Meg Bayliss, of Waihopa School, asks :-

If we stopped using toxic gases and chemicals today, how long would it take the ozone layer to recover to normal?

Deborah Keep, an atmospheric physicist with the National Institute for Water and Air at Lauder in Central Otago, responded.

About eighty years because there are still a lot of CFC molecules moving slowly up through the atmosphere and it will take a long time for all the `ozone-destroying' chlorine to convert to `safe' forms.

Global ozone depletion is the result of widespread use of chlorofluorocarbons (CFCs ie molecules made from chlorine, fluorine and carbon), in refrigerators, air conditioning, spray cans, foam production and solvents. They were originally popular because they are non-flammable and non-toxic to humans. However, because of their stability, CFCs have a very long lifetime in the atmosphere. Released at the ground, they slowly move to higher altitudes and after a few years reach heights in the atmosphere above the ozone layer (30 to 35 km). Gases do not mix very well in the atmosphere above about 10 km which is why it takes a long time for a single CFC molecule to move upwards.

CFCs release 'ozone -depleting' chlorine when they are gradually broken down by the more intense UV levels above the ozone layer (remember, we are below the ozone layer and are protected from harmful UV radiation). This chlorine is catalytic which means that one chlorine atom can destroy tens of thousands of ozone molecules in its lifetime! Currently, chlorine levels in the atmosphere are almost two times the amount scientists believe is the limit for appearance of the Antarctic `ozone hole' and several times the natural chlorine levels. For the ozone layer to go back to 'normal', chlorine levels in this `ozone depleting' form, need to be reduced.

Also old refrigerators leak CFCs as they deteriorate at rubbish dumps. The good news is that under strict international agreements, CFCs are currently being phased out and replaced by more `ozone friendlier' compounds, some countries do not use CFCs at all any more. Recent predictions are that chlorine will increase until the year 2000 and decrease gradually throughout the next century.