FAO Meeting Report No. PL/1965/10/2
    WHO/Food Add/28.65


    The content of this document is the result of the deliberations of the
    Joint Meeting of the FAO Committee on Pesticides in Agriculture and
    the WHO Expert Committee on Pesticide Residues, which met 15-22 March

    Food and Agriculture Organization of the United Nations
    World Health Organization

    1 Report of the second joint meeting of the FAO Committee on
    Pesticides in Agriculture and the WHO Expert Committee on Pesticide
    Residues, FAO Meeting Report No. PL/1965/10; WHO/Food Add./26.65.



         Methyl bromide

    Chemical name

         Methyl bromide



    Empirical formula


    Structural formula


    Relevant physical and chemical properties

    Physical state (atmospheric pressure, 20C): colourless gas

    Boiling-point: 3.56C

    Odour: sickly-sweet or musty

    Flash point: non-flammable


         Water: 1.37 g/100 ml

         Organic solvents: infinitely soluble in alcohol and ether

    Specific gravity (liquid): 1.73

    Specific gravity (gas): 3.27


         Methyl bromide is extensively used for the fumigation of a wide
    range of produce including cereals and cereal products, seeds, pulses,

    edible nuts, cheese, oil-seeds, dried fruit, cocoa beans, animal
    feeding stuffs, spices and tobacco. These commodities may be fumigated
    in special gas-tight chambers operating at atmospheric pressure or at
    reduced pressures (vacuum fumigation), in sealed warehouses, in barges
    or ships' holds, in railway box cars, and under a covering of
    gas-proof sheets. Commodities in large bulk, especially cereals, are
    treated if there is a means for forced circulation of the gas.

         It is also used, generally in chambers, for the fumigation of
    fresh fruit and vegetables and of living plants, usually for plant
    quarantine purposes.

         Methyl bromide is also used for the disinfestation of buildings,
    such as warehouses and flour mills and of empty ships' holds. Other
    uses include fumigation of soil for control of nematodes in seed beds;
    disinfestation of furniture and clothing; and control of wood-boring
    insects in buildings, timber and furniture.

         Methyl bromide is occasionally used as a component in liquid
    grain fumigants, for example with carbon tetrachloride. It is also
    used in various mixtures with ethylene dibromide for the fumigation of
    stacks of bagged grain and possibly other commodities.


         The amount of the initial sorption and of the residue immediately
    after fumigation depends upon the composition and physical character
    of the food, the usage and exposure period, the temperature and the
    moisture content. The importance of the effect of moisture content is
    shown by the results of laboratory tests on wheat fumigated at 2
    lb/1000 ft3 for 24 hours at 70F. Wheat of 9% moisture content showed
    a total bromide content of 30 ppm with no airing, 19 ppm after six
    hours' airing and 17 ppm after eight days. The corresponding figures
    for wheat of 15% moisture content were 99 ppm, 78 ppm, and 77 ppm
    (Lindgren et al., 1962).

         Sorption is higher when flour itself is fumigated and residues
    are correspondingly higher. Residues of about 50 ppm of inorganic
    bromide have been found after treatment corresponding to normal
    commercial practice (Burns-Brown et al., 1955).

         Higher residues can be expected in ground-nuts and other
    oil-seeds than in grain and much higher residues after fumigation of
    produce such as oil-seed expeller cakes and meals. Residues between
    200 and 250 ppm have been found in cotton-seed meal and flake treated
    at the high level of 8 lb/1000 ft3 for four days (Thompson, personal
    communication, 1964).

         The residue of bromide after a single fumigation of cocoa beans
    at normal levels is usually around 10 ppm but much larger residues (up
    to 150 ppm) have been reported in some commercially fumigated samples
    (Turner, 1964).

         Immediately after fumigation the residue consists partly of
    unchanged methyl bromide and partly of a fixed residne of inorganic
    bromide resulting from reaction with the food constituents. The amount
    of unchanged methyl bromide is rapidly reduced either by removal
    during airing or by continued reaction with the food.

         After fumigation of wheat of 12% moisture content at 20C at a
    measured concentration-time product of 340 mg h/l the amounts of
    inorganic bromide and undecomposed methyl bromide found one hour after
    treatment were 11.5 ppm and 14 ppm. After 24 hours the corresponding
    amounts were 12.5 ppm and 1.5 ppm. The residues of inorganic bromide
    found in the milled products were: flour 5 ppm, fine offal 30 ppm and
    bran 32 ppm (Pest Infestation Laboratory, 1947).

