The hole waxes and wanes with the South Pole's seasons, growing larger in mid-August and slowly shrinking by December.

The ozone layer protects the Earth from the harmful effects of the sun's ultraviolet radiation, which can cause skin cancer.

Over the past three years the annual ozone hole over Antarctica has peaked at about the same size in both area and ozone content, NOAA said.

Each year when the sun rises over Antarctica, chemical reactions involving chlorine and bromine from man-made chemicals known as chlorofluorocarbons (CFCs) occur in the stratosphere and destroy ozone. An international treaty calling for the phase-out of CFCs is credited with stopping a steady increase in the ozone hole.

CFCs are made by a variety of U.S. manufacturing companies in products for cooling, cleaning agents, foam insulation and aerosol propellants.

Shrinking the ozone hole to its pre-1980 size will take at least 50 years and possibly longer due to climate changes, NOAA said.

NOAA and the National Aeronautics and Space Administration have shared satellite data for the past 20 years to measure the South Pole's ozone hole.