Fumigants are gaseous forms of chemicals that are used to eliminate the pest risk. Any pest control product is NOT a fumigant. The main difference between a regular pest control product and a fumigant is that the fumigant, being in either a liquid or solid form is converted into a gas during the application process and it remains in the gaseous form during the entire fumigation period until the fumigation structure is aerated. Fumigants are very toxic and lethal to insects and in sufficient concentration, it can also be lethal to humans and animals.
Insect pests or other pests found living in or around the product, also hide in hard to reach nooks and crevices in storage premises, food processing units, equipment, etc. None of the insecticidal formulations which may be sprayed in such places will be effective in containing the pests. It is only the fumigant which could penetrate such inaccessible spots and eradicate the pests. When a pest population warrants a treatment, fumigants are the best tool to penetrate into structures and commodities without leaving a dangerous residue. However, fumigants can be hazardous and deadly and require a specialized team to apply the fumigant and carry out the fumigation process.
Hydrogen Phosphide or Phosphine is an effective and widely used fumigant for stored food products, processed foods, animal feed, tobacco and other products. Phosphine used in Singapore comes in various formulations, most common of which are Degesch plates, pellets, tablets and bags.
Phosphine is very toxic to all forms of animal life; hence exposure of human beings even to small amounts should be avoided
Properties of Phosphine
Phosphine is a low molecular weight, low boiling point compound which is generated when the metallic phosphides (aluminium or magnesium phosphide) react with atmospheric moisture. The rate of evolution of the solid phosphides to gas depends on the temperature and humidity; higher temperatures speed up this reaction while winter conditions in temperate climates slows down the process.It penetrates deeply into materials such as bulk grain stores or stored products such as rice, soya beans, wheat grains or tobacco in tightly packed bags. Each Degesch plate formulation weighing 117 gm. liberates 33 gm. of phosphine. At relative humidity of over 75%, maximum release of phosphineoccurs in 45 hours at 25°C.
Phosphine is a colorless gas and has excellent penetrative power. It has approximately the same density as air hence it does not require the air within the fumigationstructure to be circulated prior and during the gas release unlike methyl bromide.
Threshold Limit Value1 g of phosphine will generate 718 ppm in air
|Alternative name||Hydrogen Phosphide|
|Odour||Carbide or garlic-like odour may be due to impurities|
|Specific gravity gas (air = 1)t||1.214°|
|Lowest explosion point||1.79% by volume in air|
|Method of evolution as fumigant||From preparations of aluminum and magnesium phosphide|
|Pertinent chemical properties||Reacts with copper and precious metals.|
|Threshold Limit Value||0.3 ppm|
A strong odour, resembling carbide or garlic, is normally associated with the evolution of phosphine from various formulations. It can be detected by smell even at very low concentrations. While any odour associated with the evolution of phosphine may indicate the presence of phosphine, it should not be relied on for warning purposes.
How to Determine the Quantity of Phosphine to be Used
To fumigate 30 Metric Tons (mt) of rice by Magnesium Phosphide @ 2 g/mt with the requisite humidity and temperature.
To measure concentration in ppm, we must first establish the volume occupied by 30 mt of rice.
Typically an average 1mt of rice occupies 1.5 m3 of volume. Hence, 30 mt of rice would occupy 45 m3 of volume.
From this, at 2 g/m3, 90 gm. of phosphine gas would have to be released within the enclosure holding the 30 mt of rice or in the 45 m3 of volume. (45 m3 x 2 g = 90 g)
The amount of magnesium phosphide plates to be used is 2.7 Degesch plates of 33 g each would be required (90 g / 33g per plate = 2.7 plates)
Concentration in ppm would be = 2 gm X 718 ppm = 1431 ppm approximately.
Note: The minimum phosphine concentration should not drop below 200 ppm on the last day of fumigation after 72 hours of exposure period.
