The thin layer of ozone around the atmosphere that prevents entry of harmful UV rays is called ozone shield. Methane and CFCs are the two ozone depleting substances.
Chlorofluorocarbons (CFCs) release active chlorine (Cl atom) when acts with UV rays and Cl atoms degrade ozone releasing molecular oxygen. Depletion of ozone allows the entry of UV-B radiation to the earth which damages DNA causes mutation leading to skin cancer in humans and also rate of photosynthesis decreases in plants.
