A gaping hole in the ozone layer appears to be shrinking. According to scientists in New Zealand, the hole over Antarctica appears to have shrunk by about 20 per cent from last year's record-breaking size.

The National Institute of Water & Atmospheric Research said its measurements backed up NASA satellite data showing the hole peaked at about 24 million sq km compared with 29 million sq km in 2003.

The Antarctic ozone hole occurs every year during the southern hemisphere's spring between September and November.

NIWA scientist Stephen Wood cautioned against reading too much into the hole's smaller size, which he said was also influenced by natural variations. He say there needs to be smaller or less severe ozone holes over a number of years, before they can be certain the ozone is recovering.

Since the ozone-depleted air is contained over Antarctica, the only inhabited area it could affect at the moment is the very tip of South America. But Dr Wood says the hole will indirectly affect New Zealand later in the year.

"If New Zealand experiences a combination of lower ozone with high sun and few clouds, then skin-damaging UV levels can be extreme," he says.

The ozone layer sits above 15-30 km the earth and filters harmful UV rays that can cause skin cancer. NIWA says that in recent years, there has been about 10 per cent less ozone over New Zealand in the summer than 30 years ago.

In Canada, ozone depletion is usually greatest in the late winter and early spring.

Natural factors are partly to blame, such as volcanoes and the 11-year sunspot cycle. But chlorine and bromine,�man-made chemicals once widely used in aerosols and refrigerators, continue to persist in the atmosphere,�attack the ozone and thin the layer.

Those chemicals�are finally starting to decline, NIWA says, thanks to controls on their use under the Montreal Protocol. Under that agreement, more than 180 signatory states committed to phasing out the use of nearly 100 ozone-damaging substances.

But Environment Canada says that in late 1991, scientists estimated that even with the current global schedule to eliminate ozone-destroying substances, the ozone layer would not return to� pre-1980 levels until the middle of this century.