Ozone is created in the stratosphere when highly energetic solar radiation strikes molecules of oxygen (O₂) and causes the two oxygen atoms to split apart. If a freed atom bumps into another O₂, it joins up, forming ozone (O₃). This process known as photolysis. Ozone is also naturally broken down in the stratosphere by sunlight and by a chemical reaction with various compounds containing nitrogen, hydrogen and chlorine. These chemicals all occur naturally in the atmosphere in very small amounts. In an unpolluted atmosphere there is a balance between the amount of ozone being produced and the amount of ozone being destroyed. As a result, the total amount of ozone in the stratosphere remains relatively constant. The amount of ozone within the stratosphere varies according to altitude. Ozone concentrations are highest between 19 and 23 km, but there are significant amounts up to 30 km. At these levels in the atmosphere however, the air is thin. If all the ozone in the stratosphere was compressed to ordinary atmosphere pressure at ground level, it would occupy a layer only 3 mm thick. Ozone's unique physical properties allow the ozone layer to act as our planet's sunscreen, providing an invisible filter to help protect all life forms from the Sun's damaging ultraviolet (UV) rays. Most incoming UV radiation is absorbed by ozone and prevented from reaching the Earth's surface. Without the protective effect of ozone, life on Earth would not have evolved in the way it has.