Phosphine

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Phosphine or hydrogen phosphide (PH₃) is a low molecular weight, low boiling point compound that diffuses rapidly and penetrates deeply into materials, such as large bulks of grain or tightly packed materials. The gas is produced from formulations of metallic phosphides (usually aluminium or magnesium phosphide) that contain additional materials for regulating release of the gas.

AIP + 3H₂O à PH₃^ + Al(OH)₃

Mg₃P₂ + 6H₂O à 2PH₃^ + 3Mg(OH)₂

Aluminium phosphide is formulated 8S tablets, pellets or small sachets of powder with additional materials such as ammonium carbamate, ammonium bicarbonate, urea and paraffin to regulate release of fumigant and suppress flammability. The magnesium phosphide is similarly manufactured in tablets or pellets. It is also prepared in flat plates; here the formulation is embedded in a plastic matrix that regulates access of moisture and hence controls release of the gas. After the phosphine has evolved from a formulation, the residue that remains consists mainly of aluminium or magnesium hydroxide. Small amounts of undecomposed aluminium phosphide may also remain in the greywrlite dust from tablets, pellets or sachets.

PROPERTIES OF PHOSPHINE

Alternative name: hydrogen phosphide

Natural vapour pressure at different temperatures

0°C (32°F) 21.6 atmos
20°C (68°F) 34.2 atmos
40°C (104°F) 51.9 atmos

Dosages and concentrations of gas in air (25°C and 760 mm pressure)

¹Ounces per 1000 cubic feet or milligrammes per litre
²Threshold limit, ACGIH, 1981.

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. This odour seems to be due to the presence of other compounds produced along with phosphine and they may be preferentially absorbed during fumigation treatments. Under some conditions the odour may disappear, even when insecticidally effective concentrations are still present in the free space of a fumigation system (Bond and Dumas, 1967; Dumas and Bond, 1974). While any odour associated with the evolution of phosphine may indicate the presence of phosphine, it should not be relied on for warning purposes.

TOXICITY

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. A concentration of 2.8 mg/l (ca 2 000 ppm in air) is lethal to humans in a very short time (Flury and Zernik, 1931). The threshold limit value is usually set at 0.3 ppm for a 40-hour work week. Symptoms of poisoning for humans are described below under "First Aid".

Phosphine ranks as one of the most toxic fumigants of stored product insects (see Chapter 14, Table 16). It is a slow acting poison that is effective at very low concentrations if the exposure time is long enough. Usually, exposure times of four or more days are required to control insects, depending on temperature. The toxicity of phosphine to insects 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 cannot be shortened by increasing the dosage; in fact, high concentrations can have a narcotic effect on insects thereby reducing mortality (Winks, 1974a).

Phosphine has an inhibitory effect on insect respiration and is unique in that it is only toxic to insects in the presence of oxygen - in the absence of oxygen it is not absorbed and is not toxic to insects (Bond et al 1967, 1969). However, the action of phosphine is potentiated by carbon dioxide and the exposure time can be reduced when both gases are present (Kashi and Bond, 1975).

Some stages of insects are considerably more tolerant to phosphine than others (Bell, 1976; Hole et al, 1976; Nakokita and Winks, 1981). The eggs and pupae are usually hardest to kill while larvae and adults succumb more easily. Reynolds et al (1967) found that this tolerance was at least partially overcome by the development that occurred in the insects during the relatively long exposure periods. For example, a 10-day exposure of various stages of Sitophilus Granaries was found to be long enough to permit the different stages to reach a susceptible point of development at some time during the fumigation. These results suggest that all pre-adult stages, some of which are quite tolerant to the fumigant, may reach a susceptible stage of development during a 10-day fumigation, so that an exposure period of this length will lead to complete mortality. Howe (1974) discussed problems relating to the laboratory investigation of phosphine toxicity to storedproduct insects.

Observations to date on the effectiveness of phosphine on mites in bulks of grain indicate that the fumigant may be successful in bringing about immediate reduction in mite populations and thus improving the condition of the grain (Van den Bruel and Bollaerts, 1956). However, populations of some species have been seen to build up again in the grain after an interval of time. This is partially due to the fact that natural predators such as the mite, Chevletus eruditus (Schr.), are eliminated and partially because some stages of destructive species of mites are resistant to the fumigant (Heseltine and Thompson, 1957; Sinha et al, 1967). Tests on dried prunes have shown that satisfactory control of mites on this commodity can be obtained with phosphine (Cangardel and Fleurat-Lessard, 1976).

