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. Science 9, 117. Heuser, S.G. - New fumigation standards for the eighties. Proc. 5th British Pest Control Assn. Conf. (1979), Paper No. 15, 2 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. Sci. Fd. Agric. 20, 566. Larmour, R.K. and Bergsteineson, H.N. - Effect on wheat quality of long exposures to carbon tetrachloride. Can J. Res. 16c, 241. Malone, B. - Analysis of grains for multiple residues of organic fumigants. J. Ass. Off. Agric. Chem. 52, 800. Panel on Fumigant Residues in Grain. The determination of residues of volatile fumigants in grain. Analyst 99, 570. Pest Infestation Control Laboratory. Development of 1,1,1-trichloroethane as a grain fumigant. Pest Infestation Control Lab. Report 1974-76, pp.77, HMSO, London. Recknagal, R.O. et al. - Chemical mecshisms in carbon tetrachloride toxicity. Free Radicals Biol. 3, 97. Sancier, K.M. - Electron-spin resonance study of homolytic cleavage of carbon tetrachloride in rat liver. Pharmacol. Res. Commun. 8, 429. Scudamore. K.A. and Heuser, S.G. - Determination of carbon tetrachloride in fumigated cereal grains during storage. Pestic. Sci. 4, 1.
See Also: Toxicological Abbreviations Carbon Tetrachloride (EHC 208, 1999) Carbon Tetrachloride (HSG 108, 1998) Carbon tetrachloride (ICSC) Carbon tetrachloride (FAO Meeting Report PL/1965/10/2) Carbon tetrachloride (FAO/PL:1967/M/11/1) Carbon tetrachloride (FAO/PL:1968/M/9/1) Carbon tetrachloride (WHO Pesticide Residues Series 1) Carbon Tetrachloride (IARC Summary & Evaluation, Volume 71, 1999)