Sponsored jointly by FAO and WHO


    Joint meeting of the
    FAO Panel of Experts on Pesticide Residues
    in Food and the Environment
    and the
    WHO Expert Group on Pesticide Residues
    Geneva, 3-12 December 1979

    BROMOMETHANE (Methyl Bromide)


    This fumigant pesticide was evaluated at the Meeting in 1965, reviewed
    in 1966, 1967 and 1968 and re-evaluated in 1971.  These evaluations
    were related almost exclusively to post-harvest uses.  Since 1971,
    both pre- and post-harvest uses have increased considerably.

    The report of the 1971 Meeting contained a statement of general
    principles relating to residues of fumigants, to which reference
    should be made.  That of 1978 referred to problems caused in member
    countries of FAO and WHO by the use of certain fumigants and the
    re-evaluation of methyl bromide was recommended.

    Recent concern about the many sources of inorganic bromide entering
    the human diet, some of which arise from methyl bromide usage, both
    before and after harvest, has led to the need for an assessment of
    these contributions in this new context.  In addition the availability
    of more sensitive methods for determining residual methyl bromide in
    foodstuffs allows a re-appraisal of existing guideline residue levels
    for the unchanged fumigant in fumigated commodities.


    Pre-harvest Use

    Methyl bromide is applied to soil plots in glasshouses or in the open
    for control of nematodes and other pests, weeds and micro-organisms.
    It may be applied to soil under sheeting in the vaporised form or
    injected as a liquid and allowed to vaporise in situ.  Aeration of the
    soil before planting is necessary to avoid phytotoxicity from the
    fumigant.  Leaching of plots with water is sometimes undertaken to
    reduce soil bromide contents.  Its use in field plot treatments has
    been increasing in recent years.  It may be used as the pure compound
    or with 2% chloropicrin added as a warning (lachrymatory) agent.

    Post-harvest Use

    Methyl bromide is widely applied in the vapour phase to many types of
    stored dry foodstuffs and other products such as tobacco, and to a
    more limited extent to fresh fruit and vegetables in plant quarantine
    operations.  Little change in the pattern of post-harvest cereal grain
    or other stored product uses of methyl bromide has taken place since
    the 1971 evaluation.  However an awareness that bromide residues
    occurring in food as a result of fumigation with methyl bromide are
    accumulative in the food, together with attempts to minimise the
    occurrence of resistance in insect populations, has led to a reduction
    in the intentional repeated fumigations of parcels of dry stored
    commodities with the same fumigant which were an earlier feature of
    long-term tropical storage.  In international trading conditions
    though, the possibility of re-infestation and subsequent re-treatments
    with methyl bromide cannot be discounted.  Some quarantine schedules

    provide for fumigation of fresh produce with methyl bromide (e.g.
    Hargreaves et al., 1978) but because of possible damage to fruit and
    vegetables such treatments are likely to be at low dosage levels and
    unlikely to be repeated.


    Pre-harvest Use

    When methyl bromide is applied to soil, much of the gas eventually
    escapes by diffusion to the atmosphere.  A proportion is however
    hydrolysed, resulting in the deposition of inorganic bromide (bromide
    ion) in the soil.  This bromide may be leached downwards, away from
    the areas from which plants feed, by rainfall or by irrigation.  That
    which remains in the root area may be taken up by the plant and
    concentrated in the leaves and to a lesser extent in fruits (Maw and
    Kempton 1973).  So far as can be ascertained, no evidence has been
    published of any organo-bromine compounds occurring in plants as a
    result of growth on methyl bromide treated soil.

    Common rates of application of methyl bromide to soil vary between 50
    and 125 g per m2 (1-2 lb per 100 ft2).  Kempton and Maw (1972)
    found that soil applications at 50-100 g per m2 gave soil bromide
    levels of 10-60 mg/kg, in turn leading to bromide ion levels in
    lettuce leaves of 100-700 mg/kg (wet weight).  Parallel tests on
    plants grown in soil containing known amounts of potassium bromide
    confirmed a relationship between bromide content of soil and of plant
    tissue gown in it (Maw and Kempton 1973).  However Coosemans and Van
    Assche (1978) showed that the relationship between soil bromide
    content and plant bromide content was affected by the presence of
    other anions in the soil.  They reported 50-250 mg/kg bromide ion in
    the lettuce following methyl bromide soil application at levels of
    60-120 g per m2 with no leaching.  Coosemans and Van Assche (1977)
    also determined bromide ion in the fruit of strawberries and tomatoes
    grown in fumigated soil, up to 80 mg/kg and 4 mg/kg respectively, but
    much more in the leaves of both plants.

