Evaluation of Residues
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On the basis of toxicological evidence the FAD/WHO joint meeting (FAD/WHO, 1967a) estimated the acceptable daily intake for man of inorganic bromide from all sources as up to 1.0 mg/kg of body weight.
Since residues of bromide ion (inorganic bromide) in foods arising solely from methyl bromide fumigation cannot be distinguished from bromide originating from other sources and a limit of 50 mg/kg would indicate that raw cereals have not been subjected to excessive treatment with methyl bromide, this limit was maintained by the FAD/WHO committee (FAD/WHO, 1980). However, for lettuce and cabbage, where much higher levels of bromide ion have sometimes been encountered, a limit of 100 mg/kg was adopted as an aid to reduction of bromide dietary intake from all sources.
Although no reports of free methyl bromide in goods offered for sale have been published, the dangerous nature of methyl bromide as an alkylating and mutagenic agent indicates that extremely low limits at or about the lower limit of detection are necessary. Therefore guideline levels of 0.0l mg/kg are recommended for commodities at point of retail sale or when offered for consumption (FAD/WHO, 1980).
METHODS OF ANALYSIS
Determination of Vapours
Rough determinations of methyl bromide concentrations in air, for the purpose of detecting serious leaks or for protecting personnel during fumigation and subsequent aeration, may be made with a device commonly referred to as a halide leak detector or halide lamp. These lamps are easily obtainable, usually from refrigeration supply dealers, because they are also used for detecting leaks of the commonly used Freon-type refrigerants. All these lamps work on the same principle, that a flame in contact with a clean piece of copper will burn with a green to blue flame if vapour of an organic halide is present in the surrounding air.
At increasing concentrations of the halide gas, the colour changes from green to greenish-blue or blue. On the basis of this principle it is a fairly simple matter to improvise lamps, given a clean copper wire and a source of flame. Any improvised lamps must, however, he subjected to careful preliminary calibrations.
Various types of detectors are marketed and the difference in construction depends on the fuel they use. Fuels used are, among others, paraffin (kerosene), wood alcohol (methylated spirits, methyl hydrate), acetylene and propane. A lamp using a disposable or a rechargeable canister of propane gas is the most convenient, although it is not as sensitive as those burning paraffin or alchohol. Propane canisters are light, readily available, contain enough gas for about 10 hours' burning and the lamp does not require priming before it is lit.
The use of a propane lamp is illustrated in Figure 15. None of the lamps is to be relied upon for accurate quantitative determinations. They are useful for indicating immediately dangerous concentrations, for checking the effectiveness of aeration after many types of fumigation and for finding leaks during treatment. They should not be used for regular routine checks on the threshold limits for continuous daily exposure to methyl bromide; for this purpose some method of chemical analysis or an accurate recording device should be used.
Reactions of some of the lamps used are shown in Table 10.
Care and operation of lamps. Several considerations are important for the proper and reliable operation of lamps.
1. The copper ring must be kept clean, otherwise a green flame may show in the absence of methyl bromide.
2. The flame must burn through the ring, not around it; if the ring is clean, and the air is free of fumigant or dust, the flame should be invisible above the ring.
3. After some time the copper ring deteriorates and must be replaced by a new one.
4. These lamps are not specific for methyl bromide; they react to any organic halogen gas, such as the Freon refrigerants, whose presence in the air may result in erroneous lamp readings. It is advisable to test atmospheres before the fumigant is applied to ensure that no interfering substance is present.
5. Obviously, the lamps cannot be used in the presence of flammable or explosive gases, such as gasoline vapours, or with some organic halide gases, such as methyl chloride and ethylene dichloride, which are also flammable and may be used as fumigants.
6. Lamps should not be used in dust-laden atmospheres. If gas analysis is required from such areas, a sample of the atmosphere may be obtained by opening a polythene bag at the point to be checked and removing the sample to a safe location. Alternatively, a sample may be drawn from the treated area through plastic or copper tubing to a suitable outside area.
7. Colour blind persons cannot detect colour changes in a halide detector flame.
8. If used at night the gas detector flame will have a bluish cast; hence, appropriate allowance for variations in different light conditions should be made.
TABLE 10 - APPROXIMATE COLOUR REACTIONS OF HALIDE LEAK DETECTOR LAMPS
|Concentrations of methyl bromide in air||Reaction of Flame|
|Parts per million|
|10||Very faint green tinge at edge of flame|
|20||Light green edge to flame|
|30||Light green flame|
|200||Intense green, blue et edge|
|1 000||Intense blue|
Note: Owing to variations in response of individual lamps, readings below 30 ppm are unreliable.
Gas detector tubes. Where a more precise measurement of the concentration in the region of the threshold limit value is required, a simple method is the use of a gas detector tube as described in Chapter 3. Tubes for methyl bromide are available from several manufacturers in ranges from 2 - 200 ppm and they should be used strictly in accordance with instructions. A fresh tube is required for each determination.
