The ozone content of Earth's atmosphere declined about 5 percent between 1979 and 1986, according to the first report from an effort to monitor ozone levels worldwide.

The decline was as high as 30 to 40 percent over the poles -- where, during a seasonal Antarctic "ozone hole," more than 60 percent of the ozone was depleted -- and as low as zero over the tropics. Over the United States ozone levels were down between 0.5 and 1 percent.

Ozone molecules in the stratosphere, six to 20 miles up, absorb much of the sun's ultraviolet light, preventing potentially harmful amounts from reaching the Earth's surface.

Although the findings suggest global ozone is declining faster than some had predicted it would because of continued release of chlorofluorocarbons (CFCs) -- the ozone-destroying chemicals used as refrigerants and aerosol propellants -- scientists said most of the decline so far may be the result of natural processes. There is evidence going back 30 years that ozone levels have fluctuated naturally by comparable amounts in cycles of 10 to 15 years.

It is believed that CFCs released on the ground eventually find their way into the stratosphere where ultraviolet light splits off the molecule's chlorine atom.

Chlorine then acts as a catalyst that turns ozone, a molecule made of three oxygen atoms, into ordinary oxygen gas, which has two oxygen atoms. The loss of the third atom renders oxygen unable to absorb ultraviolet light. Atmospheric chemists fear even a small chlorine buildup in the stratosphere because each chlorine atom can catalyze the same ozone-destroying reaction over and over indefinitely.

Ultraviolet light can cause skin cancer, cataracts and immune deficiency. As ozone is depleted, increasing doses are expected to raise the risk of all these diseases.

The findings were reported in today's Science magazine by Kenneth P. Bowman, an atmospheric chemist at the University of Illinois at Urbana-Champaign. They are based on data from Nimbus 7, a research satellite that since 1979 has been taking daily ozone readings all over the world, except when the poles were in darkness.

Although beneficial in the upper atmosphere, ozone at ground level is a potential pollutant. In high enough concentrations, which usually extend over a relatively small area, ozone can irritate eyes, nose and throat, damage plants and corrode various nonliving materials.

Forecasts of CFC effects have varied considerably. More conservative estimates have suggested depletions from 3 to 8 percent over the next 50 to 100 years. More extreme projections hold that the ozone layer could be destroyed in a hundred years.

Although satellite ozone monitoring did not begin until 1979, earlier spot measurements begun in 1957 indicate that ozone levels were declining until 1961 when they began rising to a peak in 1970. From that peak, ozone declined again until the mid-1970s, when it began rising again, peaking once more in 1979.

"It was kind of a coincidence that the satellite monitoring began in 1979, just when we had a peak," Bowman said. "So far there's no reason to think the decline we're seeing from the satellite is anything other than part of the up and down that we've known for 30 years."

Bowman said the 1986 ozone level, the latest that has been analyzed, appears to be about the same as the low recorded in the early 1960s. He said the 1986 data suggest the decline may have bottomed out and that the ozone "may be beginning to recover."

Bowman said, however, that this was no reason to discount the CFC concerns because effects of the chemical, now widely agreed to threaten stratospheric ozone, would be added to those of natural rises and falls.

The Nimbus 7 satellite measures ozone indirectly by comparing the amount of ultraviolet light directly from the sun that strikes a white panel on the satellite with the amount reflected up from Earth, which has been filtered twice by ozone, once on the way down from the sun and once on the way up.

Last Sept. 30, countries signed an agreement to cut production of CFCs by 20 percent by 1994 and to consider a further 30 percent reduction by 1999.