Ozone-depleting substances

Content

• Overview
• Chlorofluorocarbons
• Hydrochlorofluorocarbons
• Halons
• Bromochloromethane
• Carbon Tetrachloride
• Methyl Bromide
• Methyl Chloroform
• Hydrobromofluorocarbons

Overview

Ozone-depleting substances (ODS) generally contain chlorine, fluorine, bromine, carbon, and hydrogen in varying proportions and are often described by the general term halocarbons. Chlorofluorocarbons (CFCs), carbon tetrachloride, and methyl chloroform are important human-produced ozone-depleting gases that have been used in many applications including refrigeration, air conditioning, foam blowing, cleaning of electronics components, and as solvents. Another important group of human-produced halocarbons is the halons, which contain carbon, bromine, fluorine, and (in some cases) chlorine and have been mainly used as fire extinguishers.

These substances are effective ozone-depleters for two reasons. The first is that they do not break down in the lower atmosphere - they can remain in the atmosphere from 20 to 120 years or more. Unlike most chemicals released into the atmosphere at the Earth's surface, ozone-depleting substances are not "washed" back to Earth by rain or destroyed by other chemicals, which means they drift up into the stratosphere. The second is that they contain either/both chlorine and/or bromine and thus help the natural reactions that destroy ozone. Once they reach the stratosphere, ultraviolet (UV) radiation breaks up these molecules into chlorine (for example, from CFCs, methyl chloroform, or carbon tetrachloride) or bromine (for example, from halons or methyl bromide) which, in turn, break up ozone (O₃).

Chlorofluorocarbons

Chlorofluorcarbons (CFCs) are a group of chemical substances that contain one, two or three carbon atoms and at least one atom each of chlorine and fluorine. CFCs were first developed in the 1920s to replace sulphur dioxide as a coolant gas. In the 1930s they began to replace ammonia for cooling uses. Their non-toxicity, non-flammability, stability and heat-absorption effectiveness earned them early respect as wonder chemicals of the 20^th Century. In the late 1940s they began to be used as the propellant in aerosols. This use hit its peak in the late 1970s, when CFC was identified as an ODS and aerosols became the main target of public action. CFCs were also used in the production of packaging, insulation and other foams. In the 1980s they were widely used as coolants in refrigerators and air conditioners, as solvents in degreasers and cleaners and to dilute sterilant gas mixtures, and as blowing agents in the production of foams.

The manufacture and use of CFCs in industry has been severely curtailed following the Montreal Protocol. They have been almost completely phased-out in developed countries, with remaining uses limited to metered-dose inhalers. Starting in 2010, developing countries need to completely phase out CFCs.

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• Montreal Protocol

Hydrochlorofluorocarbons

Hydrochlorofluorocarbons (HCFCs) are a group of chemical substances that contain one, two or three carbon atoms and at least one atom each of hydrogen, chlorine and fluorine. The hydrogen makes them less stable and therefore less damaging to the ozone layer. Except for a few HCFCs already in use in Canada - mainly used as refrigerants - most HCFCs have been developed for use as transitional chemicals to replace the more damaging ozone-depleting substances, mainly CFCs. HCFCs have only two to five per cent of the ozone-depleting potential of CFCs, which makes them a good temporary replacement for applications where alternatives completely safe for the ozone layer are not yet available.

HCFCs are mainly used for foam blowing, refrigeration and air conditioning, solvent cleaning and to a lesser extent, aerosols and fire protection. At the 19^th Meeting of the Montreal Protocol, it was agreed to accelerate the phase-out of HCFCs in both developed and developing countries. Developed countries agreed to phase-out the production and consumption of HCFCs by 75% in 2010, by 90% in 2015, and complete this accelerated phase-out by 2020, while allowing for the continued use of 0.5% for servicing until 2030.

Halons

Halons are a group of chemical compounds containing bromine, chlorine, fluorine or carbon in its structure. Halons are very effective for extinguishing fires. They do not leave a solid residue and are not a threat to people when used at the recommended concentrations. The combined characteristics of halons make them suitable for all types of fire-extinguishing equipment, ranging from industrial/commercial total-flooding equipment to the hand-held fire extinguishers popular for office and home use.

Although halons do not present a direct hazard to people, they have a very high ozone-depleting potential (ODP) because they contain bromine. In fact, halon 1301 - which is largely used in total-flooding systems - has the highest ODP of all known ozone-depleting substances (10 times greater than that of CFC-11).

Canada has never produced halons but has imported some to satisfy its domestic needs. The importation of newly produced halons has been banned since January 1, 1994. Developed countries no longer produce new halons but use from stockpiles for airplanes and military applications is still allowed and continues. Developing countries must completely phase-out halons by 2010.

Bromochloromethane

Bromochloromethane is a chemical substance that contains chlorine, bromine, carbon and hydrogen. It is also known as Halon 1011 or Chlorobromomethane. Total elimination of consumption and production of bromochloroethane has been in place since January 1^st, 2002.

Carbon Tetrachloride

Carbon tetrachloride is a chemical substance consisting of one carbon atom and four chlorine atoms. It was widely used as a raw material (feedstock) in the production of CFCs. CFCs are no longer produced in Canada, eliminating the leading use of carbon tetrachloride in Canada. Carbon tetrachloride is also used in the manufacture of other chemicals that do not deplete the ozone layer. Smaller quantities of carbon tetrachloride were used in fire extinguishers, as a dry cleaning agent, and as an ingredient in pesticides, pharmaceuticals, paints and solvents.

Total elimination of consumption and production of carbon tetrachloride has been in place since January 1, 1996, with the exception of feedstock for manufacturing substances that do not deplete the ozone layer - this use not being controlled by the Montreal Protocol. Developing countries are due to achieve the same phase-out in 2010.

Methyl Bromide

Methyl bromide is a chemical substance containing bromine, hydrogen and carbon and is an important contributor to ozone depletion. This substance has a wide variety of agricultural uses as a fumigant. It is used for soil fumigation and fumigation of some food production facilities. Methyl bromide is also used in quarantine applications and pre-shipment applications for commodities in trade. This use of methyl bromide is not controlled under the Montreal Protocol.

Methyl bromide is not manufactured in Canada, but is registered for use under the Pest Control Products Act. Developed countries were to achieve the phase-out of agricultural uses by 2005. However, alternatives are often either not well known, developed, accepted or allowed by regulatory authorities. Therefore, there are still a number of critical use exemptions under the Protocol, meaning that significant volumes of methyl bromide continue to be used. Developing countries are scheduled to phase-out methyl bromide by 2015.

Methyl Chloroform

Methyl chloroform is a chemical compound consisting of carbon, hydrogen, and chlorine. It was popular because of its versatility and efficiency as a solvent in cleaners, degreasers and adhesives. It first appeared as a substitute for carbon tetrachloride in the mid-1950s, and by the 1980s was widely used by the electronics and equipment manufacturing industries. Under the Montreal Protocol, its use has been phased-out in developed countries since January 1, 1996 and developing countries have until 2015 to do the same.

Hydrobromofluorocarbons

Hydrobromofluorocarbons are a group of chemical substances that contain one, two or three carbon atoms and at least one atom each of hydrogen, bromine and fluorine. HBFCs were never commercialized in Canada. They were banned in 1996 in all countries.

[1] Ozone Depleting Potential. A measure of the capability of a particular chemical to destroy ozone, measured against CFC-11 which has an ozone-depleting potential of 1.

[2] Global Warming Potential. A relative measure of the warming effect that the emission of a gas might have on the surface troposphere. It is measured as a factor relative to CO₂.