Infra-red analyser. This detector will measure concentrations of methyl bromide well below threshold limit values. It is portable, battery powered, direct reading and has no flame to cause hazards in dust laden atmospheres. This instrument can be used to determine if buildings or other spaces are safe for occupancy.
Gas chrometopraphy. The portable gas chromatograph described in Chapter 3 will readily analyse methyl bromide at working levels used in fumigation and at low levels down to 0.01 ppm and below.
Thermal conductivity analyser. The thermal conductivity analyser is fully discussed and described in Chapter 4. Instruments of this type are particularly suitable for use with methyl bromide and are, in practice, used mainly with this fumigant. However, they are not sensitive enough to determine the health hazards presented by low concentrations of methyl bromide and are not authorized for this purpose in fumigation codes of safety.
Chemical analysis. A convenient method of chemical analysis for field use is the Volhard titration after the gas has been absorbed in monoethanolamine. Chemically pure monoethanolamine, in the amount of 2 ml for each 0.45 kg (1 lb) of methyl bromide per 28 m' (1 000 ft³) in the fumigation space, is placed in a glass bottle fitted with a stopcock. The glass bottle must be able to withstand evacuation; round 1-litre (quart) bottles have been used successfully. The stopcock is connected to copper or polyethylene tubing that leads from the desired sampling points in the fumigation system. Before a sample is taken, the bottle is evacuated to a pressure of 1 to 2 cm. The flask should stand for at least 2 hours before the titration is started. The bromide is estimated after it has been precipitated by excess standard silver nitrate solution; the excess silver is titrated by standard potassium thiocyanate, ferric alum being used as an indicator. Full details of this method are given by Brown (1959) and may also be found in standard works on the nalysis of insecticides (Jacobs, 1949; Gunther and Blinn, 1955).
Other field methods. The interference refractometer described in Chapter 4 is suitable for determining concentrations of methyl bromide during fumigation.
The sachets described by Heseltine and Royce (1960) and discussed in Chapter 4 are also useful for estimating elapsed concentration x time products during an actual treatment.
Laboratory analysis. The Volhard titration method given above is suitable for laboratory determination of vapours. Dumas and Latimer (1962) described a method for analysing the vapours by a technique of coulometric titration whereby amounts as small as 17 µg of methyl bromide in a sample may be determined accurately. Berck (1965a) listed methyl bromide among the fumigants which may be determined by gas chromatography.
Determination of residues
Mapes and Shrader (1957a) gave a chemical method for determining total and inorganic bromide residues in fumigated foodstuffs. The basis of this method is that the bromides are hydrolyzed with ethanolamine and the alcohol is removed by evaporation. The residue is ashed with sodium hydroxide and sodium peroxide. The bromide is leached from the ash and determined by the KolthoffYutzy-van der Meulen method. Total bromide ion may also be determined by xray fluorescence (Cetzendaner et al, 1968) or by neutron activation analysis (Lindgren et al, 1962).
Selective determination of bromide ion may be carried out with a selective ion electrode (Banks et al, 1976) or by using gas chromatography (Heuser and Scudamore, 1970; Panel on Fumigant Residues in Grain, 1974; Stijve, 1977).
Determination of free methyl bromide in commodities can be carried out by various methods using gas chromatography (Heuser and Scudamore, 1968, 1969b; Greve and Hogendoorn, 1979; Fairall and Scudamore, 1980; Dumas, 1982).
Methyl bromide is supplied by manufacturers in the following types of containers:
- Steel cylinders with capacities from 5 to 1 800 lb (2.25 to 816 kg). For ordinary field purposes, the 50- or 100-lb (23- or 45-kg) sizes are the most convenient. The cylinders have siphons and at normal temperatures the natural vapour pressure of the fumigant is sufficient to discharge it from the cylinder. Some manufacturers inject an inert gas, such as nitrogen, under pressure into the space above the liquid before the cylinder is shipped; this helps to force the fumigant through lines of piping. Other makers supply additional valves on the heads of the cylinders so that more pressure from compressed nitrogen or air may be applied at the time of discharge.
To apply the desired quantity of fumigant from a cylinder, a set of scales is used to determine the amount being discharged (Figure 16). Spray nozzles, as described by the U.K. Ministry of Agriculture, Fisheries and Food (UK, 1974) or a sprinkler as outlined by Calderon and Carmi (1973) have been found useful for dispersing the fumigant in large spaces.
- Cans containing 1 or l.5 lb (0.45 or 0.68 kg) of the fumigant. Some manufacturers make 0.5 lb cans available on request. A special device (Figure 17) for discharging the fumigant from them under natural pressure is obtainable from the supplier and copper or plastic tubing may be attached to carry the methyl bromide to the space being fumigated, as required. This method of application is useful for small-scale operations because the cans are easy to handle and the dosages are easy to compute in terms of the number of cans required. An opened can must be completely emptied as it cannot be resealed.
