PESTICIDE RESIDUES IN FOOD - 1979
Sponsored jointly by FAO and WHO
EVALUATIONS 1979
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
CARBON TETRACHLORIDE
Explanation
This fumigant pesticide was evaluated at the Joint Meeting in 1965,
was further considered in 1967 and 1968, and was re-evaluated in 1971.
The report of the 1978 Meeting made reference to problems caused in
member countries of FAO and WHO by the use of certain fumigants and
recommended of carbon tetrachloride.
RESIDUES IN FOOD AND THEIR EVALUATION
The report of the 1971 Meeting contained a statement of general
principles relating to residues of fumigants.
USE PATTERN
Post harvest use
Carbon tetrachloride (CTC) has been used widely as a grain fumigant in
postharvest storage either alone or mixed with other fumigants which
are generally more toxic to insects. In mixtures, in addition to its
moderately insecticidal properties, carbon tetrachloride in useful as
an aid to distribution and fire suppressant for other fumigants.
Compounds with which it has been used include 1,2-dichloroethane
(EDC), 1,2-dibromoethane (EDB), trichloroethylene, acrylonitrile and
carbon disulphide. The liquid fumigant in generally sprayed on to the
surface of the grain and the vapour is distributed by gravity. As
well as its use in bulk grain fumigation in silo and floor storage,
carbon tetrachloride has been used on a small scale in developing
countries for disinfestation of single bags of grain by application of
a small quantity, typically 20 ml, from a gelatin capsule. There is
also a continuing use in small-scale farm bin storage practice where
the handling of more acutely toxic compounds is undesirable.
Concern amongst some authorities about the continued use of certain
chlorinated hydrocarbons because of the possibility of exposure to the
vapour of people engaged in, or working adjacent to fumigation
operations, coupled with the availability of alternative fumigants,
has led to a reduction in the use of liquid grain fumigants containing
carbon tetrachloride over the past few years. In some instances their
use has been abandoned or restricted (e.g. to application by
professional pest control operators). However, for deep grain silo
insect disinfestation, where the installation does not have a fumigant
circulatory facility and the grain cannot be turned to allow mixing in
of pesticide, the liquid fumigants still hold a unique place with
their ability to disinfest the whole bulk. Recent research (Heunser,
1979) has shown that the carbon tetrachloride content of liquid grain
fumigants might be replaced by 1,1,1-trichloroethane, a compound
having similar physical and pesticidal properties but which has shown
a more favourable toxicological pattern in chronic mammalian
exposures.
RESIDUES ARISING FROM SUPERVISED TRIALS
Carbon tetrachloride is usually applied to stored cereals as a liquid
ether alone or in admixture at rates equivalent to 160-1400 mg/kg
calculated on the whole bulk treated. However, the fumigant is
usually not distributed evenly throughout the commodity when applied
to the surface; after a few days the lower parts of the bulk generally
contain the higher residue levels. After the larger physical
movements of vapour within the grain bulk have stabilized, there is a
steady loss of adsorbed vapour during storage, the rate of loss
depending on the temperature, moisture content of grain and conditions
of storage e.g. the air-tightness of the enclosure.
Results from earlier trials on the rate of loss of such residues were
evaluated at the 1971 Meeting. Scudamore and Heuser (1973)
established rates of loss of residual carbon tetrachloride from maize
and wheat fumigated at normal insect control dosage levels, over
periods of up to a year, under constant temperature and air moisture
conditions. They found that maize contained about 50 mg/kg after 6
months free aeration at 10°C but less than 10 mg/kg when aired at
25°C. Wheat fumigated at the same dosage level contained 7 mg/kg
residual carbon tetrachloride after six months aeration at 20°C and 2
mg/kg when held at 25°C. The reduction in residue levels in 7 days
immediately after fumigation varied from 34-55% in maize and from
14-52% in wheat, the greater reduction in each case being recorded at
high aeration temperatures. However, Beilorai and Alumot (1975) in
confirmation of results reported earlier (Alumot and Beilorai, 1969)
gave results of aeration experiments with several liquid fumigants,
including carbon tetrachloride, which indicated a higher rate of loss
of residue at 14°C than at 32°C. The reasons for the differing
findings of these two groups of workers remains unresolved. It is
possible that under the atmospheric conditions prevailing in the
Israeli work, the moisture content of the fumigated cereals changed
during aeration and that this had a greater effect than that of
temperature alone. Since this could be the case under practical
conditions, the temperature during aeration may not be the most
critical factor.