    In a survey in the U.K. of lettuce stated to have been grown on either
    unfumigated soil or on soil fumigated at 1 - 2 lb per ft2 (Roughan
    1979) ranges of inorganic bromide contents of lettuce leaves were as

                                Fumigated soil
                 mg/kg Br- wet weight   No. of samples within range

                          0 - 5                        0
                          5 - 10                       1
                         10 - 50                       3
                         50 - 100                     12
                        100 - 200                     12
                        200 - 500                     19
                        500 - 1000                     6
                       1000 - 2000                     1

                       Soil said to have been untreated
                mg/kg Br- wet weight   No. of samples within range

                       0 - 5                       26
                       5 - 10                      2
                      10 - 50                      4
                      50 - 100                     2
                     100 - 200                     1
                     200 - 500                     5

    From a plot treated with methyl bromide in July 1977, lettuce planted
    in July 1977 and harvested in September contained 140-450 mg/kg Br-
    while spring cabbage planted in October 1977 and harvested in May 1978
    contained 300-800 mg/kg.  In the same plot, lettuce planted in July
    1978 and harvested in September contained 14-26 mg/kg.  The world data
    baseline for crops grown on untreated soil was said to be
    approximately 5 mg/kg.


    Pre-harvest Use

    A large proportion of a given amount of methyl bromide applied to soil
    diffuses away either to the atmosphere or away from the point of
    application.  Soils of high organic matter content tend to retain more
    fumigant and subsequently bear higher contents of inorganic bromide,
    probably as a result of hydrolytic decomposition.  Tonic bromide is
    taken up by some plants and concentrates in the leafy parts, e.g. in
    lettuce, cabbage, tomatoes.  So far as is known, no residues other
    than bromide ion occurring in growing plant material as a result of
    soil fumigation with methyl bromide have been reported.

    Post-harvest Use

    A major proportion of a dose of methyl bromide, generally applied by
    exposure of stored commodities to the vapour, is removed still in the
    vapour phase on ventilation and desorption.  Foods of high oil or fat
    content retain free methyl bromide for longer periods but El Lakwah
    (1978) considered the fineness of division of extracted or expeller
    products more important than the fat content in determining the amount
    originally absorbed.

    Scudamore and Houser (1970) reported detailed studies on the rate of
    loss of free methyl bromide from a range of stored fumigated
    commodities.  These studies suggested that after normal storage and
    pressing, free methyl bromide would be unlikely to be detected at the
    point of sale.  The 1971 Meeting reviewed earlier work on the
    reactions of methyl bromide with protein components of cereals
    (Bridges 1955, Winteringham 1955, Winteringham et al. 1955).  Several
    groups of workers have since studied the reactions of the methyl
    radical with proteins and amino-acids in various foods (e.g. Adomako
    1974; Asante-Poku et al. 1974; Pest Infestation Control Laboratory

    1978) but no analytical methods suitable for determining
    quantitatively such residues for monitoring or regulatory purposes
    have been published to date.  Recently isoelectric-focussing studies
    on 14C-labelled protein fractions from fumigated cereals coupled with
    HPLC analyses on fractions separated by gel-permeation chromatography
    have shown broad range methylation of protein and free amino-acids
    (Pest Infestation Control Laboratory 1980).

    In certain foods, for example navy beans heavily treated with methyl
    bromide, the formation of methyl methionine sulphonium bromide and its
    subsequent spontaneous decomposition gives rise to dimethyl sulphide
    (Fishwick 1978), which taints the commodity and can be determined
    analytically.  Dimethyl sulphide volatilises fairly rapidly though
    and, as it occurs naturally in a number of foodstuffs, it can probably
    be regarded as of little or no toxicological significance.

    Fate in Storage and Processing

    A large proportion of any free methyl bromide disappears fairly
    rapidly from most fumigated stored products by volatilisation and due
    to the reactions with plant constituents already described (Scudamore
    and Heuser, 1970).  No free methyl bromide is known to be taken up by
    plants grown on pre-fumigated soil and none should therefore be
    present unless produce is subsequently fumigated.  If, after stored
    product fumigation, small amounts of free methyl bromide persist until
    the products are milled or otherwise processed, or cooked, it is
    expected that further reductions in residue levels would then occur,
    but analytical methodology sensitive enough to determine such expected
    levels, i.e. below 0.1 mg/kg, has only recently become available
    (Fairall and Scudamore, 1980).  Any inorganic bromide produced as a
    result of decomposition of methyl bromide remains at a constant level,
    with levels likely to be highest in the high protein fraction of
    milled products.


