Ozone layer
Over the last hundred years, the ozone layer has been damaged by man-made chemicals, especially CFCs (chlorofluorocarbons), which were used mainly in aerosol sprays and refrigerants. CFCs are broken down in the upper atmosphere when they react with the ozone, causing ozone depletion. International leaders recognized this and united in banning the use of CFCs.  As a result, the hole in the ozone layer has been shrinking and the ozone layer has been recovering.