Ozone is an unstable form of molecular oxygen. About 90% of the ozone is found in the upper stratosphere. It forms a concentrated band, roughly between 19 and 23 km above the earth’s surface, and acts as a shield absorbing ultraviolet radiation, especially UV-B.
Man-made chemicals such as chlorofluorocarbons (CFCs) released into the atmosphere start chemical reactions that destroy the ozone layer.

The picture alongside shows the hole in the ozone layer (area marked grey) over the Antarctic. The affected area depicted is approximately 28 Mio km2 or 8.5 times the size of the Indian continent!! It will require many decades (scientists say 50 years or more) to repair the damage, even with the Ozone Depleting Substances phased out.

Protecting the Ozone Layer
The first international agreement to protect the ozone layer was the Vienna Convention on the Protection of the Ozone layer. This was followed by the Montreal Protocol on Substances that Deplete the Ozone layer in 1987. The Montreal Protocol was substantially revised four times, the latest being in 1999 (Beijing). The Montreal Protocol stipulates that the production and consumption of compounds that deplete the ozone layer like chlorofluorocarbons (CFCs), halons, carbon tetrachloride, and methyl chloroform--are to be phased out by 2010 (2005 for methyl chloroform).

Total ozone (DU)/ Ozone total (UD), 2006/09/25

DU - Dobson unit
Ozone Hole Antarctica
25 September 2006

Ozone Depletion has a major impact on health, agricultural products, animal and plant life as well as materials.


Ozone is a reactive gas consisting of three oxygen atoms, formed naturally in the atmosphere by the association of molecular oxygen (O2) and atomic oxygen (O). Most ozone (about 90%) resides in the stratosphere (a layer of the atmosphere between 10 to 40 km above the earth), where it acts as a shield to protect the Earth's surface from the sun's harmful ultraviolet radiation (UV-B), filtering out the high energy radiation below 0.29 µm and allowing only a small amount to reach the Earth’s surface. The high concentration of ozone in this part of the atmosphere is commonly known as the ozone layer.


In recent years, data collected in the upper atmosphere has shown a general thinning of the ozone layer over most of the globe. More dramatic damage occurs over Antarctica each spring when the ozone hole forms. This depletion in the ozone layer is far beyond seasonal variations, and the natural balance is not being restored. Research has revealed that man-made chemicals released into the air are contributing to the depletion of the ozone layer.

The consequences of the depleting ozone layer are severe – the increased levels of harmful UV-B radiation entering the Earth’s atmosphere impact adversely on human, animal and plant life on the planet.

The harmful consequences include:

a) Increased incidence of skin cancer, eye diseases and weakening of the immune system in humans.
b) Damage to marine life, particularly planktons, leading to disruption in the food chain and loss of bio-diversity.
c) Increased incidence of eye and skin disease amongst the animal population on the planet.
d) Reduction in food production as a result of damage to food crops by the UV-B radiation as well as loss of plant bio-diversity.


Research has revealed that there are several chemicals such as chlorofluorocarbons, halons and methyl bromide that contribute to ozone depletion. In the early 1930s when CFCs were first discovered,their non-toxic, non-flammable, and non-reactive properties made them ideal for many industrial and domestic applications like coolants for commercial and home refrigeration units, aerosol propellants, electronic cleaning solvents, and blowing agents. Chlorofluorocarbons were in great demand and produced in abundant quantities. It was only in 1973 that scientists discovered that the chlorine found in CFCs causes ozone destruction.When CFC molecules drift into the atmosphere, the UV-B and UV-C radiation from the sun releases their chlorine atoms. Complex chemical reactions in the atmosphere result in the formation of chlorine monoxide, which reacts with the ozone molecule to form oxygen and regenerates more chlorine atoms that carry on converting the ozone molecules. Each chlorine atom can destroy as many as 100,000 ozone molecules over 100 years. Thus, even a small amount of CFCs can cause tremendous damage to the ozone layer.


Faced with the strong possibility that CFCs and similar compounds could cause serious ozone depletion, in 1987, policy makers from around the world signed an international treaty, the Montreal Protocol on Substances That Deplete the
Ozone Layer. According to this protocol, countries would phase out CFCs and other ODS as per a given schedule, with a complete halt by 2010. More than 187 countries are signatories to the Montreal Protocol.

Under the Protocol, industrialized nations have rapidly eliminated most ozone depleting substances. Developing countries are following suit, with critical assistance from the Protocol's Multilateral Fund, which has already committed over US $ 1.5 billion to assist developing countries in the difficult transition to ozone-friendly substances.

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