    Free Methyl Bromide and Dimethyl Sulphide

    Although the studies of Scudamore and Heuser (1970) showed that after
    storage of fumigated oily or sorptive commodities at very low
    temperatures it was possible to detect low levels of free methyl
    bromide, only negative results on free methyl bromide in products at
    the point of sale have been recorded (Singh et al., 1976; Johansson,
    1976).  The presence of dimethyl sulphide in methyl bromide fumigated
    navy beans imported into the U.K. was reported by Fishwick (1978), but
    normal processing for the baked bean trade is thought to reduce this
    to insignificant amounts.

    Inorganic Bromide

    Reports too numerous to list refer to the presence of inorganic
    bromide in products or commodities fumigated with methyl bromide, but
    in commerce it is impossible to determine, except perhaps by
    inference, whether such residues arose as a result of storage
    fumigation or from the presence in the plant of bromide from another
    source such as soil fumigation with a bromine compound, or
    contamination of soil with sea water prior to growth.  There is
    however strong inferential evidence that increased incidence at retail
    outlets of lettuce and cabbage plants with raised inorganic bromide
    content e.g. greater than 100 mg/kg (wet weight) may be related to the
    increasing use of methyl bromide sterilization of soil, especially in
    glasshouses.  A few examples of lettuce plants bought at retail
    outlets in the United Kingdom in 1978 contained residues exceeding 500
    mg/kg (wet weight) but 92% of plants purchased in the South East of
    the country contained less than 6 mg/kg whilst of samples received for
    analysis from other areas, only 8% exceeded 150 mg/kg (Roughan, 1979).


    (a) Free methyl bromide

    Determination of free methyl bromide in commodities can be carried out
    by cold extraction with an acetone/water mixture followed by gas
    chromatography with flame-ionisation or electron capture detection
    (Heuswer and Scudamore 1968, 1969).  The limit of determination of
    this method is about 0.5-1 mg/kg.  Great care must be taken in the
    determination of free methyl bromide not to incur losses due to
    volatilisation or increased substrate interactions during recovery.
    Recently a method has been published (Fairall and Scudamore, 1980) in
    which extracted methyl bromide is reacted to form methyl iodide which
    is then determined by gas chromatography, with a claimed limit of
    sensitivity of 0.01 mg/kg.  Grave and Hogendoorn (1979) report a
    claimed sensitivity of 0.05 mg/kg for a head-space method for residual
    methyl bromide in grain.

    (b) Inorganic (bromide ion)

    Bromide ion may be determined selectively by selective ion electrode
    (Banks et al. 1976) or by reacting the extracted inorganic bromide
    with ethylene oxide to form ethylene bromohydrin, which is then
    determined by gas chromatography using electroncapture detection
    (Heuser and Scudamore, 1970; Panel on Fumigant Residues in Grain,
    1976; Stijve, 1977).  Bromide ion may also be determined by total
    bromide chemical assay (Mapes and Shrader 1957), by X-my fluorescence
    analysis (Getzendaner et al. 1968) or by neutron activation analysis
    (Lindgren et al. 1962) providing that other bromine compounds have
    first been removed selectively with a solvent such as methylene
    chloride.  In the absence of such pre-extinction those total bromide
    assay methods will include any organo-bromine compounds present, which
    may be of a totally different order of toxicity to mammals.


    No national residue limits for free methyl bromide in commodities are
    known to be in force.  An EEC draft directive in 1976 proposed a limit
    of 0.5 mg/kg in cereals to be put into circulation for human

    Limits for inorganic bromide in a wide range of commodities exist in
    United States and Canadian regulations and in other national
    schedules.  A figure of 50 mg/kg in raw cereals is widely adopted.  At
    present, however, there is no way of determining the source of the
    residue, which could for example also be present in plant-derived
    foods as a result of uptake from soil.


    Methyl bromide continues to be a fumigant of major importance
    world-wide, in stored product insect and mite disinfestation, in the
    control of fungi and to a more limited extent in partial sterilization
    of feeding stuffs such as poultry food.  It is increasingly being used
    as a soil sterilant, both in glasshouses and on open agricultural

    In the soil sterilization role the fumigant is rapidly dispersed, by
    seepage, by volatilization and by decomposition, leaving a residue of
    inorganic bromide.  The amount of residue appears to be greater, as a
    proportion of the dose applied, in soils of higher organic matter
    content.  This soil inorganic bromide is reduced by irrigation or by

