Ozone molecules (O₃) consist of three oxygen atoms. This poisonous gas is extremely rare in the atmosphere, representing just three out of every 10 million molecules. Ninety per cent of ozone exists in the upper atmosphere, or stratosphere, between 10 and 50 km (6 - 30 miles) above the earth. Ozone at ground-level, at the bottom of the troposphere, is a harmful pollutant resulting from automobile exhausts and other sources.

The ozone layer absorbs most of the harmful ultraviolet-B radiation from the sun. It also completely screens out lethal UV-C radiation. The ozone shield is thus essential to life as we know it. Depleting the ozone layer allows more UV-B to reach the earth. More UV-B means more melanoma and non-melanoma skin cancers, more eye cataracts, weakened immune systems, reduced plant yields, damage to ocean eco-systems and reduced fishing yields, affects on animals, and more damage to plastics.

Scientific concern started in 1970, when Prof. Paul Crutzen pointed out the possibility that nitrogen oxides from fertilizers and supersonic aircraft might deplete the ozone layer. In 1974, Professors F. Sherwood Rowland and Mario J. Molina recognized that when CFCs finally break apart in the atmosphere and release chlorine atoms they cause ozone depletion. (Bromine atoms released by Halons have the same effect.) The three scientists received the Nobel Prize for Chemistry in 1995 for their pioneering work.

The issue of ozone depletion was first discussed by the Governing Council of the United Nations Environment Programme (UNEP) in 1976. A meeting of experts on the ozone layer was convened in 1977, after which UNEP and the World Meteorological Organization (WMO) set up the Coordinating Committee of the Ozone Layer (CCOL) to periodically assess ozone depletion. Inter-governmental negotiations for an international agreement to phase out ozone-depleting substances started in 1981 and concluded with the adoption of the Vienna Convention for the Protection of the Ozone Layer in March 1985.

The 1985 Vienna Convention encourages intergovernmental cooperation on research, systematic observation of the ozone layer, monitoring of CFC production, and the exchange of information. The Convention commits its Parties to take general measures to protect human health and the environment against human activities that modify the ozone layer. The Vienna Convention is a framework agreement and does not contain legally binding controls or targets.

The Montreal Protocol on Substances that Deplete the Ozone Layer was adopted in September 1987. Following the discovery of the Antarctic ozone hole in late 1985, governments recognized the need for stronger measures to reduce the production and consumption of a number of CFCs (CFC-11, 12, 113, 114, and 115). The Protocol was designed so that the phase-out schedules could be revised on the basis of periodic scientific and technological assessments. Following such assessments, the Protocol was adjusted to accelerate the phase-out schedules in London in 1990, Copenhagen in 1992, Vienna in 1995 and Montreal in 1997. It has also been amended to introduce other kinds of control measures and to add new controlled substances to the list; the 1990 London Amendment included additional CFCs (CFC-13, 111, 112, 211, 212, 213, 214, 215, 216, 217), several Halons (1211, 1301, 2402), carbon tetrachloride, and methyl chloroform, while the 1992 Copenhagen Amendment added methyl bromide, HBFCs, and HCFCs.

Governments are not legally bound until they ratify the Protocol as well as the Amendments. Unfortunately, while most governments have ratified the Protocol, ratification of the amendments and their stronger control measures lag behind. As of 4^th August 1998, the Ozone Agreements had been ratified by countries as follows:

• Vienna Convention - 167 Parties;
• Montreal Protocol - 166 Parties;
• London Amendment - 123 Parties; and
• Copenhagen Amendment - 80 Parties.

• Halo-carbons, notably chlorofluorocarbons (CFCs) and Halons. CFCs were discovered in 1928 and were considered wonder gases because they are long-lived, non-toxic, non-corrosive, and non-flammable. They were also versatile and from the 1960s were increasingly used in refrigerators, air conditioners, spray cans, solvents, foams, and other applications. CFC-11 remains in the atmosphere 50 years, CFC-12 for 102 years, and CFC-115 for 1,700. Halon 1301 is used primarily in fire extinguishers and has an atmospheric lifetime of 65 years.
• Carbon tetrachloride is used as a solvent and takes 42 years to break down in the atmosphere.
• Methyl chloroform (1,1,1-trichloroethane) is also used as a solvent and takes 5.4 years to break down.
• Hydrobromofluorocarbons (HBFCs) are not widely used, but they have been included under the Protocol to prevent any new uses.
• Hydrochlorofluorocarbons (HCFCs) were developed as the first major replacement for CFCs. While much less destructive than CFCs, HCFCs also contribute to ozone depletion. They have an atmospheric lifetime of 1.4 to 19.5 years.
• Methyl bromide (CH₃Br) is used as a fumigant for high-value crops, pest control, and quarantine treatment of agricultural commodities awaiting export. Total world annual consumption is about 70,000 tonnes, most of it in the industrialized countries. It takes 1.3 years to break down.

