"Ozone (O3) is a positive or negative presence in our atmosphere, depending on where it is found. The atmosphere of the earth is divided into four layers, defined by their distance above the planet's surface, with ozone important in the two lower levels - the stratosphere and the troposphere. In the troposphere ozone is a damaging pollutant. In the stratosphere ozone forms a protective layer which filters out the harmful rays of the sun." (Quoted from article)