Methyl Bromide (MB)
Methyl bromide is a broad spectrum fumigant effective against most of the storage pests. It has the ability to penetrate quickly and deeply into sorptive materials at normal atmospheric pressure and at the end of a treatment, the vapours dissipate rapidly and make possible the bulk commodities safe. Methyl bromide is nonflammable and non explosive under ordinary circumstances and may be used without special precautions against fire. Another important property is the fact that many living plants are tolerant to this gas in insecticidal treatments.
National Standard for Phytosanitary Measures No. 11 (NSPM 11) – Quarantine Treatments and Application Procedures: I. Methyl Bromide Fumigation has been developed by Government of India to provide guidance on quarantine treatments and application procedures for the approved fumigation agencies (Pest Control Operators). National Standard has also been developed for accreditation procedure of these Phytosanitary Treatment Providers who have been authorized to undertake fumigation with Methyl bromide both for agricultural commodities and Wood Packaging Material in line with the requirement of ISPM 15.
Methyl bromide, in gaseous state, is released in an airtight enclosure with the required dose and specified length of time. Thereafter, the enclosure is degassed. Safety norms are kept in view while all this process is completed. The dose and exposure period is applied as per the requirement of the Importing Country. In case of Wood Packaging Material, dose at the rate of 48gm. per cubic metre at or above 21 degree Celsius is applied with the exposure period of 24 hours as per ISPM 15. On aeration the fumigated enclosure, the Threshold Level Value (TLV) should be 5 ppm or less.
Although the methyl bromide fumigation acknowledged as having potential negative impact on environment, yet so far it is considered as the most appropriate treatment method in eradication of pest infestation in agriculture products. In consideration of above, the use of methyl bromide is only allowed for pre-shipment treatments and quarantine purposes including ISPM-15 compliance under Montreal protocol and any other critical use exemptions (CUE) subject to the approval by the United Nations Environment Protection Agency.
The demerit of Methyl bromide is that it is readily photolyzed in the atmosphere to release elemental bromine, which is far more destructive to stratospheric ozone than chlorine. As such, it is subject to phase-out requirements of the 1987 Montreal Protocol on Ozone Depleting Substances. Most of the Countries have committed to phase-out by the year 2015. India has also to phase out by the year 2015.
|State: Colourless liquid
(The fumigant is liquid under pressure. When in contact with air, there is immediate vaporization. The fumigant is stored in steel cylinders or tin cans)
(It is for this reason, 2% chloropicrin (tear gas) is added as warning agent)
|Boiling Point :
|Freezing Point :
i) At gaseous stage : 3.27 at 0°C
|ii) At liquid stage : 1.732 at 0°C|
|Latent Heat of
Vaporization : 61.52 cal/g.
|Flammability : Non flammable||Solubility in water:
Affects all organic material especially natural rubber. Non-corrosive to metals.
|Relation between dosages and concentration of Gas in air
|Natural Vapour pressure at various temperatures
690 mm Hg at 0°C ; 1006 mm Hg at 10°C ; 1390 mm Hg at 20°C;
1610 mm Hg at 25°C.
(More the vapour pressure, better is the penetrating capability of the gas)
|Mode of Action:
The insects absorb fumigant through their respiratory system as they breathe (respire) through small openings in their skin. Insect breathing/respiration depends on a number of factors including temperature and humidity. The higher the temperature the faster they breathe/ respire and take in methyl bromide. Their metabolism is temperature dependent. Conversely the lower the temperature the more inactive they become. Their respiration is affected by the humidity in their environment. The drier it is, the slower they breathe/respire, to conserve body water.
However, dosages vary from commodity to commodity vis-à-vis target insect pest.
|Threshold Limit value (TLV):
|Effects on different commodities
Metals : Pure methyl bromide is not corrosive.
Paints: Not affected.
Synthetic rubber: Not affected
Natural rubber: Severely affected.
i) germination of seeds: Most of the seeds are tolerant. Germination is however affected if moisture content in seeds is 14 percent and above.
ii) Growing plants: Vitality of young plants is impaired. There is dessication and immature falling of leaves in case of orchid plants but no injury is caused to plants. Viability may be affected.
iii) Dormant plants: Tolerant but viability may be affected.
iv) Fruits: Deciduous fruits tolerant. Some of the fleshy fruits are tolerant but others are susceptible to injury both internal and external.
v) Vegetables: Fresh and dry vegetables tolerant but shelf life may be affected.
vi) Cereals and Milled products: Maida, rawa-tolerant; products with high protein are affected.
vii) Nuts and nut meals: Not affected.
viii) Dried Fruits: Not affected.
ix) Dairy products: Not affected.
Conversion: Converted into fumigant and bromide salts. Inorganic bromide is formed. Methylation of protein occurs.
Residues: Residues are formed in food stuff with MB fumigation. These residues are not important from human health point of view, if the food is consumed in normal quantity. In normal circumstances, it is well below the tolerance level fixed by the Codex Alimentarius Commission of WHO. It should also be mentioned here that many food products contain naturally occurring bromides.
|Precaution and First Aid
Produces neurological symptoms such as nausea, vomiting, dizziness, blurred vision, unusual fatigue, headache, loss of appetite, abdominal pain, impaired speech, mental confusion and convulsions
High concentration results in death through pulmonary injury and associated circulatory failure.
Toxic symptoms may appear within 30 minutes to 48 hours depending upon the intensity of exposure and personal reaction of the affected person. On skin, severe irritations, skin blisters and dermatititis. Chloropricrin, which is used as warning gas (mixed with MB (@ 2%) cause lacremation.
Artificial respiration. Oxygen if available. No mechanical resuscitation. If on skin, wash 15 min with large amounts of water. If on clothing, vapours may be released in toxic quantities. Hence the clothes should be removed and safely aerated.
Symptomatic-Artificial respiration, oxygen without mechanical resuscitation. Analysis of breath and blood may help in diagnosis and prognosis. Nausea accompanied by vomiting. Give intravenous glucose drip.
Specific precautions to be followed when using methyl bromide fumigant are listed on the label and labeling accompanying the cylinders or cans. However the following general safety procedures must be applied in all cases of fumigation:
Should always undertake fumigation operations by the approved fumigation operator at least in the presence of experienced and trained co-worker. Supervisors should avoid undertaking fumigation work by fatigue workers as they prone for risk of accidents.
|Usage as Fumigant
MB is the most preferred fumigant in Plant Quarantine work because of its:
· good penetrating power;
· quick dispersal;
· tolerance of many plants and plant products;
· high toxicity to insects;
· effectiveness on wide range of temperature;
· limited residual reaction;
However, the following precautions should be adopted:
|Concentrations are usually expressed as g/m3. However they may sometimes be expressed as parts per million (ppm), or as percentages (%). Useful methyl bromide gas conversions: