PUNTA ARENAS, Chile (Reuters) -- A wide swath of southern Chile was on alert Monday 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-percent depletion being recorded throughout most of the hole, United Nations experts said Friday.

The tip of the Americas is the only landmass 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.," said Lidia Amarales, the health minister in Chile's most southerly Magallanes and Antarctic Region, where Punta 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 indicates burning can occur 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," said Amarales.

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

Despite the alert, many local residents walked the streets unprotected Monday. "I have to go to buy bread and scarcely have money for that, so forget the sunglasses and sun cream," said Adriana Cerpa, a 28-year-old housewife.

Experts from the United Nations' World Meteorological Organization (WMO) said Friday 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, as well as bromine compounds used in firefighting halogens -- are blamed for causing depletion.

Extremely low temperatures in the stratosphere during the Southern Hemisphere's winter spark off the chemical ozone depletion, a process that accelerates as the region enters spring.

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 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 percent below the norms in the years 1964-1976, before the ozone hole was detected, the Geneva-based WMO said.

An image released by the National Aeronautics and Space Administration (NASA) on September 8 showed a hole appearing as a giant blue blob, totally covering Antarctica and stretching to the southern tip of South America.

NASA said the hole spread over 11 million square miles (28.5 million square kilometers), an area three times larger than the landmass of the United States.