If the fumigant is discharged as a gas from the can, considerable cooling of the liquid takes place, especially in cool weather, and the temperature may fall below the boiling point of methyl bromide (3.6°C). If the can is immersed in a pail of hot water (not above 77°C), the fumigant is discharged more evenly and rapidly.
- Glass ampoules, usually containing 20 ml of methyl bromide, are supplied by some manufacturers. These are useful for small chambers, such as the drum fumigator described in Chapter 8 and illustrated in Figure 27. A plunger or similar device is needed for breaking these ampoules so that the fumigant can disperse in the chamber.
Discharging methyl bromide
As already suggested, freezing of the tubes and piping carrying the fumigant sometimes occurs due to the loss of heat from evaporation. This is more likely to occur when the fumigant is carried some distance from the container. In many applications, therefore, the fumigant is led through a heat exchanger after it leaves the container. For small dosages, a suitable exchanger is a hot water bath or pail in which is coiled 15 m (50 ft) of copper tubing. (This method is unsuitable with cans because back pressure develops against the fumigant still in the can; the entire can must be immersed, as described previously.)
For large-scale operations, a greater transfer of heat is required. Several devices have been described in detail by Hammer and Amstutz (1955). The essential feature of these heaters is a vaporizing chamber of sufficient capacity to convert the liquid methyl bromide into yes as it passes through. At temperatures below 15°C or when large quantities of methyl bromide are used a vaporizer, a 15 m coil of copper tubing, 10 - 15 mm in diameter, immersed in a container of water heated to 65 C may be required. The fumigant should be passed through the tubing at a rate of 1 - 2 kg per minute and liberated as a vapour into the air stream of a fan or blower (USDA, 1976). Any convenient and safe source of heat may be utilized for these heaters. Bottled propane and live steam have proved successful. Heat from a flame or electrical heater should not be applied directly to the vaporizing chamber, which should always be surrounded by hot water. The water bath, if closed, should have a water gauge and a safety valve.
Evaporating pans When methyl bromide is discharged into a fumigation chamber, a good practice is to let the liquid flow into a shallow evaporating pan of sufficient capacity to hold the entire dosage at a depth not greater than 12 mm (0.5 in). In this way, more even distribution of the fumigant is obtained, especially if a current of air from a fan or blower is directed across the pan; also, the danger of liquid spilling directly on the commodity is eliminated. If fumigation is being effected at temperatures below 15°C, the pan may be warmed in some way, preferably with heating coils, infra-red heaters or ordinary light bulbs.
Precautions in discharge Although methyl bromide is nonflammable, in the presence of a flame it breaks down quickly to hydrobromic acid. This is highly corrosive to metals and destructive to plants and plant materials. Therefore, during methyl bromide fumigation the space should not be warmed by heaters with exposed glowing wires. Also, all pilot or other flames should be extinguished before fumigation begins. The corrosive effect is greatly increased under warm, humid conditions.
Measurement of Dosage
Fractional measuring devices are used when the dosage of methyl bromide cannot be computed in terms of whole cylinders or cans. Sometimes the approximate dosages discharged from cylinders can be determined by weighing the cylinders on portable scales. Graduated glass measuring tubes (such as the one shown on the top of the drum fumigator in Chapter 8, Figure 27) are accurate to the nearest millimetre and are used for measuring amounts up to 280 ml (the volume of l lb of methyl bromide is 260 ml at 0°C). For very small quantities a method for tapping a 454 kg (l lb) can and extracting measured amounts in a gas syringe has been described (Buckland and Bond, 1973) (see also Chapter 14).
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 suggested by the American Conference of Government Industrial Hygienists (ACGIH, 1981).
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. It is not advisable, therefore, for persons to remain in any atmosphere which gives a positive reaction for methyl bromide in the flame of the detector unless properly protected by a respirator.
It has been suggested by Torkelson et al (1966) that persons continuously engaged in fumigation with methyl bromide should have checks at least once a month to determine their blood bromide levels. Such tests would give reassurance of safe practices if no undue rise in blood bromide levels were observed. On the other hand, if a person becomes ill an immediate test would indicate whether the sickness could be attributed to the fumigant. Furthermore, any significant increase from the normal level found during the routine tests would serve as a warning of undue exposure to the fumigant and the necessary precautionary steps could be taken.
If there has been no other source of bromide, such as medications or drinking water, a level above 15 mg percent indicates dangerous exposure. A level below 5 mg percent indicates mild or no exposure and symptoms would be unlikely. Such a regime of testing should only be undertaken after consultation with a qualified industrial toxicologist. Determination of bromides in blood and spinal fluid may be made by methods of Paul et al (1952) or Hunter (1953).
