The earth's protective layer of ozone is decaying at less than half the rate of previous predictions, according to a report released by the National Academy of Sciences in Washington this week.

At the same time, a second report from the academy's National Research Council said the evidence is now much stronger that a thinner ozone layer will cause increased incidence of skin cancer, and may also cause a breakdown in the immune system which enables the body to fight disease.

The layer of ozone, a gas in the upper atmosphere, protects the earth by absorbing destructive ultraviolet rays from the sun. The sun's rays already cause thousands of cases of skin cancer annually.

In recent years scientists have expressed concern that two chemicals--fluorocarbons, which have been used as propellants in aerosol sprays, and nitrous oxide, which is present in jet exhaust--have been reacting with ozone and depleting the earth's protection against ultraviolet rays.

Three years ago, the National Research Council calculated that within the coming century, 15 to 18 percent of the ozone would be gone, which might cause as much as a 100 percent increase in skin cancer of several types.

The new estimates, using more accurate measurement techniques, put the 100-year ozone deterioration at 5 to 9 percent, still enough to cause a 25 to 50 percent increase in skin cancers.

For every 1 percent loss of ozone, the report finds, there will be a 2 to 5 percent increase in one type of skin cancer, called basal cell cancer, and 4 to 10 percent increase in another, squamous cell cancer.

These cancers are rarely fatal, and both are treatable. Another far more lethal cancer, called melanoma, may also be caused by increased exposure to ultraviolet light but the evidence is still not clear, the report said.

The report also noted another health effect of greater doses of ultraviolet light--a breakdown in the ability of the human immune system to fight off diseases of all kinds, including cancer.

In humans, for example, an amount of ultraviolet light that is enough to cause a sunburn can cause a decrease in the ability of the white blood cells to attack disease microbes for about 24 hours.