Ozone is a form of oxygen with molecules carrying three atoms instead of two. Ozone is found both in the troposphere, the lower 10 km of the atmosphere, and in the stratosphere, 10 to 50 km above the ground. Ozone acts as a shield protecting us against harmful ultraviolet radiation from the Sun.
However, ozone at ground level is a pollutant. It can trigger breathing difficulties and damage plants and crops. It is one of the principal ingredients of smog. Therefore, whether ozone is good or bad depends on its altitude in the atmosphere.
The ozone layer has been under attack from chlorine (chlorofluorocarbon, CFC) and bromine (halon) compounds abundantly used in the past in products such as aerosol spray can propellants, refrigerants, pesticides, solvents and fire extinguishers. When these substances reach the stratosphere, the ultraviolet radiation from the Sun causes them to break apart and release chlorine and bromine atoms, which react with ozone. These reactions trigger chemical cycles of ozone destruction that deplete the protective ozone layer.
It has been calculated that just a single chlorine atom can obliterate more than 1 000 ozone molecules. Bromine atoms however, are some 50 times more efficient at destroying ozone. Fortunately, the atmospheric abundance of bromine containing compounds is much lower than that of CFCs.
Countries have been adhering to international agreements currently in place, such as the Vienna Convention and the Montreal Protocol and its Amendments. After peaking around year 2000, the total amount of chlorine and bromine containing compounds in the stratosphere is now slowly going down, but it will probably take 50 years before the amount of chlorine and bromine is back at where it was before 1980 (roughly when the first Antarctic ozone hole was observed).
Recent scientific reports show that ozone reduction has stopped in most regions of the globe, but it could take years before ozone starts to increase again. The Antarctic ozone hole, which appears every year in the September to November time period has not become worse in the last 5-10 years, but there is also no sign of a significant improvement yet.