GENEVA » The depletion of the ozone layer shielding Earth from damaging ultraviolet rays has reached an unprecedented low over the Arctic this spring because of harmful chemicals and a cold winter, the U.N. weather agency said Tuesday.

The Earth’s fragile ozone layer in the Arctic region has suffered a loss of about 40 percent from the start of winter until late March, exceeding the previous seasonal loss of about 30 percent, the World Meteorological Organization said.

The Geneva-based agency blamed the loss on a buildup of ozone-eating chemicals once widely used as coolants and fire retardants in a variety of appliances and on very cold temperatures in the stratosphere, the second major layer of the Earth’s atmosphere, just above the troposphere.

Arctic ozone conditions vary more than the seasonal ozone "hole" that forms high in the stratosphere near the South Pole each winter and spring, and the temperatures are always warmer than over Antarctica.

Because of changing weather and temperatures some Arctic winters experience almost no ozone loss while others with exceptionally cold stratospheric conditions can occasionally lead to substantial ozone depletion, U.N. scientists say.

This year the Arctic winter was warmer than average at ground level, but colder in the stratosphere than normal Arctic winters. U.N. officials say the latest losses — unprecedented, but not entirely unexpected — were detected in observations from the ground and from balloons and satellites over the Arctic.

Atmospheric scientists who are concerned about global warming focus on the Arctic because that is a region where the effects are expected to be felt first.

Ozone scientists have said that significant Arctic ozone depletion is possible in the case of a cold and stable Arctic stratospheric winter. Ozone losses occur over the polar regions when temperatures drop below -78 degrees Celsius (-108 Fahrenheit), when clouds form in the stratosphere.

Average temperatures in January range from about -40 to 0 C (-40 to 32 F), while average temperatures in July range from about -10 to 10 C (14 to 50 F).

"The Arctic stratosphere continues to be vulnerable to ozone destruction caused by ozone-depleting substances linked to human activities," said WMO secretary-general Michel Jarraud. "The degree of ozone loss experienced in any particular winter depends on the meteorological conditions."

The loss comes despite the U.N. ozone treaty, known as the 1987 Montreal Protocol, which has resulted in cutbacks in ozone-damaging chemicals, such as chlorofluorocarbons, halons and other, that were used in the making of refrigerators, air conditioners, fire extinguishers and even hairspray.

The 196-nation ozone treaty encourages industries to use replacement chemicals less damaging to ozone, the atmospheric layer that helps protect against the sun’s most harmful rays.

But because these compounds have long atmospheric lifetimes, it takes decades for their concentrations to subside to pre-1980 levels as was agreed in the Montreal Protocol.

U.N. officials project the ozone layer outside the polar regions will recover to pre-1980 levels sometime between 2030 and 2040.