The ozone layer absorbs a portion of the radiation (UVB) from the sun, preventing it from reaching the surface of Earth. UVB is very harmful which can cause skin cancer, cataracts, genetic disorder, crop damage etc.

Experiments on fish suggest that 90% to 95% of malignant melanomas (a form of skin cancer) may be due to UV radiations. Plants are sensitive to UV radiation below 300 nm. Ozone depletion may also affect the climate.

With its depletion, spectrum of incoming radiation will change and hence may produce new molecular species which may change the radiation emitting and absorbing properties of atmosphere. This may cause formation of more tropospheric ozone.  In the absence of ozone layer the radiations will directly flow to the earth surface and hence change the vertical distribution of molecules in the troposphere and stratosphere which in turn change the rate of vertical circulation.

Numerous commonly used chemicals have been found to be extremely damaging to the stratospheric ozone layer. Halocarbons are chemicals in which one or more carbon atoms are linked to one or more halogen atoms (fluorine, chlorine, bromine or iodine).

Halocarbons containing bromine usually have much higher ozone-depleting potential (ODP) than those containing chlorine.

The man-made chemicals that have provided most of the chlorine and bromine for ozone depletion are methyl bromide, methyl chloroform, carbon tetrachloride and families of chemicals known as halons and chlorofluorocarbons (CFCs).In 1994, the United Nations General Assembly proclaimed September 16 as the World Ozone Day, commemorating the date of the signing, in 1987, of the Montreal Protocol.

The theme for this year's World Ozone Day-2021 is "Montreal Protocol - Keeping us, our food and vaccines cool".

As an international response in 1987 an international meeting in Montreal, Canada, produced the Montreal Protocol on phasing out most use of CFCs by 2000.

The Montreal Protocol is an international treaty designed to protect the ozone layer by phasing out the production of numerous substances that are responsible for ozone depletion. CFC production in most developed countries has fallen sharply since 1989.  The Montreal Protocol has been successful in reducing ozone depleting substances and reactive chlorine and bromine in the stratosphere. CFCs are being substituted by HCFCs (hydro chlorofluorocarbons) which release much less chlorine per molecule.

The 2019 ozone hole is indeed the smallest since recording of its size began in 1982 but the ozone is also influenced by temperature shifts and dynamics in the atmosphere through climate change.  In 50 years or so, stratospheric ozone levels are expected to be back to normal. However the final recovery is expected to require several lifetimes.

(Dr Senapati is Dean (Science), BPUT, Professor of Chemistry, Trident Academy of Technology, Bhubaneswar-751024, E-mail: dr_senapati@yahoo.com)