The ozone layer shielding Antarctica from ultraviolet radiation reached its thinnest point last month since measurements began, and government scientists said yesterday they have found the first hard evidence that the critical environmental loss can be blamed on a man-made gas.

Reporting on a six-week expedition to Antarctica coordinated by the National Aeronautics and Space Administration, the scientists noted that the effects of chlorofluorocarbons (CFCs) gas on the ozone layer in the stratosphere may be more severe in Antarctica than in the rest of the world because of the continent's weather patterns during its early spring.

But NASA program manager Robert Watson said that "CFCs can affect ozone globally," and the mission's report is expected to give impetus to an international agreement designed to halve world consumption of CFCs by early next century.

Stratospheric ozone forms a thin gaseous veil at least 12 miles above the Earth's surface, which screens out harmful ultraviolet rays and prevents skin cancer, eye disease, withering of crops and damage to aquatic life. In recent years, CFCs -- widely used as refrigerants, solvents and bubbling agents in foam products -- have been suspected as ozone depleters once they break down in the upper atmosphere.

In 1985, scientists determined that the ozone layer over Antarctica had thinned since at least 1979, and they have monitored the continent to determine the extent and cause of the problem because of its implications for the rest of the world.

The NASA expedition, using airplanes to take the first tests within the ozone sphere, found a 15 percent greater depletion in the protective layer than occurred in 1985 and about a 55 percent overall loss since 1979, Watson told reporters at a Goddard Space Flight Center briefing.

A team of 60 scientists gathered enough data to rule out theories that attributed ozone depletion to changes in the sun's output or movement of low-ozone air masses.

But the researchers found ample reason to confirm the role of CFCs. Chlorine monoxide, a byproduct of CFCs exposed to ultraviolet rays, was detected at levels 100 to 500 times higher than found at lower altitudes, Watson said. Moreover, he said, as concentrations of ozone fell, levels of chlorine monoxide rose.

"There is no longer debate as to whether {chlorine monoxide} exists within the chemically perturbed region . . . at abundances sufficient to destroy ozone," he said.

But the NASA scientist said the unusual meteorology of Antarctica, with its dehydrated atmosphere and extremely cold temperatures, especially during the Antarctic spring of August and September, is "critical" in explaining the release of active chlorine particles that so voraciously gobble up the ozone layer.

"The meteorology is important in setting up environmental conditions," he said. But he added that "chlorine has made the hole deeper," referring to the "ozone hole" concept used to describe the depleted layer.

Although Watson said the Antarctic experience could have implications for other regions of comparable climate, he emphasized that the findings were preliminary and it is too soon to project global meaning.

But environmentalists immediately interpreted the data as reason for CFC curbs and fast ratification of an international agreement tentatively approved by 46 nations earlier this month that would freeze world consumption of the chemical at 1986 levels and cut by half its use in the industrial world within a decade.