Government scientists say a global environmental threat is not lessened by their discovery that the "hole" observed annually in Earth's protective ozone layer is smaller than it has appeared in two years.

The National Oceanic and Atmospheric Administration attributes the decrease measured above the South Pole to natural phenomena affecting the layer that shields Earth from harmful levels of solar radiation. Efforts to reduce the human factors causing ozone depletion must continue, according to the agency."(The new) readings do not mean that the threat of reduced ozone levels globally no longer exist, nor does it portend the end of the Antarctic ozone hole," the NOAA said in announcing its findings Monday.

"The readings indicate that large year-to-year changes in ozone can occur from natural variations in atmosphere processes," the agency explained.

Balloons launched by NOAA scientists at the pole this month documented the latest ozone levels. Similar observations were reported by the sophisticated equipment aboard the NIMBUS-7 satellite operated by the National Aeronautics and Space Administration.

Walter Komhyr, a NOAA physicist in Boulder, Colo., said the smaller hole may have been caused by atmospheric winds that shifted more ozone from other parts of the planet and warmer temperatures that slowed the dissipation of ozone.

Discovery of the Antarctic hole in 1985 intensified concerns about the effect of industrial emissions on the ozone layer. Scientists now warn that continued depletion would have serious repercussions, including higher cancer rates.

Chlorine-containing chemicals - especially chlorofluorocarbons, or CFCs, used in refrigeration and insulation - break down ozone molecules.