When UV radiations from the sun reach the earth’s atmosphere which is heavily rich in oxygen. Then these UV rays split molecular oxygen (O₂) into free oxygen (O). Afterward, this free oxygen reacts with molecular oxygen; it forms ozone (O₃).

This ozone now protects the earth from harmful UV radiations of the sun, by allowing only some percent of rays to fall on the earth’s surface. Ozone acts as a filter of sun rays. These rays can cause skin cancer in human and can also affect other living organisms.