Environmental Problems, Their Causes, and Sustainability 1

ozone in the stratosphere. From a human standpointthe answer is that with less ozone in the stratosphere,more biologically damaging UV-A and UV-B radiationwill reach the earth’s surface. This will give humansworse sunburns, more eye cataracts (a clouding of theeye’s lens that reduces vision and can cause blindnessif not corrected), and more skin cancers (Figure 21-25and Connections, at right).Humans can make cultural adaptations to increasedUV radiation by staying out of the sun, protectingtheir skin with clothing, and applying sunscreens.However, plants and animals that help support us andother forms of life cannot make such changes exceptthrough biological evolution, a process that can take along time.Connections: What Cancer Are You MostLikely to Get? Look in the MirrorExposure to UV radiation is a major cause of skincancers.Research indicates that years of exposure to UV-B ionizingradiation in sunlight is the primary cause of squamouscell (Figure 21-25, left) and basal cell (Figure 21-25,center) skin cancers. Together these two types make up95% of all skin cancers. Typically there is a 15- to 40-year lag between excessive exposure to UV-B and developmentof these cancers.Caucasian children and adolescents who experienceonly one severe sunburn double their chances ofgetting these two types of cancers. Some 90–95% ofThis long-wavelength(low-energy) form of UVradiation causes aging ofthe skin, tanning, andsometimes sunburn. Itpenetrates deeply andmay contribute to skincancer.UltravioletAUltravioletBThis shorter-wavelength (high-energy) formof UV radiation causes sunburn, prematureaging, and wrinkling. It is largely responsiblefor basal and squamous cell carcinomasand plays a role in malignant melanoma.Thin layer ofdead cellsHairSquamouscellsBasallayerMelanocytecellsBasalcellEpidermisSweatglandDermisBloodvesselsSquamous Cell CarcinomaArising from cellsin the upper layerof the epidermis,this cancer is alsocaused by exposureto sunlight or tanninglamps. It is usuallycurable if treated early. It grows faster thanbasal cell carcinoma and can metastasize.Basal Cell CarcinomaThe most commonskin malignancyusually is causedby excessiveexposure to sunlightor tanning lamps.It develops slowly,rarely metastasizes and is nearly 100%curable if diagnosed early and treated properly.MelanomaThis deadliest ofskin cancersinvolves melanocytecells, which producepigment. It candevelop from a moleor on blemishedskin, grows quickly, and can spread to otherparts of the body (metastasize).Figure 21-25 Structure of the human skin and the relationships between ultraviolet (UV-A and UV-B) radiationand the three types of skin cancer. (The Skin Cancer Foundation)http://biology.brookscole.com/miller14487