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Methyl Bromide Questions & Answers
- What is methyl bromide? How is it used?
- Why has EPA taken action on a pesticide under the Clean Air Act?
- What about the science linking methyl bromide to ozone depletion?
- What is the Montreal Protocol? How does it regulate methyl bromide?
What is methyl bromide? How is it used?
Methyl bromide is a broad spectrum pesticide used in the control of pest insects, nematodes, weeds, pathogens, and rodents. In the U.S., methyl bromide is used in agriculture, primarily for soil fumigation, as well as for commodity and quarantine treatment, and structural fumigation.
The chemical name (IUPAC, CAS) for methyl bromide is bromomethane, and it is classified as an alkyl bromide. It is a colorless and odorless gas at normal temperatures and pressures, but the liquefied gas can be handled as a liquid (14.4 lb/gal) under moderate pressure. The specific gravity at 0ºC and 760 mm Hg is 1.732, with a vapor density of ~3.27, boiling point of 3.6ºC (38.5ºF), vapor pressure at 20ºC of 1400 mm/Hg (at 40ºC it is 2600 mm/Hg), and the viscosity is 0.22 centistokes at 0ºC. Methyl bromide is readily soluble in lower alcohols, ethers, esters, ketones, halogenated hydrocarbons, aromatic hydrocarbons, and carbon disulfide.
Methyl Bromide is manufactured from naturally occurring bromide salts which are either contained in underground brine deposits, or in highly concentrated above ground sources like the Dead Sea. Ocean water does contain bromine salts, but at such low concentrations that it is very energy intensive to use as a source in the manufacture of methyl bromide. Methyl bromide is often produced as a by-product of other bromide manufacturing processes.
When used as a soil fumigant, methyl bromide gas is usually injected into the soil at a depth of 12 to 24 inches before a crop is planted. This will effectively sterilize the soil, killing the vast majority of soil organisms. Immediately after the methyl bromide is injected, the soil is covered with plastic tarps, which slow the movement of methyl bromide from the soil to the atmosphere. Additional methyl bromide is emitted to the atmosphere at the end of the fumigation when the tarps are removed. When an entire field is fumigated, the tarps are removed 24 to 72 hours later, as can be the case in strawberry production in California. However, with row (or bed) fumigation, as is the case with tomato production in Florida, the tarps are left on for the entire growing season, some 60 to 120 days. About 50 to 95% of the methyl bromide injected in to the soil can eventually enter the atmosphere. In the United States, strawberries and tomatoes are the crops which use the most methyl bromide. Other crops which use this pesticide as a soil fumigant include tobacco, peppers, grapes, and nut and vine crops.
When used as a commodity treatment, methyl bromide gas is injected into a chamber or under a tarp containing the commodities. A high proportion of the methyl bromide used for a typical commodity treatment eventually enters the atmosphere. Commodities which use this material as part of a post-harvest pest control regime include grapes, raisins, cherries, nuts, and imported materials. Some commodities are treated multiple times during both storage and shipment. Commodities may be treated with methyl bromide as part of a quarantine or phytosanitary requirement of an importing country.
A structural pest control treatment with methyl bromide gas involves the fumigation of buildings for termites, warehouses and food processing facilities for insects and rodents, and ships (as well as other transportation vehicles) for various pests.
Methyl bromide is a toxic material. Exposure to
this chemical will affect not only the target pests
it is used against, but non-target organisms as well.
Because methyl bromide dissipates so rapidly to the
atmosphere, it is most dangerous at the actual
fumigation site itself. Human exposure to high
concentrations of methyl bromide can result in
central nervous system and respiratory system
failure, as well as specific and severe deleterious
actions on the lungs, eyes, and skin. Additional information on the health effects
of methyl bromide exposure is available from EPA's Office
of Air Quality Planning & Standards, the Centers
for Disease Control and Prevention 
, and the California
Department of Health Services .
Why has EPA taken action on a pesticide under the Clean Air Act?
Scientific assessments conducted by atmospheric
scientists under the authority of the World
Meteorological Organization with the National Oceanic
and Atmospheric Administration and the National Aeronautics
and Space Administration indicate that methyl bromide
contributes significantly to the destruction of
earth's stratospheric ozone layer. The executive
summary of the 2006 Scientific Assessment is
available from NOAA . It contains the most up-to-date understanding of
ozone depletion and reflects the thinking of
international scientific experts who contributed to
its preparation and review.
What about the science linking methyl bromide to ozone depletion?
Methyl bromide is considered to be a significant ozone depleting substance (ODS) by atmospheric scientists. The 2002 Scientific Assessment of Ozone Depletion contains a thorough discussion and analysis on the scientific understanding of ozone depletion.
Methyl bromide in the stratosphere comes from both
natural and anthropogenic sources. Natural sources of
methyl bromide, such as the ocean, plants, and soil,
can also be a sink for this material. Scientists
continue to measure atmospheric chemistry both in the
troposphere and stratosphere to better understand
this issue. Specific information on methyl bromide
and the oceans is available from NOAA.
While methyl bromide is a natural substance, many
scientists believe the additional methyl bromide
added to the atmosphere by humans may upset the
balance in the stratosphere which allows life to
exist on this planet. Emissions contribute to a
thinning of the ozone layer, allowing increased
amounts of radiation to reach the earth's surface,
with potential impact not only to human
health and the environment,
but to agricultural
cropsas well.
What is the Montreal Protocol? How does it regulate methyl bromide?
The Montreal Protocol is an international treaty developed to protect the earth from the detrimental effects of ozone stratospheric depletion. Since its initial signing by the United States and 26 other countries in 1987, virtually the whole world has signed on to the treaty (now 184 countries are “Party” to the treaty.) The Parties to the Montreal Protocol agreed to specific reduction steps that lead to the phaseout of production and import of ozone-depleting substances, including methyl bromide.