Since the 1940's, methyl bromide (MB) has been one of the most widely used fumigants in the world. Its advantages include:
However, MB has become one of the world's top environmental concerns since the discovery of its ozone-depleting action (WMO, 1991; UNEP, 1992). According to estimates published by the United Nations Environment Programme (UNEP), MB released from human activities is responsible for 5 to 10 % of the total ozone depletion in the earth's stratosphere.
The amount of MB from natural sources (emission by oceans and biomass burning), the role of the oceans as an MB sink and the amount of MB generated by man have not been precisely determined. From different sources (UNEP, 1994 and 1995) it can be calculated, however, that out of the total annual MB emissions of 105.5 to 292 kilotonnes/yr around 44 % are human-produced, agricultural uses being between one fifth and almost one third of the total amount. Other major anthropogenic sources are biomass burning (mainly for tropical forest clearance) and the use of leaded fuel.
Once in the stratosphere, high energy radiation from the sun releases a bromide atom by breaking the bond between the bromine and the methyl group. The very reactive bromine atom destroys molecular ozone (O3). The bromine atom also reacts with non-reactive molecules in the stratosphere that contain chlorine, liberating the chlorine, which will then destroy additional ozone molecules. Because of this "chain reaction", the bromine from MB is over 50 times more effective at destroying ozone than the chlorine from chlorofluorocarbons on a per-atom basis. The ozone depletion potential (ODP) for MB is listed as 0.6 (UNEP, 1994).
Bromine from MB has a comparatively short life in the stratosphere (up to two years). This means that benefits from MB control would be achieved within a few years. Failure, however, to control future use of MB could eliminate the gains made with the phase-out of other ozone-depleting substances.
Global ozone levels are decreasing steadily and, in 1992/93, were 3 - 4 % lower than the 1980's average (UNEP, 1994). Ozone depletion increases the level of UV-B radiation which has been linked to skin cancer, eye cataracts and other damage, the course and distribution of infectious diseases and immune system degradation (WHO, 1994; Gallagher, 1997). Farmers and other outdoor workers are particularly subject to these hazards (WHO, 1994). Furthermore, direct exposure to MB causes acute and chronic health problems such as respiratory difficulties, neurological disorders and, in some cases, death.
Special health hazards caused by MB use in developing countries include incorrect application by insufficiently trained staff using protective material which is often inadequate. In many countries, MB is frequently applied without using respiratory protection. Accidents occur quite regularly, and are sometimes even fatal. This problem persists in spite of considerable efforts undertaken by different parties to make fumigant application safer. This is not considered to be a major argument for the substitution of MB by many parties, but in the light of its ozone-depleting features, it supports the efforts towards a change to safer alternatives.
A series of possible implications of UV-B radiation on different ecosystems has been described (EEP, 1994) that includes yield reduction in a series of crops (Morrison, 1997), impacts on forest trees like reduced biomass production (Percy & Cameron, 1997), and on freshwater and wetland ecosystems (Vaughan & Pauli, 1997) and on marine ecosystems that result in reductions in certain fish populations essential for the fishing industry (EEP, 1994; Cullen, 1997).
Impacts on building, materials, especially reduction of the useful lifetime of polymer materials, and increases in tropospheric ozone levels of NOx-polluted regions have also been reported (EEP, 1994). The increase of tropospheric ozone concentrations has further implications for human health (eye diseases ), agriculture and forestry (possible reduction of roductivity).
MB has been included in 1992 in the Montreal Protocol's (MP) list of controlled substances. In the ninth meeting of the parties to the MP held in September 1997, the representatives of 163 nations decided on the phase-out of the production and the use of this fumigant in industrialised countries by the year 2005. The following interim reductions have been fixed: 25 % in 1999, 50 % in 2001 and 70 % in 2003. The phase-out date for developing countries has been fixed for the year 2015.
Some countries agreed voluntarily on earlier phase-outs. These countries include the USA (phase-out by 2001 pursuant to the Clean Air Act), Austria (phase-out for use in storage facilities by 1998), Denmark, Sweden, Norway, Finland and Indonesia (phase-out by 1998). The European Community (EC) and Canada plan a 25 % cut by 1998. In Germany, the treatment of soil, food crops and stored grains has already been phased out, as has all soil treatment in the Netherlands. Restrictions in the use of MB in Germany started in 1977 for toxicological reasons. Soil treatments have not been carried out since 1985. Many other industrialised countries have reduced MB use for commercial, safety or environmental reasons.
According to the 1994 Scientific Assessment of Ozone Depletion conducted by UNEP and other groups, eliminating MB is the most significant step to reducing ozone depletion.
Quarantine and pre-shipment uses (that account for about 17 % of global MB consumption) are currently exempt from the phase-out in industrialised countries. Quarantine uses are defined as treatments to prevent introduction, establishment and/or spread of quarantine pests and diseases whereas pre-shipment uses are applied directly preceding and in relation to existing phytosanitary or sanitary requirements of the importing or exporting country (Environment Australia, 1997). The Methyl Bromide Technical Options Committee (MBTOC) of the MP will periodically examine the further need for these exemptions. In 1995, the parties to the MP also agreed to identify exemptions for critical agricultural uses.
According to the MBTOC "technically feasible alternatives exist or are at an advanced state of development for more than 90 % of methyl bromide use" (Schonfield et al., 1995). Therefore, German development cooperation started with activities to support the rapid substitution of MB with alternative methods in developing countries worldwide.
