16 September the International Day or Ozone day celebrated for the past 10 years for the Preservation of the Ozone Layer, proclaimed by The United Nations General Assembly on 19 December 1994. In 1987, the Montreal Protocol was signed by many countries on that day that were apprehensive about the depletion of the Ozone Layer due to this date was chosen for the celebration of World Ozone Day.
Global stratospheric ozone levels have declined, which means that the ozone layer is changing. Stratospheric ozone has large natural temporal and spatial variations, up to 30 percent variation may be regarded as normal. However, we now have evidence of a significant thinning of the ozone layer during spring and summer. This is observed in both the northern and the southern hemispheres at middle and high latitudes. During the last 10-15 years, the ozone layer above the northern hemisphere has been reduced by 5-6 percent in spring per decade. The latest tests (January-March 1995) have shown very large reductions, with a maximum of more than 30 percent reduction compared to normal.
A depletion of the ozone layer will increase the UV-radiation at ground level. Increasing doses of UV-B may cause skin cancer, eye cataracts, damage to the immune system in animals as well as human beings, and have an adverse impact on plant growth.
The Ozone Depletion Potential; ODP, is described as a potential relative to that of CFC-11. The various ozone-depleting substances vary in the degree to which they contribute to the reduction of the ozone layer. Halons, for example, are more efficient than CFCs in depleting ozone, and therefore have a higher ODP.
Chlorofluorocarbons (CFCs), halons, methyl chloroform, methyl bromide, carbontetrachloride and several other chemicals are ozone-depleting substances.