Fumigation

DescriptionProperties of Fumigants Commonly Used in Malaysia

Fumigation

Fumigation offers one of the most effective control measures against varieties of stored product pests and for quarantine purposes for export commodities or plant quarantine requirements. In some situation, fumigation has been used for the control of drywood termite and wood borer in structural buildings.

The term “Fumigation” is often misunderstood by layman to mean pest extermination, which is the incorrect definition. Fumigation is generally defined as “ a chemical which at a required temperature and pressure, can exist in the gaseous state in sufficient concentration to be lethal to a given pest organism.” Fumigation can therefore be defined as a control technique, which involves the establishment of an atmosphere containing a lethal gas in the environment of an insect of a concentration high enough, and an exposure period long enough to kill the target insect.

Under the Malaysian scenario, the main fumigants registered and approved for use under the application of licensed fumigators are: hydrogen cyanide (HCN), methyl bromide and aluminium phosphide (phosphine gas). All forms of fumigation work carried out should be in accordance with The Hydrogen Cyanide Act and Regulations (1953) which was enacted in 1953 and later revised in 1981. The main objective of this Act is to regulate the fumigation of premises, vessels, aircrafts and articles. The purpose of this Act and the regulations under it is to protect the public from dangers in connection with the fumigation of premises and articles which include ships or vehicles. This also includes the issue of proper disposal of residue arising form the fumigation work. In order to carry out such fumigation under this Act, the Pest Control Operators or personnel must have a Fumigation License from the Ministry of Health.

Properties of Fumigants Commonly Used in Malaysia

METHYL BROMIDE (CH3Br) – is a widely used fumigant for quarantine purposes and soil fumigation for the control of noxious weed and soil borne diseases. Methyl bromide is a colourless liquid with a boiling point of 3.5°C. Hence unless under pressure, it exists as gas at normal temperature. At a concentration normally used for fumigation purposes, methyl bromide vapour is odourless. This is the reason why 2% chloropicrin is added as a lachrymatory warning agent. At high concentrations, methyl bromide is said to posses a musty or sticky sweet odour. Methyl bromide gas is 3.3 times heavier than air.


Methods of Application

Because methyl bromide is heavier than air and due to its vapour density, the gas is introduced at the top of the fumigation enclosure via an appropriate pattern of distribution piping and outlet nozzles. In small fumigations, a single tube leading into the centre of the top of the enclosure may be adequate. Methyl bromide may be introduced into the fumigation enclosure as a gas or as a liquid.


Ventilation Requirements

Upon completion of the fumigation with methyl bromide, the building structures or commodities must be thoroughly ventilated. Sometimes, in order to ventilate methyl bromide, it is necessary to provide a thorough flow of air and where possible a fan of suitable capacity should be used to provide a forced drought.


Safety Precautions

Methyl bromide is highly toxic to all forms of animal and human life. It is very dangerous to inhale the vapour and to allow the liquid or concentrated vapour to come into contact with the skin. The effect of a single inhalation depends on the concentration and the period of exposure. The new Threshold Limit Value (TLV) for the methyl bromide was set at 5 ppm in 1981 from 20 ppm in 1964; this is the maximum limit that can be tolerated at continuous exposure of 8 hr/day at 40 working hr/week. The effect of repeated exposure is cumulative. It is important to note that symptoms of poisoning are slow to appear and may take several days to a few weeks or months.

For protection against occasional exposure to low concentrations, use a full-face gas mask fitted with exhaust value and special methyl bromide filter or canister. Even if the operator is protected by a gas mask his total exposure to methyl bromide vapours should not exceed 15 minutes per day.

Phospine (PH3) is widely used as a fumigant in the stored pest products. It has reached this position because it has most of the properties desirable in a fumigant and is commercially available in easy-to apply solid formulation. The fact that methyl bromide fumigant is being slowly phased out under the Montreal Protocol Agreement and the ozone depletion substances issues has also caused PH3 to be prominent as an alternative.