Insect Resistance

The effectiveness of phosphine can be reduced considerably by development of resistance in insects. Winks (1974b) showed that Tribolium castaneum could develop 10-fold resistance to phosphine in six generations. Resistance may occur in immature stages as well as in adult insects. Bell et al (1977) found a good correlation between resistance in the adult stage of Rhyzopertha dominica and resistance in the egg stage. Champ and Dyte (1976) found evidence of resistance to phosphine in insects from several parts of the world, particularly where inadequate techniques of fumigation were employed, and they indicated that emergence of resistance to fumigants under practical conditions was a matter of great concern. There is recent evidence (Borah and Chalal, 1979; Tyler et al, 1983) of the development of resistance to phosphine in field populations of Khapra beetle and other insects infesting stored grain. Hole (1981) discussed the variation in tolerance of seven species of stored-product Coleoptera to phosphine in strains from twenty-nine countries. Further information on the nature and occurrence of resistance is given in Chapter 2.

EFFECT ON PLANT LIFE

Seeds

There is considerable evidence from studies so far conducted that phosphine in insecticidal treatments does not, under normal conditions, affect the germination of seeds. Strong and Lindgren (1960b) tested cereal, sorghum and small legume seeds with one or two (repeat) fumigations at comparatively high concentrations. Berstlief and Alexandrescu (1964) confirmed these findings for wheat sod maize under Rumanian conditions and Pinto de Matos (1961) for the germination of groundouts. When a third fumigation was done with phosphine, Fam et al (1974) found no ill effect on the germination of some varieties of tomatoes, sweet melons, cucumbers, peas and beans, but one variety of broad beans was slightly affected. However, the growth and yield of plants grown from seeds subjected to repeated fumigations with phosphine may be significantly reduced. Joubert and Du Toit (1969) reported a reduction in total yield of maize grown from seed fumigated twice sod three times with phosphine.

Living Plants

There is little information on the tolerance of growing plants to the vapours of phosphine. Monro and Upitis (1967) found that 15 varieties of glasshouse plants were tolerant to concentrations which were completely toxic to the postembryonic life stages of the mealybug Plannococcus citri (Risso), but the eggs were not killed. The nematode Meloidopyne hapla may be controlled in potted rose plants with phosphine applied to the soil, without apparent injury to the plants (Faber, 1966).

EFFECT ON PLANT PRODUCTS

Phosphine has been used for many years to control insects in a wide range of plant products throughout the world. To date there has been no report of appreciable adverse effects from recommended treatments.

Mayr and Hild (1966) concluded that normal fumigation with phosphine has no effect on vitamins, particularly vitamins A and B₂ (riboflavin), in a group of important foods which are a regular dietary source of these vitamins. Fumigation of wheat with phosphine under normal conditions has no adverse effect on the baking quality of flour made from it (Neitzert, 1953; Lindgren et al, 1958; Mayr, 1974; Matthews et al, 1970a,b). Phosphine can also be used to control insects (Nelson, 1970; Vincent and Lindgren, 1971) and mites (Cangardel and Fleurat-Lessard, 1976) in dried fruit.

Tests on fresh fruit and vegetables show that insects such as fruit flies can be controlled using gas generated from a magnesium phosphide preparation without injury to the produce. Seo et al (1979) found no injury on papaya, tomato, bell pepper, eggplant or banana fumigated with dosages that eliminated eggs and larvae of the fruit flies Dacus dorsal) Hendl. and Ceratitis capitata Wied. Ten varieties of avocado, although not injured by the treatment, did ripen more quickly than unfumigated avocados. Grapefruit and tomatoes have also been fumigated without injury at concentrations sufficient to kill fruit flies (von Windeguth et al, 1977; Spalding et al, 1978).

The use of phosphine for bulk grain fumigation is described in Chapter 10. Recommended treatments for a number of plant products, including packaged foods, will be found in Schedule P.

RESIDUES IN FOODSTUFFS

The residues resulting from the use of phosphine fumigants may be of three types: reaction products of the formulation, unchanged phosphine absorbed in commodity or products formed by chemical combination of phosphine with components of the commodity.

Formulations of aluminium or magnesium phosphide leave mainly an inert residue of the metallic hydroxide. In formulations of aluminium phosphide, a small amount of unreacted material may also remain, and hence some precautions should be taken to avoid hazards from the unspent formulation. When processed foods are Fumigated, or when space fumigations are carried out, residue from the formulation should be collected and properly disposed of.

