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Ultraviolet sunlight



Photo by Carlye Calvin.

Especially in summer, we all look forward to getting out of doors and enjoying the sunny weather. Inevitably, many of us will get sunburned. Dermatologists have been warning us about the dangers of prolonged exposure to the sun. Studies by epidemiologists show that skin cancer rates increase with greater exposure to ultraviolet (UV) light.

Today it is not just physicians who are concerned about harmful solar radiation but scientists as well. Over the last decade, atmospheric scientists have discovered significant reductions in the protective ozone layer in the stratosphere--the atmospheric layer between about 15 and 97 kilometers (9 and 60 miles) above the surface. The decrease in protective ozone, believed to be caused by human-produced chlorofluorocarbons (CFCs) and bromines, allows greater amounts of ultraviolet radiation to reach the earth's surface, raising concern about health issues. CFCs are used as the coolant in air-conditioning systems and to clean circuit boards.

The UV radiation most likely to pass through when there's less ozone, and now present in ever-greater amounts on earth, is in the wavelength range of 280-320 nanometers (one billionth of a meter), called UV-B. As ozone depletion continues, the amount of unfiltered, biologically active UV-B reaching the earth's surface increases as well. Because of its ability to chemically damage DNA, UV-B is associated with skin cancers. It also causes cataracts and immune deficiencies in humans and animals, and may damage or even kill some species of plant life. Melanoma, a type of skin cancer associated with UV-B, is the fastest-growing type of cancer in the United States today. One estimate by experts from the Environmental Protection Agency (EPA) predicts 200,000 skin cancer deaths a year in the United States by 2050.

Scientists have found that between 1979 and 1989 UV radiation levels in the Northern Hemisphere rose significantly in the 30-60 degrees latitude band during winter and early spring and north of 50 degrees in summer. Large parts of North America, Europe, and Asia are located within the 30-60 degrees band. In the Southern Hemisphere, significant UV increases occurred south of 30 degrees (a band including parts of South America, Africa, and Australia) from spring to late fall, with particularly large increases associated with the springtime ozone hole over Antarctica. The total annual UV-B reaching the earth's surface has risen by 4-25% per decade within the midlatitudes of both hemispheres, with larger increases occurring closer to the poles.

While humans can protect themselves with sunscreens and clothing and by limiting exposure time (especially during midday hours), harm to plants may be unavoidable and lead to interruptions in the food chain. Recent studies have confirmed that oceanic phytoplankton productivity inside the region under the Antarctic ozone hole decreased at least 6-12% during the spring of 1990. Lower crop yields on land are another consequence. If increased UV radiation reduces some crop yields by even a few percent, most developed nations could probably tolerate the loss, but it would significantly impact countries where food shortages are already a problem.

In addition to its harmful effects on humans, plants, and animals, increased UV radiation is also disrupting normal atmospheric chemistry, thereby complicating global change scenarios. Scientists have found that because of stratospheric ozone depletion, significantly more UV radiation is reaching the lower atmosphere, where it is responsible for the photochemical destruction of ozone. The destruction of that ozone, in turn, eventually contributes to the formation of highly reactive hydroxyl (OH) molecules. OH molecules take part in chemical reactions that influence the lifetimes of many gases, including gases that contribute to the greenhouse effect.

The repercussions of decreasing stratospheric ozone were recognized by 37 nations that, in 1987, signed an agreement, known as the Montreal Protocol, to phase out the production of CFCs. Subsequent meetings accelerated the phase-out process since research indicated that the original measures were not going to solve the problem. Ozone depletion is the one global environmental issue with international consensus--nations are working together both to control CFC production and to find viable alternatives.

Finally, take the advice of dermatologists. Protect yourself from UV radiation by wearing sunscreen, protective clothing, and sunglasses.


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Edited by Rene Munoz, munoz@ucar.edu

Prepared for the Web by Jacque Marshall
Last revised: Mon Apr 10 14:14:36 MDT 2000