• Phase out Halons by 1994;
• Phase out CFCs, carbon tetrachloride, methyl chloroform, and HBFCs by 1996;
• Reduce methyl bromide by 25% by 1999, 50% by 2001, 70% by 2003, and phase out by 2005; and
• Reduce HCFCs by 35% by 2004, 65% by 2010, 90% by 2015, and phase out by 2020 (although 0.5% is permitted for maintenance purposes only until 2030).

Developing countries have a grace period before they must start their phase-out schedules. This reflects the recognition that developed countries are responsible for the bulk of total emissions into the atmosphere and that they have more financial and technological resources for adopting replacements. The developing country schedules are as follows:

The phase-out schedules cover both the production and the consumption of the target substances (except the HCFC schedules, which apply only to consumption). However, even after phase out both developed and developing countries are permitted to produce limited quantities of most substances in order to meet the basic domestic needs of developing countries. In addition, the Parties may agree on exemptions for essential uses for which no alternatives have yet been identified, e.g. the use of CFCs in metered dose inhalers for asthma. Production is defined as total production minus amounts destroyed and used as chemical feedstock and process agents. Consumption is production plus imports minus exports. Trade in recycled and used chemicals is not included in the calculation of consumption in order to encourage recovery, reclamation and recycling.

Without the Protocol, by the year 2050 ozone depletion would have risen to at least 50% in the northern hemisphere's mid latitudes and 70% in the southern mid latitudes, about 10 times worse than current levels. The result would have been a doubling of UV-B radiation reaching the earth in the northern mid latitudes and a quadrupling in the south. The amount of ozone-depleting chemicals in the atmosphere would have been five times greater. The implications of this would have been horrendous: 19 million more cases of non- melanoma cancer, 1.5 million cases of melanoma cancer, 130 million more cases of eye cataracts.

In 1986 the total consumption of CFCs world-wide was about 1.1 million tonnes; by 1996 this had come down to about 160,000 tonnes. It has been calculated that without the Montreal Protocol global consumption would have reached about 3 million tonnes in the year 2010 and 8 million tonnes in 2060, resulting in a 50% depletion of the ozone layer by 2035. The bulk of the 1986 total, or about one million tonnes, was consumed in industrialized countries, but by 1996 these countries consumed just 10,000 tonnes (for exemptions approved by the Parties). The developing countries have increased their consumption by about 30% over the last 10 years. Twenty of the 120 developing countries account for more than 90% of this group's consumption, and of these 20, key countries including Argentina, Chile, the Islamic Republic of Iran, Republic of Korea, Malaysia, the Philippines, the Syrian Arab Republic, Thailand, and Venezuela had already started to reduce CFC consumption as of 1996.

Scientists predict that ozone depletion will reach its worst point during the next few years and then gradually decline until the ozone layer returns to normal around 2150, assuming that the Montreal Protocol is fully implemented. Despite declining CFC emissions, stratospheric concentrations are still increasing (although they are declining in the lower atmosphere) because long-lived CFCs emitted in earlier years continue to rise to the stratosphere. The atmospheric abundance of certain CFCs, (notably CFC-11, CFC-113), carbon-tetrachloride, and methyl chloroform is declining. Concentrations of HCFCs and other substitutes are of course increasing.

Consumers are recycling existing Halons to gain time for developing substitutes for fire fighting. Other extinguishing agents such as carbon dioxide, water, foam, and dry powder are now widely used. Alternative approaches, such as good fire prevention practices, use of fire-resistant materials, and appropriate designs for buildings have significantly reduced the need for Halon systems, and total phase-out was achieved smoothly by 1994.

Industrialized countries are concentrating their phase-out efforts on HCFCs and methyl bromide. They are trying to ensure that HCFCs are used only as direct replacements where other more environmentally suitable alternatives are not available. HCFCs were critical for meeting the early CFC phase-out goals but are generally considered much less important for new equipment because they do have some ozone-depleting potential.

The Global Environment Facility (GEF) was established by the world community to help developing countries deal with ozone depletion, climate change, biodiversity, and international waters. GEF supports projects and activities for phasing-out ozone-depleting substances in countries with economies in transition, as these Central and East European countries are not eligible for Multilateral Fund assistance. To date, US$111 million has been approved by GEF to assist the following countries: Belarus, Bulgaria, the Czech Republic, Hungary, Poland, the Russian Federation, Slovakia, Slovenia, and Ukraine.

The Protocol has been hailed as an extraordinary success so far, but there is no room for complacency because: