WHO Pesticide Residues Series, No. 1



    The evaluations contained in these monographs were prepared by the
    Joint Meeting of the FAO Working Party of Experts on Pesticide
    Residues and the WHO Expert Committee on Pesticide Residues that met
    in Geneva from 22 to 29 November 1971.1

    World Health Organization



    1 Pesticide Residues in Food: Report of the 1971 Joint Meeting of
    the FAO Working Party of Experts on Pesticide Residues and the WHO
    Expert Committee on Pesticide Residues, Wld Hlth Org. techn. Rep.
    Ser., No. 502; FAO Agricultural Studies, 1972, No. 88.

    These monographs are also issued by the Food and Agriculture
    Organization of the United Nations, Rome, as document AGP-1971/M/9/1.

    FAO and WHO 1972


    This pesticide was evaluated at the Joint Meeting in 1965 (FAO/WHO
    1965c) and was reviewed extensively in 1966 and 1967 (FAO/WHO 1967b,
    1968b) and briefly in 1968 (FAO/WHO 1969b).

    Reference should be made to Appendix IV, where Section 3 of the report
    on the 1971 meeting (FAO/WHO 1972a) is reproduced. It contains a
    discussion of general principles relating to the occurrence of
    residues of fumigants.


    Use Pattern

    Methyl bromide, which has been in widespread use for some 30 years, is
    used for all types of stored dry foodstuffs, particularly for produce
    in bags, cases or other packages. Its use for large loose bulks of
    foodstuff, when it may be used alone or in admixture with ethylene
    dibromide or with carbon tetrachloride, is more limited. It is also
    used for the treatment of soil before sowing or planting, especially
    in glasshouses, against nematodes, weeds and other organisms; also in
    plant quarantine operations, including disinfestation of fresh fruit
    and vegetables.

    Residues resulting from supervised trials (Post-harvest uses)

    From the extensive information previously reported it can be concluded
    that after treatment of produce the physically sorbed residue
    diminishes either by loss to the atmosphere or by reaction with the
    foodstuff. The main reaction products with cereals and cereal products
    have been characterized. In these materials methylation of nitrogen
    and sulfur containing groups in the protein fraction accounts for most
    of the reaction. In other foods the reaction appears to be similarly
    associated with the protein fraction. These reactions also leave a
    "fixed" residue of bromide which is in the water-soluble, ionic form.
    Considerable attention has been given to the determination of this
    inorganic bromide occurring in food after fumigation and many data are
    available for amounts in foodstuffs moving in commerce as well as from
    supervised trials.

    The significance of these residues of inorganic bromide is discussed
    in Appendix IV.

    Less attention has been given to study of the residue of unreacted
    methyl bromide. The earlier work supported the assumption that normal
    airing, processing and cooking of foodstuffs would ensure that no
    residue of the unreacted compound could reach the consumer. However,
    the development of analytical methods, based on gas-chromatography,
    which enable much lower levels to be detected and measured allows a
    reassessment of the persistence of these unreacted residues. Scudamore
    and Heuser (1970) present data for a wide range of dry foods showing
    the rate of disappearance of methyl bromide under controlled
    conditions from both sealed samples and well ventilated samples. The
    rate of disappearance under commercial conditions of storage and
    handling must be between these extremes.

    For most products held at 25C the amount of residual methyl bromide
    fell to below 1 ppm within a few days. In some foods such as
    groundnuts and cocoa beans the rate of loss was lower. The rate was
    also lower at lower temperatures, small amounts of methyl bromide
    being extracted from several commodities one month after treatment.
    These experiments were carried out at a constant concentration of
    approximately 14 mg per litre, which is a little above the level
    normally found in the latter part of a commercial treatment. It was
    found that the amounts of the initial residues of unchanged methyl
    bromide (one hour after fumigation) were more closely proportional to
    the concentration than to the concentration-time product. From a close
    study of these data, and considering for each commodity the normal
    extent of storage, processing and cooking to which it is subjected, it
    is reasonable to conclude that there is insignificant risk that
    residues of unchanged methyl bromide will reach the consumer.

