The thinnest area — with at least 50 percent depletion of the stratosphere's ozone layer — is the largest in history, the World Meteorological Organization said.
"The ozone hole is getting larger, deeper and is lasting longer," said Michael Proffitt, a leading expert on the ozone hole at World Meteorological Organization. "It has never stayed this large, this late."
The hole, a thinner-than-normal area in the protective layer of gas high up in the earth's atmosphere, has started forming in August at the end of the Antarctic winter every year since the mid-1980s, largely due to chemical pollution.
In recent years, the ozone hole has tended to near its largest size during mid-September. Later, it mostly gets refilled with surrounding ozone.
This year the hole peaked at 10.81 million square miles in mid-September — matching the record size set three years ago. Demonstrating its persistence, it hit the peak again in late September.
The hole could last longer still, said Proffitt.
"There is certainly no indication it is getting smaller," he said. "It looks like it could be a while."
Reduction of the ozone layer can let harmful ultraviolet rays from the sun reach the earth's surface. Too much UV radiation can cause skin cancer and destroy tiny plants at the beginning of the food chain.
WMO said although the large ozone hole is persisting, UV intensity over Antarctica will remain low until the sun rises higher in the Southern Hemisphere spring.
"The longer it lasts, the more effect you get from UV," Proffitt said. "When the sun is low, the UV rays slant through on an angle and have to go through a thicker layer.
"But as you go into the Antarctic's spring and the sun rises higher, more UV rays can get directly through the hole."
WMO also said the thinnest part — with the ozone level 50 percent below the pre-ozone hole period of 1964-76 — exceeded 5.7 million square miles for the first time.
It peaked near 6.95 million square miles Sept. 26. That is two-thirds of the size of the ozone hole.
It had only reached 3.86 million square miles four times before.
Proffitt said it was too soon to conclude what the significance of this year's larger weak spot would be.
Earth has previously lost more ozone than this year, he said. In some years, while the thin spot was smaller, it contained even less ozone.
Sunlight combined with oxygen high in the atmosphere naturally produces ozone — mostly over the tropics.
The gas also is naturally destroyed in chemical reactions over the Antarctic. In August, when the feeble sun starts to rise again over Antarctica, it triggers accelerated ozone loss following extremely cold South Pole winters when the area remains in darkness.
The natural balance has been disturbed in recent years through the release of chemical compounds such as chlorofluorocarbons contained in some aerosols and refrigerants.
For two decades the ozone hole has formed in the polar vortex, the circular wind pattern that forms annually in the stratosphere over Antarctica. This year the vortex is on the scale of the previous record year, 2000, with an area of 13 million square miles.
Emission of chlorofluorocarbons have been curbed under a global accord. As a result, measurements show they are now decreasing in the lower atmosphere and have just peaked and stabilized in the critically important ozone layer in the stratosphere.
Scientists predict it will take about 50 years for the ozone hole to stop forming.
By Erica Bulman