Berck (1974) found carbon tetrachloride residues in wheat stored in a
farm bin after fumigation with a CTC/EDC/EDB mixture at 4 gal/1000 bu
ranging from 7-73 mg/kg one week after fumigation and from 2-30 mg/kg
six weeks later.
In experiments carried out in a model silo, Jagielski et al. (1978)
fumigated wheat and maize at insect control dosage levels and
determined residual carbon tetrachloride levels up to three months
after treatment. These ranged from over 1000 mg/kg at the time of
application, falling to between 250 and 990 mg/kg in maize at 7 days
after fumigation and from 12-62 mg/kg at three months. For wheat the
corresponding figures were 7 days: 140-300 mg/kg and three months:
20-49 mg/kg.
Jagielski et al. (1978) found that a substantial proportion of
residual carbon tetrachloride present in wheat appeared in the milled
fractions, especially in the bran, with about 10% found in white flour
and 20% in wholemeal flour. A maximum of 0.03 mg/kg was found in
bread (white and wholemeal). Berck (1974) reported more substantial
reductions in initial residue content of fumigated wheat on milling;
about 1% remaining in white flour and 5% in bran. Residues in bread
baked from the flour however ranged from 0.01 to 0.13 mg/kg in Berck's
work.
FATE OF RESIDUES
General - Plant Materials
The major route of residue reduction in stored fumigated produce is by
volatilisation. No reaction products of carbon tetrachloride
occurring in plant materials under normal storage conditions have been
substantiated an far as is known, though Larmour and Bergsteinesson
(1938) suggested that deterioration of fumigated stored wheat of 24%
moisture content could be due to hydrolysis of the fumigant with
formation of hydrochlorid acid. However, during steam distillation of
residual carbon tetrachloride from wheat or maize, Malone (1969) and
Scudamore and Heuser (1973) found traces of chloroform in the
distillate which they postulated must have been formed in situ in
the boiling water, since cold organic solvent extraction methods gave
a higher recovery of carbon tetrachloride and no chloroform residue.
In Animals
There is an abundance of published material on changes in liver cells
induced by carbon tetrachloride exposure. Recknagel (1977) has
reviewed probable chemical mechanisms of carbon tetrachloride
toxicity. It is generally postulated that CCl3 free radicals are
formed and ESR studies have been claimed to identify the transient
existence of the trichloromethyl radical in exposed rat livers
(Calligaro et al., 1975; Sancier, 1976).
In Storage and Processing
The persistence of residual carbon tetrachloride in cereal grains
after application under commercial or farm storage conditions depends
on method of storage, air movement, grain moisture content and
temperature during storage. About 5-20% of the applied dose is
initially absorbed and may air off only slowly under cool storage
conditions with little air movement. This residual fumigant desorbs
considerably during milling processes and virtually disappears in the
baking process (see under Supervised trials).
EVIDENCE OF RESIDUES IN FOOD IN COMMERCE
Selective monitoring of cargoes of wheat and other grains imported
into the United Kingdom during 1978-79 showed the large majority of
wheat samples (out of 233) to contain less than 0.1 mg/kg of residual
carbon tetrachloride. 18 samples contained between 0.1 and 0.99
mg/kg, 8 between 1.0 and 9.9 mg/kg, 7 contained between 10 and 49
mg/kg and 1 sample exceeded 50 mg/kg. Residues of a similar order and
frequency distribution were also found in rice, maize and sorghum
(Fishwick and Rutter, 1979). Thus the great majority of samples
contained less than the 1971 JMPR guideline level for raw cereals. Of
66 samples of imported grain analysed in the Netherlands, only 3
contained residues exceeding 1 mg/kg and none exceeded 13 mg/kg
(Admiral et al., 1979).
METHODS OF RESIDUE ANALYSIS
Residual carbon tetrachloride in cereal grains, milled products and
other stored products, can be determined by gas chromatography after
cold extraction (Heuser and Scudamore, 1969). This method has been
collaboratively tested on wheat and maize (Panel on Fumigant Residues
in Grain, 1974) and has a limit of determination of better than 0.001
mg/kg. Grave and Hogendoorn (1979) claim a sensitivity of 0.001 mg/kg
for residual carbon tetrachloride in grain using a head-space
analytical method.