    Fate of residues

    Little, if any, new information on the nature of the reactions of
    methyl bromide with foods has been obtained since the studies on
    cereals and cereal products by Winteringham and his colleagues
    (Bridges, 1955, Winteringham et al., 1955 and Winteringham, 1955)
    reviewed at earlier Joint Meetings. In cereals, methylation, mainly of
    nitrogen and sulfur-containing groups in the proteins accounts for a
    large part of the decomposition of sorbed fumigant.

    At the same time a residue of inorganic bromide is formed and there
    have been many studies of the amounts produced in treated foods. Where
    the amount of inorganic bromide directly resulting from treatment with
    methyl bromide is known, this provides an indication of the amount of
    methylation or of other reactions that has taken place.

    Evidence of residues in food in commerce or at consumption

    Nearly all the available data relate to inorganic bromide. In most
    cases it is not possible to attribute the amount found to any
    particular treatment with methyl bromide.

    Methods of residue analysis

    Most of the earlier studies on bromide residues after fumigation with
    methyl bromide used methods which measured total bromide in all forms.
    A widely used procedure involved a preliminary alkali digestion
    process followed by ashing under controlled conditions, extraction
    with water and determination of bromide by an oxidation and titration
    method (Shrader et al., 1942, Lewis and Eccleston, 1946, Kolthoff and
    Yutzy, 1937, van der Meulen, 1931). Some workers have preferred oxygen
    combustion to the alkaline fusion procedure (Dow, 1970, Kretzschmann
    and Engst, 1968a, 1968b, 1970). Other studies of total bromide content
    have been made by the use of neutron activation analysis (Guinn and
    Potter, 1962; Lindgren, et al., 1962) or X-ray fluorescence
    (Getzendaner, 1961; Getzendaner et al., 1968; Shuez et al., 1971).

    When it was suspected that unreacted methyl bromide was present some
    workers removed this by prolonged aeration or by solvent extraction
    before determining bromide and estimated the organic bromide by
    difference from the total bromide found in an unreacted sample. This
    procedure was clearly incapable of determining small amounts of
    unreacted methyl bromide.

    Gas-chromatography now provides a ready means of direct determination
    of unreacted methyl bromide. A number of methods of extracting the
    residual fumigant from food samples have been examined. Methyl bromide
    was among six fumigants used by Malone (1969) in a comparison of three
    recovery procedures; a sweep co-distillation method based on the
    procedures of Ragelis et al., (1968) and Storherr et al., (1967); a
    steam distillation procedure adapted by Bielorai and Alumot (1966)
    from the method of Kennet and Huelin (1957); and the acid reflux
    procedure described by Ramsey (1957) for the determination of carbon
    tetrachloride. Malone (1970) further developed her preferred method,
    the acid reflux procedure, for the multi-residue determination of
    volatile fumigant residues in cereal grains. At low levels of residual
    methyl bromide (3 ppm) recoveries were low (60 to 70%).

    Heuser and Scudamore (1968) developed a simple extraction procedure
    applicable to methyl bromide in wheat and flour. The sample was soaked
    in a 5:1 acetone water mixture in a flask at room temperature and an
    aliquot of the supernatant liquid injected into the gas-chromatograph.
    The procedure was developed into a multiresidue system (Heuser and
    Scudamore, 1969) and was further evaluated in a study of the residues
    of unreacted methyl bromide in a wide range of foods (Scudamore and
    Heuser, 1970). An extraction period of 24 hours was sufficient for all
    the foods tested but for a finely divided product such as flour the
    maximum recovery was obtained after six hours. A polypropylene on
    Chromosorb W column was used with a flame ionization detector.
    Ultimate sensitivity for 1% of full-scale deflection was equivalent to
    1.5  10-10 g, or 0.1 ppm when 15 g of commodity was extracted with
    30 ml of solvent mixture and a 3 l injection was used. Satisfactory
    recoveries were obtained with all the foods tested with the exception
    of cocoa beans (about 60% recovery).

