Occurrence, sources and toxicological significance of pesticide residues in food grains

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D. Halliday

Natural Resources
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Potential hazards to consumers from contamination of food with pesticide residues is currently a major public concern in developed countries. This has led to pressure on governments to tighten legislation covering use of pesticides and increase the requirements for research data before particular uses for individual compounds can be allowed. This, in turn, has compelled manufacturers of pesticides to expand their testing programs for new products to meet the increased requirements for data from national registration authorities. The cost of introducing a new pesticide to the market has now risen to around US$100 million, because of these and other factors.

The impact of this ever-increasing demand for expanded testing programs on pesticide development costs, has sometimes led manufacturers to query the need for all the data now required. This is a valid objection because ultimately the consumer has to meet the costs involved, through higher prices for food. If these data are not really necessary then this represents money being wasted! In fact, surveys of pesticide contamination of food in Western Europe, USA, Canada, Australia and New Zealand have revealed only low levels - well within the requirements of public safety (GIFAP 1984). This indicates that if pesticides are used properly, in accordance with 'Good Agricultural Practice', the consumer is not at risk. Unfortunately, this is not always the case in developing countries where use of pesticides is frequently poorly regulated - if at all.

Against this background it appears inevitable that the public will continue to view the presence of pesticide residues in food as a problem whatever the scientific facts. Food grains are of particular importance in this regard because they are consumed as major elements of national diets.

Potential sources and nature of pesticide residues In food grains

There are three potential sources of pesticide residues in food grains, arising from:

Large quantities of herbicides, fungicides and insecticides are routinely applied to grain crops in more developed countries. However, provided they are used so that there is an adequate period between last application and harvest, it is unlikely that significant pesticide residues will be found in the crop. Pesticide registration authorities are very much aware of the need to ensure that excessive residues from preharvest application do not arise. For this, they require appropriate experimental data indicating the minimum period that can be allowed between last application and harvest - the withholding period. Pesticides are used much less to protect growing food grains in less developed countries, but the same principles apply!

Residues arising from environmental contamination are mostly due to widespread application of highly stable organochlorine insecticides, over some three decades prior to the 1980s to control migratory and agricultural pests, and insect vectors. Their use has now largely been phased out in developed countries and residues in the environment (e.g. soil and inland waters) are gradually declining, with consequent reductions in residues detected in food grains. However, organochlorines such as DDT and benzene hexachloride (HCH) are still used in large quantities in countries such as India and China, maintaining or possibly increasing the potential for environmental contamination of grains and other food crops. While it is unlikely that residues from this source are likely to pose a serious hazard to consumers of food grains they could cause problems for exports to developed countries where stringent maximum residue limits for organochlorines are imposed.

The most likely source of significant levels of pesticide residues in food grains is postharvest application of contact insecticides and fumigants. Contact insecticides are applied at levels designed to protect the commodity from attack over as much of the storage period as possible, taking into regard the need to ensure that initial residues do not exceed the limit regarded as being the maximum for consumer safety. As the insecticide is gradually lost due to breakdown or volatilisation the residue remaining on the food grain at the end of the storage period is always substantially less than that originally applied. Clearly, only insecticides of low mammalian toxicity should be applied to food commodities during storage, so the list of compounds approved for this purpose is limited. Additionally, as the market for grain protectants is very small compared with the total market for insecticides, use is invariably made of compounds developed for general application that are later also found to be suitable as grain protectants. No insecticide has ever been developed specifically to protect stored grains!

Fumigants differ from contact insecticides because they come into contact with insects in the gaseous, rather than the solid or liquid phase. While they are sorbed onto commodities during the course of fumigations, most of the sorbed fumigant is lost during subsequent aeration, unless there is some form of chemical reaction with components of the commodity. This is just as well, as most fumigants currently used to treat food grains have a high mammalian toxicity. Of the two fumigants currently mostly used to treat food grains, only methyl bromide reacts chemically with the commodity. This reaction results in fixed residues of inorganic bromide while residues of sorbed unreacted methyl bromide remain low, as do residues of phosphine, the other fumigant that is widely used.

Residues arising from postharvest treatment of food grains

Most significant pesticide residues detected in food grains arise from contact insecticides or fumigants, deliberately applied to protect the grain from postharvest insect attack. The range of compounds used for this purpose is not large: contact insecticides currently used are mostly either organophosphates of low acute mammalian toxicity, such as malathion, pirimiphos-methyl, fenitrothion and chlorpyrifosmethyl, or pyrethroids such as permethrin, deltamethrin and (in Australia) bioresmethrin. Organo-phosphates have been found to be effective against most of the beetles that commonly infest stored food grains except Rhyzopertha dominica and Prostephanus truncatus, for which treatment with pyrethroids is preferred. Treatment with a mixture of a pyrethroid and an organophosphate provides an effective way of protecting food grains against the complete range of insect pests.

