The worst of the ozone hole has pulled back once more to Antarctica this southern spring, leaving behind a shadow of uncertainty for the people living at the bottom of the Americas...

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PUNTA ARENAS, Chile — The worst of the ozone hole has pulled back once more to Antarctica this southern spring, leaving behind a shadow of uncertainty for the people living at the bottom of the Americas.

How many will develop skin cancer in years to come? How many more decades must their children live with dangerous ultraviolet rays? Will the global treaty to save the ozone survive until then?

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The people of wind-blown Punta Arenas, like the local evergreens forever bent eastward from westerly gusts, are adjusting to the intense radiation that pours each year through the gap in the ozone layer. At least that’s what some say.

“People are better informed. They’re buying more sun-block and putting it on their children,” said pharmacist Gerardo Leal.

“They’ve gotten used to it,” taxi driver Rene Bahamonde assured a visitor.

But on a “red alert” day when UV rays could have damaged eyes, Bahamonde’s dark glasses sat unused by his side. And local health chief Dr. Lidia Amarales said many of the 150,000 Punta Arenans take few precautions against a damaging sun as they go about their business on the quiet streets that slope downward to the broad, chill waters of the Strait of Magellan.

The reason is simple: It’s cool here.

“When it’s 30 degrees somewhere [86 Fahrenheit], people don’t go out into the sun. Here, with 13 degrees [55 Fahrenheit], they go outside,” Amarales said.

This is a gray, drizzly corner of South America, but clouds are no protection against UV. It rarely exceeds 70 Fahrenheit, and “without the heat, they don’t ‘feel’ the radiation,” Amarales said. “We need to change habits.”

The stratosphere’s layer of ozone, a form of oxygen, for countless millennia filtered out almost all the sun’s cancer-causing ultraviolet-B rays. But in the 1970s, scientists warned that manmade chemicals such as chlorofluorocarbons (CFCs), used in aerosol sprays and refrigerants, were destroying ozone through chain reactions high in the skies. By the 1980s, satellite images showed the world, an “ozone hole” had formed over Antarctica.

Air currents and intense cold in the polar region, combined with chlorine from CFCs, created a vast expanse of ozone-thin atmosphere that briefly reached the tip of South America each southern spring. Measured in Dobson units, ozone here was found in October 1992 to have thinned to 147 units, less than half the normal 333. Ultraviolet radiation, in its most damaging wavelengths, multiplied many times.

The world’s nations took action in 1987, signing the Montreal Protocol, phasing out some CFCs and other ozone-damaging compounds. As a result, chlorine has declined in the lower atmosphere since the mid-1990s, while the rate of growth of bromine has slowed.

It will take decades to purge the atmosphere. Experts watch year by year for positive signs, and in fact this September’s maximum ozone hole, at 8 million square miles, was markedly smaller than last year’s huge 11-million-square-mile hole, though similar in size to the one in 2002.

But Dutch climatologist Henk Eskes, a leading ozone analyst, cautioned that climatic changes make it hard to draw conclusions.

“It’s still very difficult to say that it’s really at a turning point. There’s a lot of variability from one year to the next because of wind patterns and dynamical situations,” he said.

Fernando Carbacho wears his $100 UV-guard sunglasses rain or shine. “But not everyone can afford good glasses,” the retired soldier said. Besides, he said, “people are contrary,” resisting being told what to do.

“They know what the problem is,” Amarales said. “They just haven’t changed their habits.”