6 Shade Planning for America’s Schools Chapter 1 infrared radiation. Clouds may make us feel cooler and block our view of the sun, but they do not fully protect us from UV radiation. Atmospheric Ozone The stratosphere’s ozone layer provides us with an enormous amount of protection against the damaging effects of UV radiation. Unfortunately, in certain areas, ozone has been depleted to a dangerous extent, primarily due to the release of chlorofluorocarbons (CFCs) and other ozone-depleting chemicals, such as carbon tetrachloride and methyl chloroform, into the atmosphere. Both carbon tetrachloride and methyl chloroform are solvents that have been used in industrial applications, and CFCs have, in the past, been used as refrigerants and aerosol propellants. The use of all three of these chemicals has since been restricted or prohibited.2 2 United Nations Development Programme, Montreal protocol on substances that deplete the ozone layer. Available at http://www.undp.org/seed/eap/montreal.htm. Nonetheless, much of the damage that has been done remains. What Are Direct and Indirect UV Radiation? Direct UV radiation, or UV radiation that travels from the sun in a straight line, may pose the greatest risk to our health, but we are also at risk from exposure to indirect (scattered and reflected) UV radiation. Scattered UV radiation results from being bounced around by atmospheric dust and water droplets in clouds. Throughout the day, the level of indirect UV radiation varies, as does the level of direct UV radiation. In the early morning and late evening when the sun is low on the horizon, indirect UV radiation may exceed direct. Likewise, on a cloudy day, UV radiation scattered by atmospheric particles may result in greater exposure to indirect than to direct UV radiation. UV radiation can also be reflected off buildings and the terrain. Smoother surfaces, such as concrete or asphalt, whether they are dark in color or not, typically result in greater reflectance of UV radiation than irregular surfaces. Surface irregularities, such as that found in grass or bark nuggets, reduce the level of reflectance, thereby reducing exposure to reflected UV radiation. One exception, however, is water. Smooth water absorbs almost all UV radiation, whereas the irregular surface of choppy water reflects a considerable amount of UV radiation. The following table lists surfaces and terrains commonly encountered on school grounds and their UV radiation reflectance. Materials with a lower reflectance are more desirable. Ultraviolet Radiation Reflectance for School Grounds Surfaces Surface UV Radiation Reflectance3,4,5 Grass 1% – 4 % Still water 3% – 8 % Soil 4% – 6 % Asphalt 4% – 9 % Concrete 7% – 12 % Choppy water 8% – 13% Dry sand 15% – 18 % Fresh snow 85% – 88 % 3 Williams ML, Pennella R. Melanoma, melanocytic nevi, and other melanoma risk factors in children. J Pediatr 1994;124:833-45 4 Moore LA. Ocular protection from solar ultraviolet radiation (UVR) in sport: factors to consider when prescribing. The South African Optometrist 2003;62(2):72-79. 5 Sliney, DH. Physical factors in cataractogenesis: ambient ultraviolet radiation and temperature. Invest Ophthalmol Vis Sci 1986; 27(5):781-790.
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