Fresh warnings about shrinkage of the ozone layer that shields the earth from the sun's ultraviolet rays will be sounded early next year by the National Academy of Sciences.

The new warnings will say that in spite of the ban on nonessential uses of the aerosol gas Freon, enough of the gas is still being used and leaked into the atmosphere that it will cause a 5 percent depletion of the ozone layer in the next 20 years. By the year 2010 the academy will say, the gas will reduce the ozone shield by as much as 15 percent.

Academy sources said the report stating all this will not come out until early next year because the scientists writing it are still unsure about the effects of a 5 and even a 15 percent reduction of the ozone layer.

"We have to take a much closer and longer look at the ultraviolet question," one academy source said. "We don't understand it all and we're not in agreement on what we think we understand."

What scientists do agree on, the report will say, is that if present levels of fluorocarbon use persist the ozone layer will continue to shrink before the loss stabilizes at 15 percent.

Four years ago, scientists said that a 5 percent loss in the ozone layer might be enough to trigger a worldwide increase of 3 to 4 percent in skin cancers among light-skinned people. Most skin cancers are caused by over-exposure to the sun's ultraviolet light, more of which would reach the earth if the ozone layer were to shrink.

The rise in Freon usage even as it was curtailed for nonessential purposes has come from its role as a refrigerant. Air conditioners also use Freon. No substitutes have been found for either.

Scientists say that the great virtue of Freon is also its Achilles heel. It's inert, odorless and fireproof, but because it's inert the gas persists and rises above thestratosphere into the ozone layer at an altitude of 20 miles. There, it's attacked by ultraviolet light and broken down into its constituents, one of which is chlorine.

It's the chlorine that acts to break up the ozone. There is no way to reverse this reaction, scientists say, even though many scientists have tried adding other chemicals to fluorocarbons to prevent its attack on the ozone layer.

The academy report due next year will say that if the ozone layer is reduced by 15 percent in the next 30 years the temperature of the upper stratosphere could fall by as much as 10 degrees Celsius. Serious as that sounds, it is not viewed seriously by scientists who believe it will not cause any change at the earth's surface.