Backgrounder: Ultraviolet Radiation and Sun Protection Why cover UV Radiation? As vacation-goers head for the beaches and mountains, they may need to be reminded that the sun can sneak up on them. Over-exposure to the sun’s ultraviolet (UV) rays can cause more than a nasty sunburn -- it can cause skin cancer. Weather forecasters may routinely mention the "UV Index" along with other weather conditions, but they rarely pause to explain exactly what it means or what to do about it. A significant portion of the roughly one million cases of skin cancer diagnosed in the United States are caused by overexposure to damaging UV radiation. This kind of "environmental cancer" is more common and better established scientifically than many of the cancer risks from toxic chemicals which get more media attention. Sunlight is one cancer-causing agent to which people can easily limit their exposure. If they are properly informed. Story Ideas 1. Explain to your readers some of the local conditions which affect their exposure to ultraviolet radiation. How far is your town or county above sea level? What’s its latitude? What are prevailing cloud conditions? How much more UV might people get at the beach? 2. Explain to your readers what the UV Index means and how it is calculated. Why does it change from day to day? 3. Explain the key precautions people can take to limit their exposure to UV radiation. 4. Can anyone quantify the effect of the thinning stratospheric ozone layer on the amount of UV radiation reaching your locality? 5. What do SPF ratings on sunscreen products mean? How much is enough? How should sunscreen be worn? Background The sun which makes life possible on Earth can also be lethal. Visible sunlight is only a fraction of the huge amount of energy given off by this continuous thermonuclear reaction. Some is in the form of ultraviolet radiation. Some kinds of ultraviolet can be very harmful to living things, because they directly damage cells and DNA, which carries genetic information. Fortunately, Earth’s atmosphere normally protects living things from most incoming UV. Oxygen as it normally exists in the atmosphere comes in molecules consisting of two oxygen atoms attached chemically. A small fraction is in molecules containing three oxygen atoms -- called ozone. Both oxygen and ozone are virtually transparent to visible light. They block ultraviolet not by shading or reflecting but by taking up its energy in various chemical reactions. Key among those reactions are the transformation of two-atom oxygen into three-atom ozone and the transformation of ozone back to oxygen. Ozone is particularly abundant in a certain layer of the lower-mid stratosphere, at an altitude of about 19 miles. The largest part of incoming UV radiation is absorbed by this so-called "ozone layer." The ozone layer has been eroded by the large-scale release of certain man-made chemicals into the atmosphere. The most common ozone-destroying chemicals are the chlorofluorocarbons -- or CFCs. In the Montreal Protocol of 1987, the nations of the world agreed to phase out and eventually ban such chemicals. The ABC’s of UV The wavelengths of ultraviolet radiation (220-400 nanometers) are shorter than the wavelengths of visible light (400-790 nm). A nanometer (nm) is one-billionth of a meter. Ultraviolet radiation is not all the same. The shorter the wavelength, the more energy it contains, and the more damage it can do to living things. The oxygen, ozone, and other gases in the atmosphere do not block all wavelengths equally. Scientists find it convenient to break the UV portion of the spectrum down into three smaller bands — or “types” of UV radiation. UV-A. The wavelengths of UV-A radiation range between 320 and 400 nm -- the closest to visible light. These longer-wavelength UV rays carry less energy than other kinds of UV. UV-A rays are often considered less damaging to human skin, because they do not cause appreciable sunburn. But UV-A penetrates to the lower layers of skin, and can damage its supportive elastic tissues (collagen). Years of UV-A exposure tend to cause skin to wrinkle and sag. Scientists also believe UV-A is a contributing cause of skin cancer. UV-A rays are generally not absorbed by the ozone layer. UV-B. UV-B ranges in wavelengths between 280-290 and 315-320 nm. These more energetic rays are the ones that cause sunburn, or erythema. They also damage the skin’s immune response and promote formation of cataracts in unprotected eyes. Small doses of UV-B can benefit people by promoting the body’s production of vitamin D, but larger doses raise the risk of skin cancer. Most, but not all, UV-B is absorbed by the ozone in the atmosphere. UV-C. This most energetic -- and dangerous -- band of ultraviolet is generally considered to include waves shorter than 290 or 280 nm and range to as short as 220-150 nm. UV-C is the most damaging to humans, plants, and animals. Fortunately, nearly all of the UV-C coming from the sun is absorbed by ozone and other gases. For many years, sunscreens were only designed to block UV-B rays, since they were the ones causing sunburn. The SPF rating of sunscreens only measures their ability to block UV-B. Dermatologists today recommend use of "broad spectrum" sunscreens, which