Methyl Bromide Ban

Prof. Peter Saunders gives timely warning for maintaining the ban on a powerful ozone depleter

Six years ago, 160 countries agreed in the Montreal Protocol to phase out substances that are known to deplete the ozone layer. The most important of these were the chlorofluorocarbons (CFCs), which were being extensively used as refrigerants and as propellants in aerosols. Also included was a plant pesticide, methyl bromide, which developed countries are supposed to have phased out by 2005. Developing countries were allowed a further ten years.

Methyl bromide (CH3Br) is widely used for fumigating agricultural products (including wood) and to kill the organisms in soil in which crops such as tomatoes and strawberries are to be planted. It tends to be used more in warmer climates where soft fruits are more susceptible to attack.

When methyl bromide reaches the stratosphere it is split by ultra-violet radiation. This releases bromine, which is approximately 40 times as effective as chlorine at breaking down ozone. Only about a fifth of the methyl bromide that enters the atmosphere comes from human activity, but it is such a powerful ozone depleter that banning it makes an important contribution to restoring the ozone layer. It is also highly toxic, and because it is volatile at 20 C, it is hard to prevent it from drifting beyond the fields on which it is being applied or to escape from greenhouses. When it enters the soil, it tends to penetrate to the deeper soil layers where it breaks down only slowly, which makes it dangerous to use in water collection areas.

Methyl bromide persists for less than a year in the atmosphere, so measures taken now can have an immediate effect. Following the signing of the Montreal Protocol, production of methyl bromide has declined and so has the level in the atmosphere.

The Montreal Protocol allows exemptions for “critical users”. This is a category that was intended to allow banned substances to be used in small quantities in applications in which there is no suitable alternative: CFCs are still permitted as propellants in inhalers for asthmatics but the amounts involved are relatively small. There is now, however, pressure to grant large-scale critical use exemptions to farmers, especially in the USA, and so that raw wood can still be used in packaging. If these are allowed, they could halt, or even reverse, the decline in atmospheric methyl bromide.

A decision was expected in November but the delegates to the meeting at the Nairobi headquarters of the United Nations Environment Programme (UNEP) agreed they needed more time to consider the issues. There is now to be an extraordinary meeting in Montreal in March 2004. With only a year to go before the ban is meant to come into force, any users who do not gain the exemptions they are hoping for will have very little time to comply. So it’s likely to be a very difficult meeting.

Montzka, S.A., J.H. Butler, B.D. Hall, D.J. Mondeel, and J.W. Elkins, A decline in tropospheric bromine, Geophysical Research Letters, 30 (2003) 1826

Further reading can be found on the following web sites:
US Environmental Protection Agency www.epa.gov/ozone/mbr/
Methyl Bromide Alternatives Outreach www.mbao.org/

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