Residue from magnesium phosphide in the plate preparations remains in the plastic matrix in which it is embedded as magnesium hydroxide. The reaction with water vapour is substantially complete, so that no abreacted material remains, and elimination of the residue simply involves collection and disposal of plastic trays at approved sites.

Unchanged phosphine does not remain in fumigated commodities in appreciable amounts. Tolerance levels of 0.1 mg/kg for raw grains and 0.01 mg/kg for processed foods have been established by many agencies and numerous investigations have shown that the gas desorbs rapidly during aeration to levels well below the tolerances (see review by Dietrich et al, 1967). It is interesting to note, however, that minute but detectable traces of phosphine can remain in fumigated commodities for very long periods of time. Dumas (1980) detected phosphine desorbing from fumigated wheat 220 days after the treatment.

Some reaction products may form by combination of phosphine with components of a commodity. Several investigations have shown that small quantities of innocuous phosphites and phosphates from phosphine remain in fumigated materials (Robinson and Bond, 1970; Disney and Fowler, 1972; Tkachok, 1972; Underwood, 1972).

Recommendation for Tolerances

Taking into account the fact that phosphine aerates rapidly from foodstuffs, and that a residue of 0.1 mg/kg in a raw cereal would yield a much lower residue in bread or other food ready for consumption, the FAD/WHO joint meeting (1967a) considered that there was no necessity to establish a figure for acceptable daily intake. For cereals in international trade a tolerance of 0.1 mg/kg expressed as PH₃ is recommended.

METHODS OF ANALYSIS

Determination of Vapours

For the determination of concentrations of phosphine used in fumigation, glass detector tubes (Figure 18) are available from a number of manufacturers in various ranges from 15 to 3 000 ppm. These tubes, described more fully in Chapter 3, are reasonably reliable, easy to use and are of sufficient accuracy for monitoring approximate concentrations achieved at various stages in a fumigation. For protection of personnel from low levels of phosphine around the threshold limit value (TLV), similar tubes with ranges down to 0.025 ppm can be obtained. A mixed indicator paper strip that will give rapid, sensitive and reliable detection of phosphine around the TLV has been developed by Kashi and Muthu (1975).

Infra-red analysers have been used on an experimental basis for analysis of phosphine and found to have good stability, reproducibility and sensitivity (Webley et al, 1981). The concentration of phosphine is determined by measurement of absorption bands at either 4.2 µm or at 9.0 µm unless carbon dioxide is present; carbon dioxide interferes with analysis of phosphine at 4.2 µm and may need to be absorbed from the sample or the analysis may be done at 9.0 µm. In normal use, sampling can be done by pumping fumigant - air mixture through the analyses but there is also a closed loop injection system for calibration of the instrument or for experimental work. Here small samples can be injected through a septum by a gas syringe. With the infra-red analyses, concentrations up to 2.7 mg/l have been measured under field conditions and, with proper manipulation, levels down to 0.0004 mg/l (0.25 ppm) can be detected.

Gas chromatography has been developed and used extensively for analysis of phosphine in experimental work (Dumas, 1964, 1969; Chakrabarti and Wainman, 1972; Bond et al, 1977). Small, portable gas chromatography suitable for analysing phosphine in commercial treatments are now available on the market. One such instrument that is simple, easy to use and virtually unaffected by air and high moisture levels can measure phosphine below 0.02 ppm (Barker and Leveson, 1950).

Determination of Residues

The problem of determination of residual phosphine in fumigated foodstuffs has been reviewed by Dietrich et al (1967). A sensitive method, developed by Bruce et al (1962) and modified by Sullivan and Murphy (1966) has been widely used for analysis of phosphine residues in many commodities. Kroeller (1968) described a sensitive method with a simplified procedure for plotting the calibration curve using high purity potassium dibydrogen phosphate. These methods are sensitive to less than 0.005 mg/kg.

For analysis of desorbing residual phosphine by gas chromatography the method of Dumas (1978) can be employed.

FORMULATIONS

Both aluminium and magnesium phosphide are manufactured in several different formulations for a variety of applications. Aluminium or magnesium phosphide powder is compressed into hard round or flat tablets about 3 9 in weight or pellets of 0.6 9, which yield approximately 1 and 0.29 of phosphine, respectively. Aluminium phosphide powder is also prepared in permeable paper bags or sachets. Additional materials such as paraffin and ammonium carbamate or ammonium bicarbonate are included in the formulations to regulate moisture uptake and to dilute the phosphine as it is generated. The products are supplied in sealed metal tubes, cans or flasks, which are packed in cases. As long as the containers remain sealed the storage life of the product is virtually unlimited. The pellets and round tablets are supplied in flasks that can be resealed after opening.

For convenience and safety, pellets or sachets are sometimes supplied in predetermined quantities for specific applications. Pellets are prepared in special prepacks of 165 pellets each for treatments such as railway box car fumigations. Similarly, sachets are joined together in ropes or in "blankets". In these prepared packages the pellets or sachets are separated sufficiently to avoid a build up of excessive heat and concentrations of the gas in small spaces.

Magnesium phosphide is also marketed in the form of a flat plate about 280 x 170 x 5 mm and weighing 2069. The active ingredients of the formulation are embedded an inert polyvinyl acetate matrix fabricated in the form of a semi-rigid plate covered on both sides with moisturepermeable paper. Every plate is individually sealed in a gas-impermeable foil pouch, or 16 plates interconnected to form a 4 480 mm strip, are similarly sealed in foil and packaged in tins - 32 plates per tin or two strips of 16 plates each.

Once the plates or strips are removed from the foil pouches, they start evolving phosphine within one half to one hour. This formulation is intended to be used for fumigation of bulk goods and packaged and processed commodities. It can be applied successfully under almost nil space storage conditions provided that the structure is tightly sealed. According to the manufacturer, this formulation can also be used to fumigate fruits and vegetables at recommended dosages without any adverse effects, such as phytotoxicity.

The plates and strips provide ease of application and collection after the treatment; there is no danger of contaminating goods with spent fumigant as the plastic matrix retains all such material. Magnesium phosphide formulations release the phosphine more rapidly than aluminium phosphide products, with the maximum gas reading usually being achieved within the first 24 hours.

Magnesium phopshide made in discs weighing 10 9 for control of burrowing rodents and moles is for outdoor use only.

Handling of Phosphine Formulations

Containers of aluminium or magnesium phosphide formulations have labels that give important information concerning use, hazards and precautions. The instructions on the labels should be carefully adhered to during fumigation. Containers should be opened in the open air where any released gas can readily diffuse away. Caution: DO NOT OPEN IN A DUSTLADEN OR EXPLOSIVE ATMOSPHERE. The formulation should be kept away from liquid or water, as this causes immediate release of the gas with possible spontaneous ignition. Also, piling of the formulation or residual dust may cause a temperature increase so that a flash may occur. Gloves should be worn by the person handling the phosphine formulation and smoking or eating should be avoided until the hands are washed after application. Containers with screw type caps are made to be gas-tight and may he resealed if only part of the contents are used. When containers are emptied they should be triple-rinsed with water to remove any traces of abreacted product and then disposed of in an approved disposal site.

APPLICATION

Because phosphine is highly toxic, inhalation of even small quantities of the dust from the formulation, as well as the evolved yes, should be avoided. Pellets or tablets may be applied directly to a grain stream by hand (protected by gloves) or by means of automatic applicators. The rate that the yes evolves from the formulation varies, depending on type of formulation, moisture and temperature. In grain, for example, if the moisture content and temperature are high, all of the gas from aluminium phosphide formulations is evolved within three days. Special probes are used for applying tablets below the surface of bulk grain. Sachets may be applied directly to the grain stream, pushed into the grain bulk or inserted into specially designed, permanently installed pipes in grain bins (Anon, 1980). For some treatments the sachets are laid out in blankets on the surface of grain to allow the gas to evolve and diffuse into the grain mass. Methods of application of phosphine formulations for bulk grain are described and illustrated in Chapter 10.

For the treatment of bagged grains and other raw commodities in transport facilities, such as railway wagons, pellets or tablets may be spread evenly over the load or placed in moisture permeable envelopes to fit in some convenient location near the door before closing. When fumigating packaged commodities under gas-proof sheets the tablets or pellets can be spread out on trays to lay under the sheet before it is secured. In warehouses, after the structure is adequately sealed, the tablets or pellets are spread out on trays or sheets of Kraft paper so that residual material can be easily collected at the end of the treatment. The tablets or pellets should never be piled on top of each other or in a mass.

On completion of the fumigation, all windows and doors should be opened and the space aerated for at least two hours. A gas reading should be taken with a suitable analyses before entering the fumigated area. If it is necessary to enter the fumigated space to open doors and windows a gas mask with a canister designed for phosphine must be worn.

Disposal of Spent Reaction Products

After a fumigation, any residual material left from the reaction process should be disposed of in an approved manner. This can be accomplished by burying or by slowly adding the dust to a container of water (with detergent as a wetting agent) and stirring into the water until a slurry is formed and the residue sinks. If prepacks have been used the entire strip should be submerged in the water-detergent mixture and allowed to soak for 36 hours before disposal. For purposes of safety the disposal procedure should be carried out in the open air, where any generated phosphine can rapidly disperse.

Spent plates or prepack strips may be held out of doors in locked wire containers and moved to an approved disposal site at monthly intervals, or whenever the container is full.

In fumigation treatments of raw agricultural commodities such as grain or bulk animal feeds, no special disposal procedures are needed because any of the phosphide formulation that may remain is further decomposed and removed along with grain dust in the handling and turning that accompanies further processing of the grain (liscombe, 1963).

PRECAUTIONS

Reaction with Metals

Phosphine is practically insoluble in water, fats anti oil and is stable at normal fumigation temperatures so that it has no appreciable reaction with most fumigated commodities. It may, however, react 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 (Bond et al, 1984).

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 or using techniques that will keep the concentration of phosphine and the humidity low.

Concentrations Toxic to Humans

The threshold limit value-time weighted average (TLV-TWA) for an eighthour daily exposure in a five-day week is set at 0.3 ppm (ACGIH, 1981). 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.

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 (see below) are absolutely imperative before and until the arrival of a doctor.

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 alcholic beverages after any poisoning is strongly recommended.

Respiratory Protection

For personal protection against the vapours of phosphine at concentrations above the threshold limit, a respirator, gas blouse or other similar equipment for supplying uncontaminated air must be used. Respirators with a special canister for phosphine vapours will give protection up to 0.5 percent phosphine by volume in air (Kloos et al, 1966). Above this concentration, air must be supplied by an air-line or self-contained breathing equipment. Appropriate detection equipment for measuring concentrations of phosphine in air should be used in conjunction with respiratory protective devices to ensure adequate protection.

General Precautions

Full precautionary instructions are supplied by the manufacturers of the proprietary materials used for generating phosphine. Some of the more important precautions are listed here.

1. Gloves should be worn when tablets or pellets are being dispensed by hand.

2. Respirators need not be worn when tablets or pellets are being dispensed under conditions where the operator does not breathe the vapours of phosphine. Under normal conditions, there is a delay in evolution of the fumigant from the formulations described in this manual. Respirators equipped with a canister designed for protection against phosphine (see above) or other appropriate respiratory equipment 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. Detection equipment such as glass detector tubes or other detectors should be used to monitor concentrations of the gas and to determine when an area is free of fumigant after a treatment.

4. Do not smoke or touch food at any time during the applicaion of this insecticide.

5. Any spaces adjoining silo bins or close to other 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 the grain is turned, or aeration of a structure is undertaken, full precautions must be undertaken to ensure that no person is exposed to residual vapours of the fumigant.

FIRST AID

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 or reaching), anoxia (subnormal blood oxygen content), unconciousness 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.

Advice to the Physician

The following measures are suggested by the manufacturer for use by the physician in accordance with his own judgement.

In its milder forms, symptoms of poisoning may take some time (up to 24 hours) to make their appearance, and the following measures are suggested:

1. Complete rest for one or two days, during which the patient is kept quiet and warm.

2. Should the patient suffer from vomiting or increased blood sugar, appropriate intravenous solutions should be administered. Treatment with oxygen breathing equipment is recommended as is the administration of cardiac and circulatory stimulants.

In cases of severe poisoning intensive care in a hospital is recommended:

1. Where pulmonary oedema is observed, steroid therapy should be considered and close medical supervision is recommended. Blood transfusions may be necessary.

2. In case of manifest pulmonary oedema, venesection should be performed under vein pressure control, and intravenous administration of glycosides (in case of haemoconcentration, venesection may result in shock). On progressive oedema of the lungs, perform immediate incubation with constant removal of oedema fluid and establishment of oxygen positive pressure respiration, as well as any measures required for shock treatment. In Case of kidney failure, extracorporeal haemodialysis is necessary. There is no specific antidote known for this poison.

3. Suicide may be attempted by taking solid phosphides by mouth. In such a case, empty the stomach by inducing vomiting and flush it with a dilute potassium permanganate solution or a solution of magnesium peroxide until the flushing liquid ceases to smell of carbide. Thereafter, administer medicinal charcoal.

4. Scientific research has shown that phosphine poisoning is not chronic; the action of phosphine is reversible and symptoms will disappear by themselves.

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