom Campbell)

    A publication-quality photograph is available at


    Estimation of Pedestrian Level UV Exposure Under Trees

    Richard H. Grant, Gordon M. Heisler and Wei Gao

    Trees influence the amount of solar UV radiation that reaches pedestrians. A three-dimensional model was developed to predict the ultraviolet-B (UV-B) irradiance fields in open-tree canopies where the spacing between trees is equal to or greater than the width of individual tree crowns. The model predicted the relative irradiance (fraction of above-canopy irradiance) under both sunlit and shaded conditions under clear skies with a mean bias error of less than 0.01 and a root mean square error of 0.07. Both model and measurements showed that the locations people typically perceive as shady, low-irradiance locations in the environment can actually have significant UV-B exposure (40-60% of the amount compared with being in direct sunlight). The relationship of tree cover in residential neighborhoods to erythemal UV-B exposure for children and adults was modeled for the four hours around noon in June and July. Results showed that human exposures (on the horizontal) in cities located at 15- and 30-degree latitudes are nearly identical. For latitudes between 15 and 60 degrees, ultraviolet protection factors (UPF) were less than 2 for less than 50% tree cover. A UPF of 10 was possible at all latitudes for tree cover of 90%.

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