The ozone layer is showing signs of recovering, thanks to a drop in ozone-depleting chemicals, but it is unlikely to stabilise at pre-1980 levels, researchers said. Depletion of the earth's protective ozone layer is caused by the chemical action of chlorine and bromine released by man-made chlorofluorocarbons (CFCs), which are used in aerosol sprays and cooling equipment. Ozone-depleting chemicals were banned by the 1987 Montreal Protocol which has now been ratified by 180 nations. "We now have some confidence that the ozone layer is responding to the decreases in chlorine levels in the atmosphere due to the leveling off and decrease of CFCs,' said Betsy Weatherhead, of the University of Colorado in Boulder.