    Effect of fumigant on treated crop

         Methyl bromide reacts mainly with the protein fraction of
    foodstuffs. In wheat the protein fraction was shown to account for 80%
    of the decomposition of sorbed fumigant (Bridges, 1955; Winteringham
    et al., 1955), the mechanisms being that of methylation, mainly of
    nitrogen- and sulfur-containing groups with the formation of N-methyl
    derivatives (methylated histidines), dimethyl sulfonium derivatives
    and lesser amounts of methoxyl and thiomethoxyl derivatives. The
    sulfur-containing derivatives tend to decompose spontaneously,
    possibly producing "off" odours. These reactions are not thought
    sufficient to reduce the nutritive value of the food (Clegg and Lewis,
    1953; Winteringham, 1955).

         Wheat flour treated with methyl bromide at normal levels of
    dosage was found to produce foreign odours in bread at the time of
    removal of loaves from the oven, but these are not likely to be
    detected by the consumer and there is no effect on flavour
    (Burns-Brown et al., 1961). The effect of repeated fumigation is

         When whole wheat was fumigated at a measured concentration-time
    product of 340 mg h/l no taint was detected in loaves baked from the
    flour produced from the wheat (Pest Infestation Laboratory, 1947).
    There was no effect upon the content of riboflavin and nicotinic acid.


    Biochemical aspects

         After absorption, methyl bromide appears to be broken down by the
    mammalian body, at least in part, for the level of non-volatile
    bromide in the blood and tissues rises significantly. There is also
    some evidence that volatile bromide, presumably unchanged methyl
    bromide, is stored in tissues which are rich in lipoid material. It is
    doubtful, however, whether these findings are relevant to the
    consideration of the residues to be found in foodstuffs fumigated with
    methyl bromide, because in this process as commercially practised no
    unchanged methyl bromide persists (von Oettingen, 1946).

    1. The fumigant

    Acute toxicity

         For rabbits the minimum acute lethal dose of methyl bromide has
    been estimated at 60-65 mg per kg body-weight, when given by
    stomach-tube as a solution in olive oil (Dudley et al., 1940).

         100 mg/kg of methyl bromide given by stomach-tube as a solution
    in olive oil proved to be fatal to rats in five to seven hours (Miller
    and Haggard, 1943).

    2. The fumigated foodstuff

    Short-term studies

         Rat.    When an unspecified number of rats was fed on a diet
    fumigated with methyl bromide so as to give a residue in the diet
    equivalent to 5760 ppm, calculated as bromide, the animals developed
    diarrhoea after three weeks and, over eight weeks in all, their weight
    gain was slight, their general condition was poor and reproduction
    ceased (Dudley et al., 1940).

         In a further experiment extending over 16 weeks in which, again,
    the number of animals was not disclosed, a residue equivalent to 1304
    ppm bromide in the diet gave rise to increased sleepiness, reduced
    activity, increased death-rate, decreased weight gain and cessation of
    activity. Return to a normal diet then led to complete restoration to
    health (Dudley et al., 1940).

         A third experiment was carried out with groups each of 36 young
    white rats. They were fed on a methyl bromide fumigated diet for 20
    weeks. In those receiving the diet in which the residue was equivalent
    to 5760 ppm bromide adverse effects were observed - partial limb
    paralysis, scaly tails, a form of xerophthalmia, loss of hair, reduced
    food intake, inactivity, impaired reproduction and increased
    mortality. By contrast, those animals on a diet in which the residue
    was equivalent to 206 and 223 ppm bromide remained apparently healthy,
    although their weight gain was slightly reduced. Finally, where the
    diet was a mixed one containing fresh vegetables, cheese, peanuts, and
    dried fruits, in addition to the rat pellets, all fumigated with
    methyl bromide at a concentration of 3 lb per 1000 ft3 (48 g/m3) so
    that the residue was equivalent to a maximum of 240 ppm calculated as
    bromide, no deleterious effects were seen. In all the experiments
    there was no significant histopathology (Dudley et al., 1940).

         Two groups, each of 25 males and 32 females, were fed for one
    year on a modified Sherman diet, previously fumigated with methyl
    bromide at the rate of 2 lb and 6 lb respectively per 1000 ft3 (16 g
    to 48 g/m3) and having an average bromide content of 262 ppm and 637

    ppm. A group of 10 male and 15 female rats was fed the same basic diet
    unfumigated but with the addition of 0.1% (i.e., 1000 ppm) sodium
    bromide. The control group received the unmodified diet. At the end of
    12 months none of the animals showed any evidence of adverse effect in
    growth rate, haematological, histopathological, organ weight and
    analytical studies (Spencer et al., 1944).

         Rabbit. Groups, each of 12 rabbits, were fed for one year on
    diets which had been fumigated with methyl bromide for 24 hours. When
    the residue was equivalent to 3254 ppm bromide the animals developed a
    progressive paralysis and urinary retention and all died within two
    weeks. In the others, for which the residue was equivalent to 67 and
    75 ppm bromide, there were no abnormalities, apart from a slight
    reduction in weight gain and some polydypsia and polyuria. Post
    mortem, bronchopneumonia was common in the high dose group and changes
    were found in the brain (Dudley et al., 1940).

