Ozone is a colorless combination of three oxygen atoms. High in the atmosphere, about 7 to 25 miles above the Earth, ozone shields Earth from ultraviolet rays that cause skin cancer, crop damage and other problems.

Scientists at the United Nation a few years ago determined that without the 1987 treaty there would have been an extra 2 million skin cancer cases by 2030. They said overall the ozone layer is beginning to recover because of the phase-out of chemicals used in refrigerants and aerosol cans.