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Properties

Phosphine is a highly toxic colourless gas. It has a boiling point of 87.4°C and a molecular weight of 34.04. It is highly flammable and slightly soluble in water. The safety limit for prolonged exposure is 0.3 ppm in air or 0.4 milligram/cubic meter of air. Phosphine is slightly heavier than air (20%)


Mode of Action

Aluminium phosphide is available in a number of different forms including tablets, pellets, blanket and sacket. All these forms contain a mixture of aluminium phosphide and other ingredients. On contact with water vapour in the air these preparations liberate a gaseous mixture of phosphine and other gases, which act to lower the combustibility of other mixtures and allow fine grinding of the aluminium phosphide, which results in quicker and more complete decomposition of the preparations. Commencement of the process involving the evolution of hydrogen phosphide (phosphine) is accompanied by a change in external appearance of the tablets or pellets. The shiny gray-green surface turns matter rough and whitish and finally commences to bloom. Grey white dust appears on the surface, the structure becomes brittle, volume of the tablets and pellets increase finally resulting in the pile of grayish dust (composed of aluminium oxide hydrate, a constituent of clay) 5 times the original volume of the tablet or pellet. This process takes 3-4 days for complete decomposition of the formulation (although depending on the temperature and humidity once the tablets are exposed to the atmosphere, decomposition may take 48-72 hours or in the case of pellets 12-48 hours).


Methods of Application

Aluminium phosphide tablets and pellets can be applied directly to a grain stream being loaded into sealed silo bin or flat storage either by an automatic dispenser or by hand. Natural gas movement will then distribute the gas throughout the storage. Care should be taken to ensure that no tablets or pellets touch each other in order to avoid the risk of localized heating, explosion or spontaneous combustion.

Alternatively, tablets or pellets can be placed on trays that will facilitate the removal of residues at the completion of fumigation and ventilation. With this method, precaution must be taken to avoid piling of tablets or pellets on the trays, i.e. it should be spread out in a single layer. This mode of application is particularly applicable when fumigating bag stacks under sheet covers. Likewise, it is extremely important that the sheeting materials to be used will provide sufficient gas tightness to ensure maximum retention of fumigant vapours and for safety reasons.


Ventilation Requirements

There are the minimum ventilation periods for the following structures containing treated commodities.

(a) With through flow and forced drought (flash proof fan) operated 2 hours on and 2 hours off in period of 12-24 hours, depending on the size of the structure.

(b) With thorough flow and natural draught (wind) for structure of 300 tonnes or greater capacity for a period of 2-5 days; depending on size, for structure of less than 300 tonnes capacity, ventilation period is 5 days


Safety Precautions

Exposure to phosphine is the reported cause for depression of the central nervous system and impairment of the respiratory function. Inhalation of phosphine may produce the following symptoms: nausea, vomiting, headache, diarrhoea, shock, abdominal pain chest pain or intense thirst. The current Threshold Limit Value for phosphine in the air is 0.3 ppm.

A full-face mask equipped with the correct phosphine-absorbing canister must be worn during fumigation. Also, gloves should be worn when the phosphine generating tablets or pellets are handled.

HYDROGEN CYANIDE (HCN) – Hydrogen cyanide is used to control insect in stored grains and seed. This fumigant has been used mainly in Malaysia for the ship fumigation and de-railing certificate requirements. HCN may be generated in fumigation practices by the action of moisture on sodium or calcium cyanide, by dispensing of gaseous HCN from a cylinder or by release of HCN absorbed into an inert material (discoids).

In disinfestation of grains, hydrogen cyanide is used in the form of pressurized liquid in gas cylinders or as a liquid absorbed into special cardboard disks.

A full-face mask equipped with the correct phosphine-absorbing canister must be worn during fumigation. Also, gloves must be worn when dispensing of gaseous HCN material (discoids) are handled.