         Dog. Twenty-two beagles of both sexes, in groups of four to
    six animals, were fed for one year on methyl bromide fumigated diets.
    The residues of 1389, 2975 and 6097 ppm led to a bromide intake of
    approximately 35, 75 and 150 mg per kg body-weight daily. Another
    group of dogs was given sodium bromide in the diet at the rate of 100
    mg per kg body-weight daily of bromide. Lethargy was seen only in
    those animals on the high residue intake and there was occasional
    salivation and diarrhoea both in this group and that on sodium
    bromide. Otherwise the animals remained substantially healthy and,
    post mortem, no significant changes were found (Rosenblum et al.,

    Comments on experimental studies reported

         No work has been done on the chronic oral toxicity of methyl
    bromide as such. However, it is probable that by far the major part,
    if not all, of the residue which persists in foodstuffs fumigated with
    methyl bromide in accordance with ordinary commercial practice is in
    the form of inorganic bromide. If this is so, the safety of treated
    products could be assessed simply on the basis of their bromide

         The question still remains whether the small interaction that
    takes place between the fumigant and the food constituents could lead
    to a significant decrease of nutritional value or to the formation of
    toxic compounds. The answer to this should be provided by the feeding
    studies with animals given a diet which had previously been fumigated.
    These experiments have been short-term and covered three species of
    animals. From these it is clear that diets fumigated with methyl
    bromide at the usual rates adopted commercially had no deleterious
    effects on any of the animals. If, however, the attempt is made to
    take the maximum no-effect level demonstrated in animals and then to
    apply the conventional safety factor of 100 for extrapolation to man,
    the resulting permitted bromide level would be extremely low. It
    might, indeed, be no more than the natural level of bromides in a
    number of foodstuffs (World Health Organization, 1964).


         Since nothing is known about the chronic oral toxicity of
    unchanged methyl bromide no acceptable daily intake can be assigned to
    it as a residue.

         To judge by the work on cereal products the interaction of the
    compound with the food is unlikely to have hazardous consequences.

         The risk to the consumer from the fixed residue of inorganic
    bromide could be assessed, if need be, on the basis that the total
    daily intake of this ion from all sources should not exceed 10 mg Br
    per kg body-weight (World Health Organization, 1964).

    Further work required

         Chemical nature of the residue in foods other than cereals.

         Biochemical studies on the changes produced by methyl bromide in

         Long-term studies, at least in rats, with inorganic bromide.

         Chronic oral toxicity of methyl bromide.


    Bridges, R. G. (1955) J. Sci, Food Agric., 6, 261

    Burns-Brown, W., Coppock, J. B. M., Edwards, G. H., Greer, E. N.,
    Gordon Hay, J. & Heseltine, H. K. (1955) Chem. and Ind., 19 March,
    p. 324

    Burns-Brown, W., Heseltine, H. K., Devlin, J. J. & Greer, E. N. (1961)
    Milling, 137, 401

    Dudley, H. C., Miller, J. W., Neal, P. A. & Sayer, R. R. (1940) Publ.
    Hlth Rep. (Wash.), 55, 2251

    Clegg, K. M. a Lewis, S. E. (1953) J. Sci. Food Agric., 4, 548

    Lindgren, D. L., Gunther, F. A. & Vincent, L. E. (1962) J. econ.
    Ent., 55, 773

    Miller, D. P. & Haggard, H. W. (1943) J. industr. Hyg., 25, 423

    von Oettingen, W. F. (1946) Nat. Inst. Hlth Bull., 185, United
    States Public Health Service

    Pest Infestation Laboratory (1947) Pest Infestation Research Board
    Report No. 11 (unpublished document)

    Rosenblum, I., Stein, A. A. & Eisinger, G. (1960) Arch. environ.
    Hlth, 1, 316

    Spencer, H. C., Rowe, V. K., Adamo, C. M. & Irish, D. D. (1944) Food
    Res., 9, 11

    Turner, A. (1964) J. Sci. Food Agr., 15, 265

    Winteringham, F. P. W. (1955) J. Sci. Food Agr., 6, 269

    Winteringham, F. P. W., Harrison, A., Bridges, R. G. & Bridges, P. R.
    (1955) J. Sci. Food Agr., 6, 251

    World Health Organization (1964) Wld Hlth Org. techn. Rep. Ser.,

    See Also:
       Toxicological Abbreviations
       Methyl Bromide (EHC 166, 1995)
       Methyl bromide (ICSC)
       Methyl bromide (PIM 340)
       Methyl bromide (FAO/PL:CP/15)
       Methyl bromide (FAO/PL:1967/M/11/1)
       Methyl bromide (FAO/PL:1968/M/9/1)
       Methyl bromide (WHO Pesticide Residues Series 1)
       Methyl Bromide (IARC Summary & Evaluation, Volume 71, 1999)