This image was compiled by the Ozone Monitoring Instrument on NASA's Aura satellite from Sept. 21 to 30, 2006. The average area of the ozone hole was the largest ever observed, at about 17 million square kilometres. ((NASA))

The size of the hole in the ozone layer over the Antarctic is the largest ever seen, U.S. scientists reported on Friday.

The average area of the hole was more than 17.4 million square kilometres between Sept. 21 and 30, researchers at NASA and the U.S. National Oceanic and Atmospheric Administration (NOAA) said.

Under normal atmospheric conditions, the hole would be about the size of North America, or between 14 million and 15 million square kilometres.

The ozone layer blocks harmful ultraviolet light produced by the sun from reaching the Earth's surface. The hole is created by the release of chlorine and bromine gas-related pollutants produced through human activity, which react with ozone.

The hole also set a record for depth, with measurements indicating that nearly all of the ozone had been destroyed between about 13 kilometres above the Earth to an altitude of about 21 kilometres.

Instruments carried by balloon showed a reading of only 1.2 Dobson Units (DU) by Oct. 9 — down from an average 125 DU outside the hole in July and August. Dobson Units (DU) are a standardized measure of the amount of ozone over a fixed point in the atmosphere.

"These numbers mean the ozone is virtually gone in this layer of the atmosphere,"David Hofmann, director of the Global Monitoring Division at the NOAA Earth System Research Laboratory, said in a statement.

"The depleted layer has an unusual vertical extent this year, so it appears that the 2006 ozone hole will go down as a record-setter."