Toxicity to Insects
Phosphine ranks as one of the most toxic fumigants of stored product insects. It is a slow acting poison that is effective at very low concentrations with sufficient exposure time. Usually, exposure time of four or more days are required to control insects, depending on temperature. The toxicity of phosphine to insect’s declines as the temperature falls to 5°C, so that longer exposure times are required for it to exert its effect. It is not recommended for use below 5°C. The exposure time however cannot be shortened by increasing the dosage; in fact, high concentrations can have a narcotic effect on insects thereby reducing mortality and increasing the chances of resistance in insects and consequently causing the effectiveness of phosphine to be reduced considerably by development of resistance in insects.
Some stages of insects are considerably more tolerant to phosphine than others. The eggs and pupae are usually hardest to kill while larvae and adults succumb more easily. Hence the need for a sufficient exposure period to ensure that all life stages are killed.
Monitoring Gas Concentration
To monitor gas concentrations achieved at various stages in a fumigation process, various instruments are used:
- Drager X-AM 5000 – from 0 pm to 2000 ppm – to measure the gas concentration within the fumigation enclosure. This is critical to enable the fumigator to monitor that the gas concentration is maintained above the required levels to ensure the efficacy of the treatment. Should the concentration drop below the stipulated threshold of 300 ppm, sufficient top-up of the fumigant must be applied to raise the concentration and extend the exposure time.
- Drager PAC 7000 – from 0.00 ppm – to 20 ppm to measure the gas concentration within the fumigation risk area is kept below the TLV of 0.3 ppm
Disposal of Spent Fumigant Residue
After a fumigation, the residual material should be disposed of in an approved manner.
For magnesium phosphide plates, this to done by immersing the spent plates or strips in locked wire containers into a drum of water and allowed to soak for 36 hours before they can be discarded as general waste.
For aluminium phosphine, the residue powder from the tablets, aluminium hydroxide powder should be slowly added to a drum of water with detergent as a wetting agent and stirring it into the water until a slurry is formed and the residue sinks.
For purposes of safety the disposal procedure should be carried out in the open air, where any generated phosphine can rapidly disperse.
The waste water used for treating the spent plates must conform to local government regulations on the treatment of waste water before in can be discarded in the sewage or waterway.
Reaction with Metals
Phosphine reacts with certain metals, particularly copper, copper compounds, silver and gold to cause corrosion. This reaction is enhanced by the presence of ammonia, which is given off during the decomposition of some proprietary formulations. High humidity and temperature appear to favour the reaction, particularly in air with a salt content as found near the sea.
As a result of this reaction any copper-containing equipment, especially electrical apparatus, may be severely damaged. During fumigation of buildings with phosphine special attention should be given to electric motors, electric wiring, switches, fire alarm systems, electronic systems or other pieces of equipment that contain copper.
If equipment that is liable to damage cannot be removed from the area being treated some protection may be afforded by coating copper materials with a thin layer of paraffin, spraying with a light lubricating oil.
Toxicity to Humans
Phosphine is very toxic to all forms of animal life, hence exposure of human beings even to small amounts should be avoided. Poisoning can result from ingestion or inhalation; however, the gas is not absorbed through the skin.
The threshold limit value-time weighted average (TLV-TWA) for an eight-hour daily exposure in a five-day week is set at 0.3 ppm. The maximum concentration to which workers should be exposed for a period up to 15 minutes is 1 ppm, with the stipulation that at least 60 minutes should elapse between such exposures and provided the daily TLV-TWA of 0.3 ppm is not exceeded. A concentration of 2000 ppm is known to be lethal to humans.
For personal protection against the vapours of phosphine at concentrations above the threshold limit, a respirator with an A2B2 filter must be used. Such respirators however will give protection up to 0.5 percent phosphine by volume in air. Above this concentration, air must be supplied by a self-contained breathing equipment. The Drager PAC 7000 must always be available on hand with the Fumigator to keep a tab on the level of phosphine concentration to decide on the appropriate respiratory device to be used.