    Plants grown in soils containing inorganic bromide from whatever
    source take up bromide; this is particularly concentrated in the leafy
    parts of plants such as lettuce and cabbage but smaller amounts are
    found in fruits, e.g. strawberries, tomatoes.  Residual inorganic
    bromide from plant uptake appears to be the sole cause of concern for
    food residues occurring as a result of using methyl bromide for soil
    sterilization.  In goods in commerce it cannot be specifically related
    to pesticide usage and it must be borne in mind that inorganic bromide
    is taken into the human diet from a number of food additive sources,
    importantly in bread and beer manufacture, and in medication.  Any
    acceptable limits must therefore be set with a view to limitation of
    the total dietary intake of bromide, rather than relating to a
    toxicological assessment of the consequences of pesticide usage in
    isolation.  However, further data on the possibly beneficial effects
    of soil irrigation on bromide ion levels in crops are required. 
    Without post-fumigation leaching it may sometimes be difficult to
    produce leafy vegetables containing bromide ion within legally
    acceptable levels.

    In stored product disinfestation, including animal feeding stuff
    treatments, general methylation of proteins and free amino-acids takes
    place with concurrent formation of an inorganic bromide residue.
    Laboratory electrophoretic studies have indicated changed patterns of

    enzymes and other proteins in fumigated cereals, but no routine
    analytical methods are at present available to measure these reaction
    products as residues.  Inorganic bromide formed can be determined
    specifically and the same considerations apply as to those discussed
    in relation to bromide from plant uptake.  The amounts of reaction
    products and the bromide residues increase roughly in proportion to
    the severity and the number of treatments.

    Whilst small amounts of residual free methyl bromide can be determined
    in fumigated stored products, these generally disappear relatively
    quickly on storage and/or processing.  Although no reports of free
    methyl bromide in goods offered for human consumption have been
    published, the dangerous nature of methyl bromide as an alkylating and
    mutagenic agent indicates that extremely low limits at or about the
    new lower limit of detection should be set.  The meeting therefore
    recommended that the guideline levels for free methyl bromide should
    be reduced as given below.

    From available information it appears that no MRLs can be set for
    bromide ion (inorganic bromide) in foods arising solely from methyl
    bromide fumigation because no distinction can be made from other
    sources of bromide acquisition.  However there is a considerable body
    of evidence that cereals not grown on fumigated soil or subjected to
    fumigation with bromine compounds in storage generally contain less
    than 5 mg/kg bromide ion.  The limit of 50 mg/kg already widely
    adopted therefore indicates that raw cereals have not been subjected
    to excessive treatments with methyl bromide and hence suffered major
    changes in protein/amino acid content.  Consequently the meeting
    concluded that the previously recommended limit for bromide ion in raw
    materials should remain unchanged.  MRLs for bromide ion in lettuce
    and cabbage were adopted as an aid to the reduction of bromide dietary
    intake from all sources.


    Guideline levels are recorded for bromomethane and MRLs recommended
    for bromide ion, as shown below.

    (Guideline Levels): Nuts, peanuts                        10a
                        Cereal grains, cocoa beans            5a
                        Dried fruits                          2a
                        Milled cereal products                1a


    a  To apply at point of entry into a country, and in the case of
    cereal for milling, if product has been freely exposed to air for a
    period of at least 24 hours.

    Bromomethane        Bread and other cooked cereal
    (cont'd)            products, milled cereal products
                        cocoa products, dried fruits,
                        nuts, peanuts                         0.01b

    Bromide ion (MRLs)  Cereal grains, wheat flour
                        (wholemeal)                          50c,d
                        (These MRLs replace "Raw cereals"
                        and "wholemeal flour" at the
                        same level)
                        Lettuce, cabbage                     100d


    Required: (by 1982)

    Further data on levels of bromide ion in crops grown in soil fumigated
    with bromomethane with special attention to the effects of soil
    irrigation on such levels.


    1.  Data on levels of free bromomethane in fumigated commodities and
    processed products, using newly available more sensitive analytical

    2.  Further information on reaction products of bromomethane with
    fumigated commodity constituents.


    Adomako, D.  Final report on bromide residues and methylated compounds
    in fumigated cocoa. IAEA Vienna 1976. ISBN 92-0-111576-8.

    Asante-Poku, S., Aston, W.P. & Schmidlt, D.E.  Site of decomposition
    of methyl bromide in cocoa beans., J.S. Fd. Ag. 25, 285.

    a  To apply at point of entry into a country, and in the case of
    cereal for milling, if product has been freely exposed to air for a
    period of at least 24 hours.

    b  To apply to commodity at point of retail sale or when offered for

    c  Relates exclusively to bromide ion, not to bromine present as
    unchanged fumigant.

    d  Relates to bromide ion from all sources.

    Bridges, R.C.  (The fate of labelled insecticide residues in food
    products III) N-methylation as a result of fumigating wheat with
    methyl bromide., J.S. Fd. Ag. 6, 261.

    Coosemans, J.& Van Assche, C.  Influence of organ differentiation on
    the bromide concentration in tomatoes and strawberries. Med. Fac.
    Frandbouw. Rijksuniv. Gent 42, 1719.

    Coosemans, J.& Van Assche, C.  Factors affecting the bromide
    concentration in green house lettuce after methyl bromide soil
    fumigation.  4th Int. Congr. Pesticide Chem. (IUPAC) 1978.

    El Lakwah.  Investigations on the sorption of methyl bromide by
    various expeller and extracted by-products of oil seeds. Nachricht des
    Deutschen Pflanzens. 30, 1.

    Fairall, R.F. & Scudamore, K.A.  Determination of residual methyl
    bromide in fumigated commodities using derivative gas-liquid
    chromatography. Analyst 105 (In press).

    Fishwick, F.B.  Chemistry Department Report No. 40. Levels and
    significance of dimethyl sulphide resulting from fumigation of certain
    commodities with methyl bromide. Pest infestn. Contr. Lab., U.K. Min.
    Ag. (1978), Unpublished.

    Getzendaner, M.E., Dogy, A.B., McLaughlin, E.L. & Lindgren, D.L.
    Bromide residues from methyl bromide fumigation of food commodities.
    J. Agric. Fd Chem. 16, 265.

    Grove, P.A. and Hogendoorn, E.A., Personal communication.

    Hargreaves, P.A., Wainwright, D.H., Swains, G. and Corcoran, R.J.
    Residual ethylene dibromide and inorganic bromide levels in some fruit
    and vegetables after fumigation with ethylene dibromide or methyl
    bromide.  Austr. J. Exptl. Ag. & Animal Husb., 18, 586.

    Heuser, S.G. and Scudamore, K.A.  Determination of residual methyl
    bromide and ethylene oxide in flour and wheat. Analyst 93, 252.

    Heuser, S.G. and Scudamore, K.A.  Determination of fumigant residues
    in cereals and other foodstuffs: a multi-detection scheme for gas
    chromatography of solvent extracts. J.Sc.Fd. Ag. 20, 566.

    Heuser, S.G. and Scudamore, K.A.  Selective determination of ionized
    bromide and organic bromides in foodstuffs by gas-liquid
    chromatography.  Pestic. Sci. 1, 244.

    Kempton, R.J., & Maw, G.A.  Soil fumigation with methyl bromide:
    bromide accumulation by lettuce plants.  Ann. Appl. Biol. 72, 71.

    Johansson, C.E.  Determination of residues of methyl bromide in oats,
    wheat flour and groundnuts.  Var. Foeda. 28, 126.

    Mapes, D.A. & Shrader, S.A.  Determination of total and inorganic
    bromide residues in fumigated products.  J. Ass. Off. Chem. 40, 189.

    Maw, G.A. & Kempton, R.J.  Methyl bromide as a soil fumigant.  Soils
    and Fertilisers, 36, 41.

    Panel on Fumigant Residues in Grain.  Determination of residues of
    inorganic bromide in grain. Analyst, 101, 386.

    Pest Infestation Control Lab. Report 1977-1979 HMSO London (in press).

    Roughan, J.A.  Personal communication (1979).

    Scudamore, K.A., & Heuser, S.G.  Residual free methyl bromide in
    fumigated commodities Pestic. Sci. 1, 14.

    Singh, G., Rippon, L.E., Gilbert, W.S. & Wild, B.L. Inorganic bromide
    residue in bananas, bell capsicums (sweet peppers) cherries and apples
    following fumigation with ethylene dibromide and methyl bromide.
    Austr. J. Exptl. Agric. and Anim. Husby., 16, 780.

    Stijve, T.  Improved method for the gas chromatographic determination
    of inorganic bromide residues in foodstuffs fumigated with methyl
    bromide. Deutsche Lebensm. Rund. 73, 321.

    Winteringham, F.P.W.  The possible toxicological and nutritional
    significance of fumigating wheat with methyl bromide. J. Sci. Fd
    Agric. 6, 269.

    Winteringham, F.P.W., Harrison, A., Bridges, R.G. and Bridges, P.M.
    The nature of methyl bromide residues in fumigated wheat., J. Sci. Fd
    Agric. 6, 251.

    See Also:
       Toxicological Abbreviations