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. It is important to remember that methyl bromide may be odourless and, if a canister-type gas mask is used, the length of time the correct canister (organic vapours) can afford protection must be estimated. This time should be calculated from the concentration corresponding to the actual dosage applied and a high rate of breathing. Table 11 is drawn up on this basis. The suggested lengths of time given in this table apply only to methyl bromide or a mixture of 98 percent methyl bromide and 2 percent chlaropicrin.
It is recommended by manufacturers of safety equipment that with methyl bromide the respirator canisters be discarded after only one exposure to this fumigant. This precaution is due to the fact that methyl bromide may continue to diffuse through the activated charcoal filling of the canister even when inspired air is not being drawn through it. The only possible exception is that, if the canister is used in concentrations of less than 50 ppm in air and the concentration never exceeds this, it would be permissible to use it during a continuous 8-hour period. Such provision would permit the use of the canister during an aeration procedure when only low concentrations, below 50 ppm, would be encountered (Merkle, 1967, personal communication). Regardless of the number of exposures, the canister should be discarded at the end of 8 hours.
Because 80 g/m³ (5 lb/1000 ft³) are equivalent to 2 percent by volume in air, the respirator cannot be guaranteed3to give protection from a dosage of more than 64 g/m (4 lb/1000 ft ). When interpreting the figures given in Table 11 the dosages and times should be overestimated in order to allow the greatest possible margin of safety.
TABLE 11 - SUGGESTED MAXIMUM TIME A RESPIRATOR CANISTER SHOULD BE USED IN METHYL BROMIDE FUMlGATION(CANISTER OF THE TYPE USED I-OR ORGANIC VAPOURS)*
|Concentration of methyl bromide g/m³ (oz/l 000 ft 3)||Maximum time** Minutes|
|16 or less||60|
|16 - 32||30|
|32 - 48||22|
|48 - 64||15|
* This applies to a standard size canister for only one
exposure to the fumigant vapours (see text)
** Based on recommendations of Dow Chemical Company (undated).
As pointed out previously, -in sound fumigation practice there should be little exposure to fumigant concentrations. Self-contained respirators, airline respirators, safety blouses and head-to-toe protective suits may also be used to give protection. 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 of a building, 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.
Absorption of Vapours through Skin
Medical literature contains references suggesting that methyl bromide poisoning may follow absorption through the skin. It is, however, probable that this may not occur in exposure to concentrations and times given in the preceding paragraph, for which protection is given by the respirator (Butler et al, 1945).
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 petrolatum dressing. When working with methyl bromide, gloves, bandages or occlu sive dressings 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 (von Oettingen, 1955).
Possible symptoms of methyl bromide poisoning are:
nausea and vomiting
loss of appetite
double or blurred vision
impaired, slurred speech
If the affected individual is seriously poisoned, it is advisable for all other members of the crew to place themselves under medical observation immediately.
The following measures are recommended for administration under medical supervision (Dow Chemical Company, 1964).
Notes to the Physician
Nausea and vomiting can be a most distressing part of the symptom complex from methyl bromide exposure and may require one of the anti-emetic drugs. Support of the respiratory system will dictate a semi-recumbent position, maintenance of a free airway and possible tracheostomy. Oxygen should be used at the first sign of respiratory embarassment and if pulmonary oedema develops. Intermittent positive pressure breathing may be helpful. If respiration fails, artificial respiration by an appropriate means may be necessary. Central nervous system effects are extremely difficult to control. tlyperexcitability and convulsions may require either a barbiturate, such as pentobarbital (Nembutal), or diazepam (Valium). Respiratory depression must be guarded against. Diazepam may be given in 5 to 10 mg doses by slow intravenous injection.
Severe seizures may be controlled by slowly giving pentobarbital intravenously in doses up to 5 my/kg of body weight. Caution: Respiratory depression must be watched for in the use of these drugs.
Circulatory failure may be combated by intravenous solutions and levarterenol bitartrate .
Individuals surviving the first two or three days will probably recover. Central nervous system symptoms may persist for weeks or months, occasionally even as much as a year. However, eventual recovery is the rule.
Although the treatment of methyl bromide intoxication is usually symptomatic, there is some evidence that early haemodialysis may be helpful. Dimercaprol (BAL) is sometimes recommended as an antidote, but the rationale for this approach is unclear. There are unconfirmed reports from Japan suggesting that pantothenic acid in large doses may be helpful in treating residual neurological symptoms.
Burns resulting from skin contact with the liquid material should be treated like thermal burns following decontamination.
Liver and kidney damage is quite unlikely in the absence of severe respiratory or central nervous system effects.
A blood bromide level should be obtained immediately on any individuals suspected of being exposed to methyl bromide.
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