PUNTAARENAS, Chile WIDE SWATH of southern Chile was on alert this week as dangerous levels of ultraviolet radiation hit peaks because of the depletion of the protective ozone layer over the Antarctic.

Health authorities warned the 120,000 residents of this wool and fishing city - one of the few populated areas beneath the ozone hole in the southern hemisphere - not to go out in the sun during the day. The ozone hole over the Antarctic this year has reached its deepest since scientists began measuring it 15 years ago, with more than 50 per cent depletion being recorded throughout most of the hole, United Nations experts said late last week.

That has left this windy city 2,250 kilometres south of Chile's capital, Santiago, - and also the Argentine city of Ushuala on the nearby island Tierra del Fuego - open to harmful ultraviolet radiation, which can cause skin cancer and destroy tiny plants in the food chain.

The tip of the Americas, south of the Patagonia wilds, where Britain's Prince William is on a character-building expedition, is the only land mass outside the Antarctic exposed to ultraviolet radiation from the ozone hole.

"We are warning people throughout the region not to go out in the sun between 11 a.m. and 3 p.m.," says Lidia Amarales, the health minister in Chile's Magaflanes and Antarctic Region, where Flunta Arenas is the provincial capital.

Health authorities called an orange alert - the second most dangerous level in a scale of four - in which ultraviolet (UV) exposure can cause skin burns in 7 minutes. A red alert can provoke burning in 5 minutes.

"If people have to leave their homes, they should wear high-factor sun creams, UV protective sunglasses, wide brimmed hats and clothing with long sleeves," Amarales says.

Dr. Claudio Casiecia, head of the ozone department at the University of Magallanes, says ultraviolet radiation levels hit an all-time peak last Saturday. "We are slightly below that level now but still on alert."

Despite the alert, many local residents continue to walk the streets unprotected.

"I have to go to buy bread and scarcely have money for that, so forget the sunglasses and sunblock," says Adiiana Cerpa, a 28-year-old housewife.

Experts from the United Nations World Meteorological Organization (WMO) say the ozone hole is at its deepest level on record and that "near total destruction" of the ozone in some layers of the stratosphere had been observed since the middle of September, much earlier than in previous years. Chemicals - including chlorine compounds used in refrigerants, aerosol sprays and solvents and bromine compounds used in firefighting halogens - are blamed for causing depletion.

Extremely low temperatures in the stratosphere during the southern hemisphere's winter spark the chemical ozone depletion, a process that accelerates as the region enters spring- time.

For more than a decade, the hole has appeared in late August or early September, with the phenomenon peaking in the first week or two of October, a clear sign that greenhouse gases are eating away the Earth's protective layer.

All 12 monitoring stations around the rim of the Antarctic have reported measurements of ozone this spring that are 50 to 70 per cent below the norms in the years 1964 to 1976, before, the ozone hole was detected, the Geneva-based WMO says.

An image released by the National Aeronautics and Space Administration last month showed a hole appearing as a giant blob, totally covering Antarctica and stretching to the southern tip of the Americas.

NASA says the hole spread over 28 million square kilometres, about three times the size of Canada.