In 1974, chemists F. Sherwood Rowland and Mario Molina warned that chlorofluorocarbons (CFCs), widely used as refrigerants, were destroying the ozone layer.
Using a spectrometer, British researchers discovered the ozone hole in the lower stratosphere over Antarctica in the mid-1980s.
The hole swells and contracts; it grew largest in Sept. 2000, spanning about 17.5 million square miles.
Ozone in the stratosphere is created and destroyed through natural cycles. CFCs and other substances speed its destruction and cause lower-than-normal levels.
Stratospheric ozone filters the sun's ultraviolet rays. Overexposure to these rays can cause skin cancer.
The first global restriction on CFCs was the 1987 Montreal Protocol on Substances that Deplete the Ozone Layer.
Some scientists predict the ozone layer will not recover for 60 years at the earliest.