The Environmental Protection Agency unveiled plans yesterday for a staged, 50 percent cut in ozone-depleting chemicals, but environmentalists denounced the proposal as too weak to preserve the stratospheric layer vital to life on Earth.

Producers of the chemicals, meanwhile, predicted that prices would double in the 10-year phase-down, squeezing manufacturers of the myriad products in which the pollutant -- chlorofluorocarbons (CFCs) -- is an ingredient.

The plan announced by EPA Administrator Lee M. Thomas conforms with an international protocol signed by 24 nations in September. The agreement aims to stop CFC erosion of the ozone barrier, a gaseous veil that shields the Earth from ultraviolet radiation of the sun and prevents skin cancer, eye disease and crop damage.

CFCs, used in refrigerants, plastic foam and solvents, do not break down in the lower atmosphere like other pollutants. As they waft into the stratosphere, however, they release chlorine that eats up ozone.

"What we can prevent is depletion from occurring," Thomas said of the plan, which, in line with the protocol, would freeze U.S. consumption of CFCs at 1986 levels, cut back 20 percent after four years and another 30 percent six years later.

The five U.S. producers of CFCs would be required to report 1986 production levels to the EPA after the agreement goes into effect Jan. 1, 1989, if ratified by nations representing two-thirds of the world's consumption of CFCs. The agency would then allocate production quotas to each company, requiring CFC cutbacks consistent with the timetable agreed upon in September.

Thomas said the CFC controls would prevent a 40 percent depletion in ozone, as well as three million extra deaths from skin cancer that experts predict by the year 2075 if production of the gaseous chemical proceeded at the current pace. Despite decades of CFC use, he said, "if you don't add to that load" in the stratosphere, the protective shield over the United States will remain intact.

But his plan came under attack from environmentalists, who cited recent reports from Antarctica, where ozone levels have dropped 60 percent since 1979, as support for more stringent cutbacks than the protocol requires.

David Doniger of the Natural Resources Defense Council said that halving CFCs in 10 years "is not far enough or fast enough" to prevent ozone erosion and that an almost total phase-out is needed. He said that despite the protocol, the EPA is independently obligated to save the ozone layer, adding, "The agency has not done that."

At a news conference, Thomas said that since CFC production is worldwide, deeper U.S. cuts would have an "insignificant impact." The United States accounts for 29 percent of world CFC consumption.

For U.S. producers of CFCs, led by Du Pont, the plan means hefty price increases for the popular product as supplies dwindle. Manufacturers most dependent on CFCs, including plastic foam, would be hardest hit. Some are expected to go out of business, lay off workers and pass along added costs to the consumer.

"We are expecting a very severe economic impact," said Margaret Rogers of the Society of the Plastics Industry, adding that CFCs, used to puff up foam insulation and cushions, represent 20 to 30 percent of production costs.

Industry officials predict at least five years would be needed to develop environmentally acceptable alternatives to CFCs, which were involved in $28 billion worth of goods and services in 1986.