Aug. 29 (Bloomberg) -- A hole over Antarctica in the ozone layer that protects the Earth from cancer-causing radiation may be smaller than usual this year, the United Nations said.

The World Meteorological Organization said, although the hole appeared earlier than usually registered, initial readings show it is no bigger or deeper than previous years at this stage.

The area ``observed so far could indicate that the 2007 ozone hole will be relatively small,'' the organization said yesterday in its first Antarctic Ozone Bulletin of the year. It's too early to say how much of the gas that filters out ultraviolet radiation will be lost, it said.

The ozone layer, found mainly in the stratosphere about 25 kilometers (16 miles) above the Earth's surface, shields the planet from the sun's harmful radiation. The less ozone there is to protect the Earth, the higher the risk to humans of contracting illnesses such as skin cancer and cataracts.

Researchers in the 1980s tied ozone depletion to chemicals known as chlorofluorocarbons used in aerosols and refrigerators. Their use in household items was banned by the Montreal Protocol, an international treaty reached in 1987.

While the amount of ozone-destroying chemicals in the atmosphere is starting to fall, scientists have warned that it may be two decades before there is a clear decline in the size and depth of the annual holes.

The Antarctic ozone hole appears every year at the beginning of spring in the Southern Hemisphere as the sun returns to the continent after the polar night and continues to grow until October, the UN said.

Ozone loss over the South Pole reached a record in October last year as the size and depth of the hole approached all-time highs, the European Space Agency said at the time.

Ozone Thickness

The size of last year's hole was about 28 million square kilometers while the depth of the hole stood at 100 Dobson Units, which are used to measure the thickness of the ozone layer.

A hole is deemed present anywhere the thickness falls below 220 units.

The overall ozone loss last year stood at 40 million metric tons, (88 billion pounds) exceeding the previous mark of 39 million metric tons in 2000, the space agency said at the time.

The WMO and other scientists use data from satellites and weather balloons to measure the rate of depletion.

To contact the reporter on this story: Ed Johnson in Sydney at ejohnson28@bloomberg.net.

Last Updated: August 28, 2007 21:21 EDT