1. Gloves should be worn when tablets or pellets are being dispensed by hand.
2. Respirators need not be worn when phosphine plates, tablets or pellets are being dispensed under normal conditions where there is a delay in evolution of the fumigant from these formulations and the operator does not breathe the vapours of phosphine. However, respirators equipped with an A2B2 canister designed for protection against phosphine and the Drager PAC 7000 should always be on hand in case of emergency.
3. Odour of the fumigant cannot be relied upon as an indication of whether or not the operator is breathing poisonous concentrations. The Drager PAC 7000 must always be available on hand with the Fumigator to keep a tab on the level of phosphine concentration in the surrounding area.
4. Do not smoke or touch food at any time during the application of this insecticide.
5. Any spaces adjoining or close to structures undergoing treatment with phosphine should be kept continuously aired by leaving windows open or by providing artificial ventilation by means of fans or blowers.
6. All persons working, or likely to work, in any place near the fumigation area must be notified that fumigation is in progress. Warning notices should be posted to prevent exposure of employees or the public at large to the gas.
7. When the fumigation is completed and aeration of a structure is undertaken, full precautions must be undertaken to ensure that no person is exposed to residual vapours of the fumigant.
Should a person become exposed to phosphine as a result of inattention, negligence, failure to follow proper procedures or some other reason and, as a result, symptoms consisting of fatigue, ringing in the ears, nausea, or pressure in the chest appear, he should go immediately into the open fresh air.
Symptoms of poisoning by a small quantity of phosphine will normally disappear when a person is removed to the fresh air. However, despite the seeming insignificance of even mild cases of poisoning with symptoms as described above, first aid measures are absolutely necessary before and until the arrival of a doctor.a. Remove patient from exposure area into open area with fresh air. Keep at rest.
b. If the patient is unconscious, place in semi-prone recovery position or otherwise maintain the airway.
c. If patient is conscious but has difficulty in breathing, treat in a seated position and give oxygen if available.
d. Allow patient to recline with the legs slightly elevated.
e. If breathing stops, immediately ventilate the patient artificially
f. If the heart stops, begin cardiopulmonary resuscitation-CPR Under no conditions should an affected person resume work during the next 48 hours, particularly work dealing with fumigation, as it takes time for the body to eliminate the poison completely. Complete abstinence from alcoholic beverages after any poisoning is strongly recommended.
Symptoms of Poisoning
According to the amount of phosphine inhaled, symptoms may occur immediately or several hours after exposure.
Slight or mild poisoning may give a feeling of fatigue, ringing in the ears, nausea, pressure in the chest and uneasiness. All of these symptoms will normally disappear in fresh air.
Greater quantities will quickly lead to general fatigue, nausea, gastrointestinal symptoms with vomiting, stomach ache, diarrhoea, disturbance of equilibrium, strong pains in the chest and dyspnoea (difficulty in breathing).
Very high concentrations rapidly result in strong dyspnoea, cyanosis (bluish-purple skin colour), agitation, ataxia (difficulty in walking), anoxia (subnormal blood oxygen content), unconsciousness and death. Death can be immediate or occur several days later due to oedema and collapse of the lungs, paralysis of the respiratory system or oedema of the brain. Disturbances of kidney and liver functions (hoematuria, proteinuria, uraemia, jaundice) and cardiac arrhythmia may occur.
Methyl Bromide (CH3BR)
Methyl Bromide (MB) was a widely used fumigant for decades but since it depletes the stratospheric ozone layer the use of this fumigant has greatly decreased in use. At this time, the only permitted uses for Methyl Bromide in Singapore are for:
1.Quarantine and Preshipment (QPS) to eliminate quarantine pests.
