Ozone, a form of oxygen, is a natural gas found in the atmosphere of the earth. About ninety percent of ozone is found in a layer of the stratosphere located 10 to 50 km above the surface of the earth. This is called the ozone layer. This layer acts as a natural shield in the upper atmosphere. Ozone prevents most of the harmful ultraviolet rays of the sun to reach the earth’s surface. Thus it helps to reduce sun burn, cataract and skin cancer and also helps the plants and animals reducing the risk of mutation. The depletion of ozone layer is a natural process resulting from the cycles of the sun, changes in the wind, volcanic eruption and seasonal changes. But the human activities have been proven to be the greatest cause of upset of this delicate component of our environment.

When ozone depleting substances (ODS) release in the air several chemical reactions take place and break the ozone apart. A number of free radical catalysts can destroy the ozone among them the most important substances are hydroxide radical (OH•), the nitric oxide radical (NO•), the atomic chlorine ion (Cl•) and bromine ion (Br•). Though all of these have both natural and man-made sources but human activity has dramatically increased the levels of chlorine and bromine. Most of the OH• and NO• in the stratosphere is of natural origin. Chlorofluorocarbons (CFCs) go to the stratosphere without being destroyed in the troposphere as this is a low reactive gas. The Cl and Br atoms then destroy ozone molecules and as a result the amount of ozone is decreasing. More complicated mechanisms have been discovered that lead to ozone destruction in the lower stratosphere as well.

Only one chlorine atom would keep on destroying ozone for up to two years. They are not removed after the chemical reaction is done. They remain with other atoms, such as, hydrogen chloride, chlorine nitrate etc. Bromine is more efficient than chlorine at destroying ozone, but there is less bromine in the atmosphere at present. So, both chlorine and bromine play a significant role to the overall ozone depletion. In the Earth's stratosphere, fluorine atom reacts rapidly with water and methane to form strongly bound hydrogen fluoride. But organic molecules which contain iodine react very rapidly in the lower atmosphere and do not reach the stratosphere in significant quantities. A single chlorine atom may react with 100,000 ozone molecules and the amount of chlorine released into the atmosphere by chlorofluorocarbons (CFCs) per year tells how dangerous CFCs are to the environment. These are mainly used as refrigerants, propellants and foaming agents in plastic manufacturing, fire extinguishing etc.

In 1987, in the Montreal Protocol, 43 nations signed the protocol to freeze consumption and production of CFCs at 1986 levels by 1990, reduce them 20% by 1994, and another 30% was expected to be reduced by 1999. At least 183 counties are now signatories on the Montreal Protocol. The Montreal Protocol is working, as NASA estimates that levels of ozone-depleting substances peaked in 2000, and had fallen by 3.8% by 2008. From this information Antarctic ozone hole is expected to disappear by 2050.