The "ozone hole" has formed again over the Antarctic and is showing signs of growing toward record size, the UN's weather organization said on Friday.
The so-called "hole," actually a thinner-than-normal area in the protective layer high up in the earth's atmosphere, has been forming at the end of Antarctic winter every year since the mid-1980s.
"The ozone hole is developing quite rapidly this year, in a very similar manner to the record-breaking year 2000," said Carine Richard-van Maele, spokeswoman of the World Meteorological Organization.
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.
One cause of ozone depletion is the chlorine and bromine released by manmade chemical compounds such as chlorofluorocarbons, which were contained in some aerosols.
Although emissions of the chemicals have been curbed under a global accord, scientists predict it will take about 50 years for the ozone hole to close.
This year the ozone hole had developed "some time during the month of August," said Michael Proffitt, a leading expert on the ozone hole at WMO. "It's a gradual process."
The hole now appears to be 25 million square kilometers in area, about 10 percent below the record size recorded in mid-Sept. 2000, Proffitt said.
He said he was reluctant to put a figure on the record size because different ways used to measure it yield "slightly different numbers."
"They're all within a million square kilometers of each other."
US government scientists have said the hole briefly approached 30 million square kilometers in 2000.