2.Critical Use Exemptions (CUE) designed for uses with no technically or economically feasible alternatives
MB is a preferred fumigant for most of the quarantine authorities around the world, because of its:
- good penetrating ability
- vapours disperse quickly upon aeration
- tolerance of many plants and plant products
- high toxicity to insects; it is very effective in controlling insects at all stages from egg to the adult stage
- effectiveness on wide range of temperature
- limited residual reaction
- non-flammable and non-explosive
However, the following precautions should be adopted:
- since the gas is heavier than air, a good circulatory system is essential for even distribution of gas
The effectiveness of methyl bromide is based on the following:
- Dosage of the fumigant
- Duration of exposure
Fumigants only control existing infestations in the cargo they do not provide any residual protection against subsequent re-infestation. Consequently, timber treated by MB fumigation must be packed in container or shipped within 21 days of treatment. During this 21 day period it should be stored safely or cross infestation may occur. After completion of Methyl Bromide fumigation ,care should be taken to store the treated timber and the packing area pest free and good hygienic practices should be followed in order to avoid cross infestation between fumigated and unfumigated cargo.
Methyl bromide is absorbed by oils, fats and finely ground materials. It is also known to react with materials containing sulphur, including food stuffs and proteins to produce objectionable discoloration and odours which may persist even after prolonged aeration.
Fumigation of Wooden Crates Fumigation of wooden crates should be carried out with methyl bromide at the dosage of 48 grams per meter cube. In this process the wooden packing crates are placed in the fumigation covers under a smooth floor and then it converted completely. The gas monitoring lines for measuring gas concentration during the exposure period is placed at different locations as per requirements. The fumigation covers are sealed with the help of sand snakes and an air tight enclosure is made. The total cubic area of the enclosure is measured and the required dosage of methyl bromide is administrated. Gas monitoring is carried out during the exposure period. After the exposure period of 24 hours the fumigation covers are removed and the empty wooden crates are aerated
Methyl Bromide fumigation should be carried out for packing materials such as wood crates, skids and other packing materials. The maximum thickness of the packing wood must not exceed 200 mm.
Properties of MB
|Odour||Nil at low concentrations; strong musty or sickly sweet at high concentrations|
|Boiling point||3.6°C (38.5°F)|
|Specific gravity gas (air=1)||3.27 at 0°C|
|Flammability limits in air||Nonflammable|
|Pertinent chemical properties||Powerful solvent of organic materials, especially natural rubber. When pure, noncorrosive to metals. Liquid reacts with aluminium|
|Method of evolution as fumigant||From steel cylinders under natural or added pressure.|
|Threshold Limit Value||5 ppm|
|Dosage||128 g -Giant africal snail
80 g – Khapra Beetle
48 g – timber/wood packaging materials
32 g – stored food prdocts
At normal fumigation concentrations methyl bromide is odourless. This disadvantage is sometimes overcome by mixing it at the time of packaging with a warning gas such as chloropicrin. The chloropicrin usually constitutes 2 percent of the mixture.
The effect of methyl bromide on humans and other mammals appears to vary according to the intensity of exposure. At concentrations not immediately fatal, this chemical produces neurological symptoms. High concentrations may bring about death through pulmonary injury and associated circulatory failure. The onset of toxic symptoms is delayed, and the latent period may vary between 0.5 to 48 hours, according to the intensity of the exposure and the personal reaction of the patient. Contact of the human skin with the liquid or strong concentrations of the gas may cause severe local blistering.
Against insects, methyl bromide exerts its principal toxic effect on the nervous system. As in humans, the onset of poisoning symptoms may be delayed, and with many species of insects definite conclusions as to the success of the treatment should be delayed for at least 24 hours.
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.
Monitoring of MB Gas Concentration For the purpose of monitoring methyl bromide concentrations within the fumigation enclosure to check the efficacy of the treatment as well as so as to detect any serious leaks in the air within the risk area for the purpose of protecting personnel during fumigation and subsequent aeration, the devices to be use dare:
- Riken FI-21 – for measuring MB gas concentration in the fumigation enclosure
- Riken GH 202F -for measuring MB gas concentration in the surrounding area
Discharging methyl bromide In Singapore, MB is supplied in liquid form in 20 kg & 50 kg steel cylinders. To discharge the fumigant, the liquid from the cylinder is led through a heat exchanger or vaporizer which is a special container with copper coils immersed in water heated to 65 C. This causes the liquid MB as to be immediately vaporised by the heat.