NATIONAL RESIDUE LIMITS REPORTED TO THE MEETING
A number of countries including Australia, Canada and the United
States have hitherto taken the view that no residue will be present in
foods ready for consumption when carbon tetrachloride is applied to
raw commodities and require that residues must not be present
(Australia), or exempt foods from a tolerance (eg USA). An EEC draft
directive in 1976 proposed a limit of 0.1 mg/kg in cereals put into
circulation for human consumption. Dutch tolerances were reported to
the meeting as: cereals, 50 mg/kg, flour 10 mg/kg fruit and
vegetables, nil.
APPRAISAL
Although carbon tetrachloride has been widely used for many years as a
grain and animal feedstuff fumigant on its own or mixed with other
liquid fumigants, it is considered that this use is declining and in
some countries such use is prohibited. In the United Kingdom tests
have shown that 1,1,1-trichloroethane is a satisfactory substitute for
carbon tetrachloride in fumigation practice (Pest Infestation Control
Laboratory, 1978) and has a much more satisfactory toxicological
record, especially in relation to the effects of carbon tetrachloride
on mammalian liver.
Although very little carbon tetrachloride residue from grain
fumigation persists through to prepared food such as bread or
fermented products highly sensitive analytical methods now available
can detect the presence of minute amounts. If bran or wholemeal
products subjected to little or no processing are consumed there is a
possibility of ingestion of larger residues.
In view of the difficulty of assessing the toxicological significance
of even small residues of carbon tetrachloride in the diet, the
Meeting concluded that guideline levels for residues in food as
offered for consumption should be set at lower levels than hitherto
recommended. They recommended a new category of cereal foods not to
be subjected to cooking or baking for which a low guideline residue
level should be set. The meeting further expressed the view that
because the hazards of ingestion of even small amounts of residual
carbon tetrachloride could not be adequately evaluated its use as a
food fumigant should be phased out as and when practicable.
From available information on residue levels occurring in raw
commodities in storage and transit practice, it is recommended that
the levels given below be used as guidelines only.
The following residue levels may occur after currently recognized
fumigation practice.
Cereal grains 50a
Milled cereal products
(to be subjected to baking
or cooking) 10
Milled cereal products intended
for consumption without cooking 0.01b*
Bread and other cooked cereal
products 0.01c*
The meeting concluded that these levels are suitable for use as
guideline levels and should not be exceeded if good fumigation
practice, including adequate aeration, is followed.
a) 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 comply with the guideline level proposed for milled cereal
products intended for consumption without cooking, the cereal grain
must be selected from lots which have not been treated with carbon
tetrachloride.
c) To apply to commodity at point of retail sale or when offered for
consumption.
* At or about the limit of determination
FURTHER WORK OR INFORMATION
Desirable
Development of alternative fumigants for cereal grain disinfestation
for use in circumstances where carbon tetrachloride is presently
employed.
REFERENCES
Admiral, P., de Bruin, A., Cats, H., Dornseiffen, J., Greve, P.A.,
Hogendoorn, E.A. and Mulders, E.J. - Rept. No. 95/79 (1979), Rijts
Instituut voor de Volksgezondheid Bilthoven Netherlands.
Alumot, E. and Beilorai, R. - Residues of fumigant mixture in cereals
fumigated at two different temperatures. J. Agric. Fd. Chem. 17, 869.
Beilorai, R. and Alumot, E. - The temperature effect on fumigant
desorption from cereal grain. J. Agric. Fd. Chem. 23, 426.
Berck, B. - Fumigant residues of carbon tetrachloride, ethylene
dichloride and ethylene dibromide in wheat, flour, bran, middlings and
bread. J. Agric. Fd. Chem. 22, 977.
Calligaro A. et al. - Electron spin resonance study of homolytic
cleavage of carbon tetrachloride in rat liver. Trichloromethyl free
radicals. Chem. Abstr. 83 (15), 12130.
Fishwick, F.B. and Rutter, I.R. - U.K. Survey of pesticide residues in
cereals, 1979. Unpublished report Ministry of Agriculture, Slough Lab.
U.K.
Jagielski, J., Scudamore, K.A. and Heuser, S.G. - Residues of carbon
tetrachloride and ethylene dibromide in cereals and processed foods
after liquid fumigant grain treatment for pest control. Pestic.
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Heuser, S.G. - New fumigation standards for the eighties. Proc. 5th
British Pest Control Assn. Conf. (1979), Paper No. 15, 2
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Pest Infestation Control Laboratory. Development of
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Lab. Report 1974-76, pp.77, HMSO, London.
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