Residues arising from fumigation are generally negligible when phosphine is used but can reach quite high levels in commodities that have been repeatedly fumigated with methyl bromide.

Contact insecticides may be applied to grains as sprays, mists, fogs or dusts. Spraying, misting or fogging of stores containing bag-stacked commodities is commonly used as a means of holding insect populations down to sub economic levels, or to minimise re-infestation after fumigation. Residues arising from such treatments generally have low overall significance and are confined to the peripheral layers of stacks. Significant residues are likely to be found only when insecticides are admixed with bulk grain or as they are loaded into silos, or farmers' stores. Application rates used for such treatments are determined by laboratory experiments to establish the minimum dose needed to control the target pests. The rate at which the insecticide subsequently breaks down is established and a figure is then set for the maximum residue of a particular compound that is likely to be found when the commodity is given the recommended treatment. This figure, called the maximum residue limit (MRL), is used by regulatory authorities as a guide as to whether the treatment has been properly carried out in accordance with good agricultural practice.

Most developed countries have established maximum residue limits for food grains and their processed products, in statutory schemes that control marketing and use of pesticides. In addition, the Codex Alimentarius Commission of the FAO recommends maximum residue limits for commodities which move in international trade, in the hope that these MRLs will be accepted by national governments. Less developed countries which do not currently possess facilities to develop their own maximum residue limits often use those set by the Codex Commission as a basis for their own regulations. Current Codex recommendations for maximum residue limits of the more important grain protectants and fumigants on cereals and cereal products are given in Table 1. It should be noted that these include some proposals still to be adopted as full Codex maximum residue (FAO 1989).

Toxicological evaluation of pesticides

Pesticides must be subjected to two types of toxicological evaluation before they can be registered for particular uses. The first is a determination of acute toxicity to mammals by means of laboratory feeding trials. In these, single doses of the pesticide are fed to groups of animals and the dose required to kill 50% of the group is determined (as mg of pesticide per kg body weight). This is the LD50, which is used as an indication of likely acute toxicity to man. Pesticides with an LD50 less than 50 mg per kg may be regarded as highly toxic; those between 50 and 500 mg per kg as moderately toxic; and those with an LD50 of more than 500 slightly toxic. Table 2 gives the LD50 values for insecticides commonly used to protect food grains (British Crop Protection Council 1987). Most may be regarded as only slightly acutely toxic to mammals while none is highly toxic.

There has been criticism of the continued use of the LD50 value as an index of acute toxicity as it depends on the experimental conditions and the way in which test material is fed to test animals (GIFAP 1988a). These uncertainties are reflected in the spread of values reported for some pesticides in Table 2. However, the LD50 continues to be widely used. Fumigants, which as respiratory poisons are not covered by the concept of the LD50, are always highly acutely toxic to all forms of animal life.

Except in very rare cases of accidental contamination of foods, possible chronic toxicity through ingestion of small quantities of pesticide over a long period is generally regarded as the major potential hazard to consumers of food grains treated with grain protectants insecticides. Assessment of potential chronic toxicity requires major investment in long-term studies with laboratory animals aimed especially at identifying possible carcinogenic, mutagenic or teratogenic potential. Such studies also may identify neurotoxic or reproductive effects. The amount of pesticide ingested daily over a long period which has no significant toxicological effect on test animals, is determined as the 'No Observed Adverse Effect Level' (NOAEL). The NOAEL is then used as a basis to calculate the acceptable daily intake (ADI) which is the amount of pesticide (as mg/kg body weight) that a healthy adult can consume daily over his, or her, complete life-span without any adverse effect to health. The ADI is normally calculated by applying a safety factor of 100 to the NOAEL of the most sensitive species of animal used in the testing program. It is sometimes possible to use information obtained on the direct effect of pesticides on humans when calculating the ADI but, in general, it is based on the extrapolation of data from laboratory animals. Table 3 gives ADls for insecticides commonly used to protect stored food grain (FAO 1989).