    Heuser and Scudamore (1970) have developed a method for the selective
    determination of inorganic bromide by gas-chromatography with
    simultaneous measurement of residual methyl bromide if desired. The
    method is based upon the quantitative reaction of inorganic bromide
    present in the commodity with excess of ethylene oxide in an acidified
    solvent to form ethylene bromohydrin. Methyl bromide (and also
    ethylene dibromide) remains intact under these conditions. The
    bromohydrin is determined by gas-chromatography with electron-capture
    detection, with a sensitivity of 0.5 ppm of ionic bromide.

    National tolerances (as reported to the meeting)

    Residues of unreacted methyl bromide

    Australian and Canadian regulations assume that after post-harvest use
    of methyl bromide, residues of the unreacted fumigant will disappear
    before food reaches the consumer.

    Residues of inorganic bromide

    Many countries have established tolerances of inorganic bromide, or
    total bromide, in foods fumigated with methyl bromide. The United
    States and Canadian regulations provide extensive lists of tolerances
    related to different treatments. The tolerance of 50 ppm in raw cereal
    grains is widely adopted.


    For many years methyl bromide has been widely used for dry stored
    foods especially those in bags, cases or other packages. It is also
    used for quarantine purposes, on a wide range of fresh fruits and
    vegetables, and in the treatment of soil against nematodes and
    soil-borne insects and fungi.

    After fumigation the physically sorbed fumigant is rapidly given off
    into the atmosphere: some may react with the food and it has been
    generally assumed that none of the unreacted compound will reach the
    consumer. Recent work employing more sensitive and specific analytical
    methods based upon gas-chromatography has demonstrated that this
    residue can, in some circumstances, be more persistent than has been
    hitherto supposed. For any given food the amount depends upon the
    concentration during treatment, the moisture content of the food, the
    duration and temperature of storage and the amount of ventilation

    A multi-residue method is available using cold solvent extraction and
    gas-chromatography by which unreacted methyl bromide can be detected
    to a limit of about 0.1 ppm. The method is suitable for regulatory
    purposes when a limit of Ca. 0.5 ppm should be determined with

    The reactions of methyl bromide with cereals and cereal products have
    been characterized as mainly methylation of nitrogen- and
    sulfur-containing groups in the proteins with simultaneous formation
    of a "fixed" residue of inorganic (ionic) bromide. Reaction in other
    foods appears to be similarly associated with the proteins. A
    considerable amount of attention has been given to the amounts of
    inorganic bromide formed in this way and to the setting of tolerances
    for inorganic bromide. Unfortunately, in regulatory work it is not
    usually possible to assign a particular fraction of the total
    inorganic bromide as being derived from methyl bromide. Most
    analytical methods determine the total bromide in the sample, but a
    method using gas-chromatography is now available which specifically
    determines ionic bromide in the presence of methyl bromide (or
    1,2-dibromoethane) with a limit of determination of 0.5 ppm. The
    general problem of residues of inorganic bromide in food is discussed
    in the Report (see Appendix IV).

    Food moving in commerce shortly after fumigation with methyl bromide
    may contain high levels of unreacted fumigant. This treatment
    sometimes takes place immediately before shipment or during transit in
    ship holds or in containers. The amount of unreacted methyl bromide
    usually diminishes rapidly during handling, storage and processing.

    From the available information on the occurrence of unreacted methyl
    bromide in or on the foods listed below, after fumigation in
    accordance with good practice the following amounts need not be
    exceeded (provided that the commodity is freely exposed to air for a
    period of at least 24 hours after fumigation before sampling). It is
    recommended that these residue levels be used as guidelines:

    In the following raw
    agricultural products at
    point of entry into a country:  nuts, peanuts,                 100 ppm
                                    raw cereals, cocoa beans        50 ppm
                                    dried fruits                    20 ppm

    In milled cereal products
    which will be subject to
    baking or cooking:                                              10 ppm

    In the following foods at the
    point of retail distribution
    or when offered for
    consumption:                    nuts, peanuts,                 0.5 ppm
                                    bread, other cooked        (i.e. at or
                                    cereal products, cocoa       about the
                                    products, dried fruits   present limit

    Even though the content of bromide ion per se may be considered to
    be of minor importance, nevertheless, to ensure that the content of
    methylated reaction products is not excessive it is recommended that
    the previously recommended tolerance of 50 ppm of bromide ion in raw
    cereals and wholemeal flour should stand.

    It is recommended that temporary tolerances previously recommended for
    residues of bromide ion in other foods be suspended (see Report).

    Further work desirable

    1.   Additional data on residues of unchanged methyl bromide occurring
         in food in commercial practice including data for fresh fruits
         and vegetables.

    2.   Information on the nature of the reaction products of methyl
         bromide with foods other than cereals and cereal products.

    3.   Additional data on amounts of "background" bromide ion occurring
         in foods before post-harvest fumigation.


    Alumot, E. and Bieloria, R. (1969) Residues of fumigant mixture in
    cereals fumigated and aired at two different temperatures. J. Agr.
    Food Chem., 17: 869

    Bieloria, B. and Alumot, E. (1966) Determination of residues of a
    fumigant mixture in cereal grain by electron-capture gas
    chromatography. J. Agr. Food Chem., 14: 622

    Bridges, R. G. (1956) The fate of labelled insecticide residues in
    food products. V. The nature and significance of ethylene dibromide
    residues in fumigant wheat. J. Sci. Food Agr., 7: 505-313

    Burns-Brown, W., Coppock, J. B. M., Edwards, G. H., Greer, E. N.,
    Hay, J. G. and Heseltine, H. K. (1955) The fumigation of flour with
    methyl bromide. Chem. and Ind., 324-325

    Dow, M. L. (1970) Oxygen combustion method for determination of
    bromide residues in foods. J. Assoc. Offic. Agr. Chem., 53:

    Getzendaner, M. E. (1961) Bromide residues in cereal products
    resulting from experimental fumigations with methyl bromide. Cereal
    Sci. Today, 6: 268-270

    Getzendaner, M. E., Doty, A. E., McLaughlin, E. L. and Lindgren, D. L.
    (1968) Bromide residues from methyl bromide fumigation of food
    commodities. J. Agr. Food Chem., 16: 265-271

    Guinn, V. P. and Potter, J. G. (1962) Determination of total bromine
    residues in agricultural crops by instrumental neutron activation
    analysis. J. Agr. Food Chem., 10: 232-236

    Heseltine, H. K. (1970) Pest Infestation Research 1969. Agricultural
    Research Council, London

    Heuser, S. G. and Scudamore, K. A. (1967) Determination of ethylene
    chlorohydrin, ethylene dibromide and other volatile fumigant residues
    in flour and whole wheat. Chem. and Ind., 1557-1560

    Heuser, S. G. and Scudamore, K. A. (1968a) Fumigant residues in wheat
    and flour: solvent extraction and gas-chromatographic determination of
    free methyl bromide and ethylene oxide. Analyst, 93: 252-258

    Heuser, S. G. and Scudamore, K. A. (1969) Determination of fumigant
    residues in cereals and other foodstuffs: a multidetection scheme for
    gas-chromatography of solvent extracts. J. Sci. Food Agr., 20:

    Heuser, S. G. and Scudamore, K. A. (1970) Selective determination of
    ionized bromide and organic bromides in foodstuffs by gas-liquid
    chromatography with special reference to fumigant-residues. Pesticide
    Sci., 1: 244-249

