Various substances containing chlorine or bromine can break down in the stratosphere, triggering chemical reactions that deplete the ozone layer. These substances evaporate easily and are stable at lower levels in the atmosphere, but they can easily rise into the stratosphere without being broken down. Their stability at ground level also means that they pose little direct risk to humans and the terrestrial environment. At one time, these chemicals were used in many applications, but now they are strictly controlled to help protect the ozone layer.
The substances described below deplete stratospheric ozone at various rates, as described by their ozone-depleting potential (ODP) factors.
The first CFC compounds (fully halogenated chlorofluorohydrocarbons) came onto the market in the 1950s.They have been used as propellants in aerosol sprays, as refrigerants, in the production of plastic foams, in dry cleaning and grease removal processes, and in laboratory analyses. The most commonly used CFCs have an ODP of 1.
Halons (fully halogenated compounds containing bromine, fluorine and/or chlorine) have mainly been used in fire extinguishers. They are the most harmful substances involved in ozone depletion, as their ODP varies between 3 and 10. The usage of halons is banned in Finland, with the exception of certain applications where no safe alternatives are available.
Carbon tetrachloride (ODP 1.1) and 1,1,1-trichloroethane (ODP 0.1) have both been used as solvents and in laboratory analyses, but their usage today is limited to certain laboratory applications where they are irreplaceable.
Methyl bromide is a toxic gas used as an agricultural pesticide and during the transportation and protective storage of foodstuffs and other products. It is also used as a pesticide to preserve museum exhibits and historic buildings. In Finland methyl bromide was also previously used as a pesticide gas in flour mills, but it was banned for this purpose in 1999. Metyhyl bromide has an ODP of 0.6.
When controls were imposed over the usage of CFCs, they were often replaced with HCFC compounds (partially halogenated chlorofluorohydrocarbons). HCFCs have relatively low ODP factors of about 0.1 on average. HCFCs have mainly been used in the manufacture of foams and as refrigerants.
Bromochloromethane is used as a solvent and in fire extinguishers. It has never been used in Finland, and its manufacture and use was banned globally in 2002.