Table 1 Codex maximum residues limits for insecticidal protectants and fumigants in food grains and their processed products

Table 2 Acute mammalian toxicity of contact insecticides used as protectants for food grains

Oral LD50 (mg/kg body weight)

  Rats Mice Rabbits Guinea pigs Dogs
Bloresmethrin 7070-8000a - - - -
Bromophos 3750-8000 3311-5900 720 - -
Carbaryl 850b - - - -
Chlorgyrifos- 1630-2140 - 2000 2250 -
Deltamethrin 135-5000 - - - -
Dichlorvos 56-108 - - - >300
Etrimfos 1800b - - - -
Fenitrothion 800a - - - -
Fenvalerate 451 - - - -
Lindane 88-270 59-246 - - -
Malathion 2800 - - - -
Methacrifos 678 - - - -
Methoprene >34600 - - - 5000-10000
Permethrin 430-4000 540-2690 - - -
Pirimiphos-methyl 2050a 1180 1150-2300 1000-2000 -
Pyrethrins 584-900 - - - -
Piperonyl butoxide 7500 - 7500 - -

a Female;
b male

Toxicological significance of maximum residue limits In relation to acceptable dally Intakes

Full Codex maximum residue limits are never assigned unless it has been possible to calculate an ADI for the pesticide concerned. However, the basis for estimating MRLs is unrelated to the toxicity of the pesticide. This has at times caused considerable problems for national governments which fear that acceptance of a Codex MRL may result in daily ingestion of residues at levels approaching or even exceeding the ADI. This was expressed in relation to residues on staple items of national diets such as food grains (and their processed products) and was based on the calculation of theoretical maximum daily intakes (TMDI) by multiplying maximum residue limits by estimated daily consumption of the commodity and its products.

This problem was reviewed by a joint FAD/WHO Consultation which developed guidelines for predicting dietary intake of pesticide residues arising from treatment of produce in accordance with good agricultural practice (WHO 1988). The consultation pointed out that it was inappropriate to predict daily intake of pesticides on the basis of the TMDI, and it was necessary to take into account a range of factors that diminish residues at the point of consumption.

Table 3 Acceptable Daily Intakes (ADis) of contact Insecticides used to protect food grains

Insecticide mg/kg body weight
Bloresmethrin -*
Bromophos 0.04
Carbaryl 0.01
Chlorpyrifos - methyl 0.01
Deltamethrin 0.01
Dichlorvos 0.004
Etrimfos 0.003
Fenitrothion 0.003
Fenvelerate 0.02
Lindane 0.01
Malathion 0.02
Methacrifos 0.0003
Methoprene 0.1
Permethrin 0.05
Pirimiphos-methyl 0.01
Pyrethrins 0.04
Piperonyl butoxide 0.03

* No ADl currently assigned.

These include the following.

1. Only a proportion of daily consumption will be derived from treated grain
2. Only a small proportion of treated grain is likely to contain pesticide residues at the MRL
3. A proportion of the residue at time of application will dissipate during storage, transport, preparation, commercial processing, and cooking
4. Some of the residue may be discarded with inedible portions.

The consultation has suggested that an estimated maximum daily intake (EMDI) be calculated which takes factors 3 and 4 into account and provides a more realistic assessment of potential hazards to consumers. This may be further refined into an estimated daily intake (EDI) which also takes account of factors 1 and 2 together with other factors affecting the amount of residue which may be consumed. Calculation of the EDI is only likely to be possible for countries possessing an adequate data base on the following factors:

It has been suggested that the TMDI be calculated in the first instance and that the EMDI should be calculated only if the TMDI is found to exceed the ADI. The much more onerous task of assessing the EDI would only be undertaken if the EMDI were also found to exceed the ADI.

So far, all these assessments of potential hazards to consumers have been for single pesticides. Concern has also been expressed about possible additive or interactive affects of pesticide combinations. This is of particular importance with regard to grain protectants, as combinations of pyrethroids and organophosphates are being used increasingly to provide protection for food grains. If data were to be required on the long-term toxicity of such combinations it could cause problems for registrants, though there are suggestions that such data may be unnecessary (GIFAP 1988b).


British Crop Protection Council (1987). The Pesticide Manual, 8th Edition.

FAO (1989) Guide to Codes Maximum Limits for Pesticide Residues, Part 2.

GIFAP (1984) Pesticide Residues in Foods; Is there a Real Problem?

GIFAP (1988a) Position Paper on Acute Toxicity Tests.

GIFAP (1988b) Position Paper on Toxicology of Crop Protection Products in Combination.

WHO (1988) Guidelines for Predicting Dietary Intake of Pesticide Residues.

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