A Story of Two Substances

Real-World Reading Link Have you ever moved a piece of furniture to a new location, only to discover that the new location won t work Sometimes, moving furniture creates a new problem, such as a door will not open all the way or an electric cord will not reach an outlet. Solving a problem only to find that the solution creates a new problem also occurs in science. 

If you have ever had a sunburn, you have experienced the damaging effects of ultraviolet radiation from the Sun. Overexposure to ultraviolet radiation is harmful to both plants and animals. Increased levels of a type of ultraviolet radiation called UVB can cause cataracts and skin cancer in humans, lower crop yields in agriculture, and disrupted food chains in nature. 

Living organisms have evolved in the presence of UVB, and cells have some ability to repair themselves when exposed to low levels of UVB. However, some scientists believe that when UVB levels reach a certain point, the cells of living organisms will no longer be able to cope, and many organisms will die. 

Earth s atmosphere Living organisms on Earth exist because they are protected from high levels of UVB by ozone. Ozone, which is made up of oxygen, is a substance in the atmosphere that absorbs most harmful radiation before it reaches Earths surface. A substance, which is also known as a chemical, is matter that has a definite and uniform composition. 

The stratosphere is the layer above the troposphere. It extends from about 10 to 50 kilometers (km) above Earths surface. The ozone layer that protects Earth is located in the stratosphere. 

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