The ozone layer over the Arctic has suffered unprecedented damage over the northern hemisphere's winter due to cold weather and pollution in the upper atmosphere, scientists said.

The United Nations weather agency said 40 percent of the layer — which protects against harmful ultraviolet rays — had been lost between the start of winter and the end of March, exceeding last year's loss of 30 percent.

"Depletion of the ozone... has reached an unprecedented level over the Arctic this spring because of the continuing presence of ozone-depleting substances in the atmosphere and a very cold winter in the stratosphere," World Meteorological Organization said.

The layer of ozone gas is destroyed by reactions with industrial chemicals once found in refrigerators and spray cans that have been restricted by U.N. protocol but remain in the atmosphere where they are expected to cause damage for decades, the BBC said.

Colder than usual temperatures 15 miles above the Earth's surface increased the reactions, it said.

While ozone depletion occurs annually over the Antarctic, giving rise to the term "ozone hole," variable weather conditions over the Arctic mean that patterns of destruction are more unpredictable.

"The degree of ozone loss experienced in any particular winter depends on the meteorological conditions," the U.N. agency's secretary-general, Michel Jarraud, said in a statement.

"The 2011 ozone loss shows that we have to remain vigilant and keep a close eye on the situation in the Arctic in the coming years."

The agency said that the depleted ozone area could drift towards lower latitudes, increasing the risk of exposure to ultraviolet radiation, linked to skin cancer and cataracts.

It described the ozone loss as "unprecedented but not unexpected."

-- Barry Neild