also protect against UV-A. Sunscreen products containing zinc oxide (Z-cote) or avobensome (Parsol 1789®) will also protect against UV-A. Sources U.S. Environmental Protection Agency - Global Programs Division. Houses EPA’s programs on stratospheric ozone depletion and UV protection. Division Director: Drusilla Hufford, (202) 564-9101, hufford.drusilla@ epa.gov. Press contact: Dave Ryan, (202) 564-7827, ryan.dave@epa.gov. Web: http://www.epa.gov/ozone/. - Stratospheric Ozone Protection Hotline toll-free at (800) 296-1996; (301) 614-3396 from outside the United States and Canada - UV Index: http://www.epa.gov/ozone/uvindex/uvover.html. - Sunwise School Program: http://www.epa.gov/sunwise/index.html. - EPA UV Monitoring Network: http://www.epa.gov/uvnet/. Further Links (especially medical and research organizations): http://www.epa.gov/sunwise/links.html, http://www.epa.gov/ozone/othlinks.html National Oceanic and Atmospheric Administration (NOAA) - Climate Prediction Center: http://www.cpc.ncep.noaa.gov/products/stratosphere/index.html - Daily UV Index (forecasts) for selected U.S. cities in map and text form: http://www.cpc.ncep.noaa.gov/products/stratosphere/uv_index/index.html - Weekly ozone hole report: http://www.cpc.ncep.noaa.gov/products/stratosphere/sbuv2to/ozone_hole.html. Hole usually begins to form by mid-August or a few weeks later. - WMO/UNEP Scientific Assessment of Ozone Depletion: 1998 (executive summary): http://www.al.noaa.gov/WWWHD/pubdocs/Assessment98.html. The most recent and authoritative scientific consensus overview on the status of ozone depletion. This text published online only by NOAA’s Aeronomy Lab (not UNEP). Assessment was co-headed by Daniel Albritton, director of Aeronomy Lab., (303) 497-5785, Daniel.L.Albritton@noaa.gov. - NOAA national press contacts: http://www.publicaffairs.noaa.gov/reporters.html. Carmeyia Gillis (301) 763-8000 ext. 7163, Carmeyia.Gillis@noaa.gov or Greg Hernandez, (202) 482-3091, Gregory.Hernandez@noaa.gov. National Safety Council/Environmental Health Center - Sun Safety Page: http://www.nsc.org/ehc/sunsafe.htm SPF -- Sun Protection Factor SPF ratings are a standard measure of how well a suntan lotion protects a person against UV radiation. The rating is a number that typically ranges from as low as 2 to as high as 60. The number is a multiplier -- representing how much longer a person using the lotion could stay in the sun without burning. For example, a person wearing SPF 4 lotion could stay in the sun four times longer without burning than could a person wearing no lotion at all. If a lotionless person would take 30 minutes to start burning, a person wearing SPF 4 lotion would take 120 minutes. That’s the theory, anyway. The sensitivity of people’s skin to solar UV also varies widely according to how darkly pigmented it is and other factors. SPF ratings only apply to UV-B radiation, the shorter wavelengths that cause sunburn. Longer UV-A rays can also harm skin. The UV Index Index Number Exposure Level 0 to 2 Minimal 3 to 4 Low 5 to 6 Moderate 7 to 9 High 10+ Very High Calculation of the daily UV index is complex -- but some understanding of how it is done may help your audience understand what it means. The UV Index is not calculated for everywhere in the U.S. Currently it is calculated for 58 U.S. cities. The index is not a measurement of actual conditions, but a prediction of what conditions will be in that locality over the next 24 hours. It is pegged to predicted UV at noon (the peak time for UV) the following day. It is based on the following variables: - Elevation/Altitude. A mile-high city like Denver has a mile less atmosphere to protect it than a sea-level city like Washington, DC. UV exposure increases by about 6 percent for every kilometer of altitude. - Angle of Sun. The lower the angle of the sun in the sky, the more protective atmosphere its rays pass through before they reach us. Even at noon, the sun is often not directly overhead. The angle of the noonday sun in the sky varies according to time of year and latitude of the location in question. - Amount of stratospheric ozone. Satellites measure global atmospheric ozone daily, and this data is fed into computer models to produce a prediction of total ozone for the next day. - Cloudiness. Based on conventional weather forecasts. More clouds filter out more UV. A clear sky transmits 100% of UV to the surface, while an overcast sky transmits as little as 32%. - UV Wavelength. Shorter wavelengths in the UV portion of the spectrum (290 to 400 nm) do more harm to human skin than longer ones. But the atmosphere blocks different amounts of the different wavelengths. UV Index calculations estimate the amount of the different wavelengths reaching the surface and then weight them according to how much they harm human skin. There are important variables which the UV Index does not consider. One is the reflectivity (albedo) of the surface. Sea and sand reflect more UV onto people than a green lawn. Another is pollution. Smog, haze, soot, and other particulates can reduce the amount of UV reaching the surface. Reprinted with permission. Published in Environment Writer newsletter July/August 2001, by the National Safety Council's Environmental Health Center.