    Kolthoff, I. M. and Yutzy, M. (1937) Volumetric determination of
    bromide after oxidation to bromate in the presence of much chloride.
    Ind. Eng. Chem. (Anal. Edn.), 9: 75

    Kretzschmann, F. and Engst, R. (1968a) (Protometric determination of
    bromide in methyl bromide-containing food and fodder) Nahrung,
    12: 135-136

    Kretzschmann, F. and Engst, R. (1968b) (Determination of traces of
    bromides in vegetables after treatment with methyl bromide.
    I. Analytical studies). Nahrung, 12: 603-610

    Kretzschmann, F. and Engst, R. (1970) (Micro-determination of bromide
    residues in food following treatment with gaseous methyl bromide).
    Mikrochim. Act., (2): 270-278 (Chem. Abstr., 1970, 73: 23953)

    Lewis, S. E. and Eccleston, K. (1946) Residues in wheat flour
    fumigated with methyl bromide. J. Soc. Chem. Ind. Lond., 65: 149152

    Lindgren, D. L., Gunther, F. A. and Vincent, L. E. (1962) Bromide
    residues in wheat and milled wheat fractions fumigated with methyl
    bromide. J. Econ. Entomol., 55: 773-776

    Lindgren, D. L., Sinclair, W. B. and Vincent, L. E. (1968) Residues in
    raw and processed foods resulting from post-harvest insecticidal
    treatments. Residue Rev., 21: 1-121

    McMahon, B. Malone. (1971) Analysis of commercially fumigated grains
    for residues of organic fumigants. J. Assoc. Offic. Analyt. Chem.,
    54: 964-965

    Malone, B. (1969) Analysis of grains for multiple residues of organic
    fumigants. J. Assoc. Offic. Analyt. Chem., 52: 800-805

    Malone, B. (1970) Method for determining multiple residues of organic
    fumigants in cereal grains. J. Assoc. Offic. Analyt. Chem., 53:

    Mapes, D. A. and Shrader, S. A. (1957) Determination of total and
    inorganic bromide residues in fumigated products. J. Assoc. Offic.
    Agr. Chem., 40: 189-191

    Ramsey, L. L. (1957) Colorimetric determination of carbon
    tetrachloride in fumigated cereal products. J. Assoc. Offic. Agr.
    Chem., 40: 175-180

    Scudamore, K. A. and Heuser, S. G. (1970) Residual free methyl bromide
    in fumigated commodities. Pesticide Sci., 1: 14-17

    Scudamore, K. A. and Heuser, S. G. (1971c) Effect of methyl bromide on
    dried egg. (Personal communication)

    Shrader, S. A., Beshgetoor, A. W. and Stenger, U. A. (1942)
    Determination of total and inorganic bromide in goods fumigated with
    methyl bromide. Ind. Eng. Chem. (Anal. Edn.), 14: 1-4

    Shuey, W. C. Youngs, V. L. and Getzendaner, M. E. (1971) Bromide
    residues in flour streams milled from fumigated wheats. Cereal Chem.,
    48: 34-39

    Winteringham, F. P. W. (1955) The possible toxicological and
    nutritional significance of fumigated wheat with methyl bromide. J.
    Sci. Food Agr., 6: 269-274

    Winteringham, F. P. W., Harisson, A., Bridges, R. G. and Bridges, P.
    R. (1955) Nature of methyl bromide residues in fumigated wheat.
    J. Sci. Food Agr., 6: 251-261

    See Also:
       Toxicological Abbreviations
       Methyl Bromide (EHC 166, 1995)
       Methyl bromide (ICSC)
       Methyl bromide (PIM 340)
       Methyl bromide (FAO Meeting Report PL/1965/10/2)
       Methyl bromide (FAO/PL:CP/15)
       Methyl bromide (FAO/PL:1967/M/11/1)
       Methyl bromide (FAO/PL:1968/M/9/1)
       Methyl Bromide (IARC Summary & Evaluation, Volume 71, 1999)