Ozone, OH zohn, is a form of oxygen that is present in the earth's atmosphere in small amounts. Ozone in the upper atmosphere is a major factor in making life on the earth possible. The highest concentration of ozone is reached, depending on the latitude, between 9 and 18 miles (15 and 30 kilometers) above the earth's surface. This concentration is 10 parts per million by volume--that is, 10 parts ozone per 1 million parts air. This ozone layer in the upper atmosphere shields the earth from 95 to 99 percent of the sun's ultraviolet rays. Overexposure to these rays is a cause of skin cancer.