Measurement of Dosage To accurately measure the quantity of fumigant to be released, digital weighing scales are used when the dosage of methyl bromide cannot be computed in terms of whole cylinders.
Concentrations Toxic to Humans
Persons should not be exposed continuously to concentrations of this gas in excess of 5 ppm. This is the threshold limit for an 8-hour daily exposure.
From experiments with animals and records of accidents to human beings, it appears that daily exposure to concentrations of 20 to 100 ppm of methyl bromide can quickly bring about severe neurological symptoms, described below under “First aid”. Exposure for only a few hours to concentrations of 100 to 200 ppm may cause severe illness or death.
Methyl bromide is listed as a compound that may be absorbed through the skin as well as by the respiratory system. In a properly planned fumigation, the operator releasing the gas should keep moving away from the initial high concentration. During aeration, the operators usually wait for some time after as many doors and windows as possible have been opened and the ventilators or blowers started. After entering the structure, the operator is thus exposed to rapidly falling concentrations.
During any phase of a fumigation operation where there is likelihood of exposures to methyl bromide above 5 ppm, appropriate respiratory protection must be taken either in the form of a full-face gas mask with an AX filter or a self-contained breathing aparatus if required.
Contact of Liquid with the Skin
Prolonged contact of liquid methyl bromide with the skin produces severe blisters similar to those caused by burns or extreme chilling. Great care should be taken to avoid spilling liquid methyl bromide on clothes or footwear. Leather or rubber boots, shoes and gloves are likely to retain the liquid and hold it in contact with the skin. Since there is no particular sensation produced by such contact, methyl bromide may be maintained in contact with the skin for extended periods without an awareness that this has occurred.
As soon as possible after methyl bromide is spilled on clothing or footwear, it should be removed and thoroughly aired. If methyl bromide has remained in contact with the skin so that blisters form, the blisters should be left intact and the area covered with a sterile dressing. When working with methyl bromide, gloves should not be worn. If liquid is spilled on the hands, lower arms or other exposed areas of skin, it will evaporate quickly. However, the parts touched should be washed with soap and water immediately.
There is no known antidote for methyl bromide poisoning. Also, because the onset of symptoms is usually delayed, there are no specific procedures to bring about immediate recovery. However, there are certain well-defined symptoms which, except in cases of exposure to high and rapidly fatal concentrations, may serve as preliminary warnings of initial poisoning. If, on experiencing these symptoms, the fumigator immediately abstains from further contact with methyl bromide and places himself under medical supervision, there is every prospect of complete recovery.
If any of the symptoms listed below are experienced during or after exposure to methyl bromide, the person affected should leave the vicinity and report immediately to a physician.
Important. These symptoms may be delayed for periods up to 45 hours.
Possible symptoms of methyl bromide poisoning are:
- nausea and vomiting
- loss of appetite
- abdominal pain
- double or blurred vision
- impaired, slurred speech
- unusual fatigue
- mental confusion
If the affected individual is seriously poisoned, it is advisable for all other members of the crew to place themselves under medical observation immediately.
A blood bromide level should be obtained immediately on any individuals suspected of being exposed to methyl bromide.
General safety procedures to be applied when using methyl bromide fumigant:
- Make sure the fumigation enclosure is air tight
- Wear protective chemical suits, plastic gloves and full-face gas mask fitted with AX filter
- The area surrounding the fumigation enclosure should be well aerated
- If it is necessary to stay in the area of a treatment, the air should be monitored to determine if harmful levels of the fumigant are present
- A first-aid kit equipped with the proper materials should be readily available at the fumigation site
- Do not eat, drink, smoke in an area where fumigation is carried out
- To display danger sign boards at the fumigation area indicating skull with cross bones with the information on ‘AREA UNDER FUMIGATION. DO NOT ENTER’