Sampling for safety

Work by the UK Pesticides Safety Directorate (PSD) has shown that there can be unexpectedly wide variations in residue levels in fruit. PSD held an international conference at York in December 1998 to consider these implications for residue analysis and for health(1).

Residue variability
In 1996, an average Cox's desert apple tree would have been sprayed 16 times with 36 different pesticide active ingredients(2).  However professionally sprays are applied, apples near the inside of the tree are unlikely to receive as much spray as apples nearer the outside of the tree. PSD found that the greatest amount of residue found in an individual apple, compared to the mean average for the batch could vary by a factor of 13: in other words, a 'hot' apple could contain 13 times as much pesticide as an ordinary apple.
    Carrots in 1995 were sprayed with an average of four insecticides, three herbicides and two fungicides(3). PSD found that a 'hot' carrot could contain 29 times as much pesticide as an 'ordinary' carrot.
    This work has also been repeated with other fruit and vegetables, including peaches, bananas, oranges, kiwifruit, pears, plums, nectarines, parsnips and tomatoes - these are all consumed in the diet fresh, unprocessed and as single items as opposed to foods processed, stored and sold in bulk, such as cereals or juices.
    It has always been known that the amount of residue uptake per individual fruit or vegetable would vary. What has been surprising is the amount of the variation, and in spite of the research, no one yet knows why particular units should receive or take up so much more pesticide than others.

Health implications of hot items
Apples are healthy foods. The risk of adverse health effects from eating pesticides in food is considered on the basis of how much of a particular pesticide can be eaten every day for a lifetime with no adverse effect-the ADI (the amount of chemical that can be consumed  every day for an individual's entire lifetime in the practical certainty, on the basis of all known facts, that no harm will result).  However, it is possible that eating a 'hot' fruit with a much higher level of residues may cause a short-term, acute adverse effect. Not all pesticides leave residues in food, but of those that do, organophosphate (OP) and carbamate insecticides are likely to be of most concern, as they have the potential to cause acute toxic effects. For a number of pesticides, therefore, there needs to be urgent consideration of the possibility of short-term toxic effects and a calculation of the acute reference dose (Arfd) or safe daily dose.
    The Department of Health considers the effects of OPs and carbamates at residue doses are likely to be local on the gut and produce colicky pain and diarrhoea; but also notes that impacts on the developing brain may give cause for concern, together with the additive effects of pesticides with similar modes of action. Vulnerable groups are likely to be children, infants, pregnant mothers and nursing women and the elderly.

Regulating hot food
The idea of monitoring pesticide residues in food is to ensure that pesticides are properly applied according to GAP and that only legally permitted pesticides are used. But the fact that individual residues may vary greatly highlights another issue-compliance with MRLs can only be tested satisfactorily when the entire lot that is analysed has been subject to the same pesticide treatment. And this means that  monitoring and analysis protocols can only really guarantee to show what pesticides were applied to the crop if the analysis takes place in the field, at the point of harvest.
    If sampling takes place later in the trading chain, when two or more 'lots' are mixed-when the produce from two growers is combined at the co-operative, or at the wholesalers warehouse-current sampling protocols are not sensitive enough to give a correct picture of what pesticides have been used on the separate groups of produce. Different growers may use different pesticide regimes depending on their expertise and local pests; the same grower may use different treatments on different fields of produce. An unscrupulous supplier may 'hide' produce containing residues of an illegal pesticide, or high residues, by mixing it in a more compliant batch.
    Alan Hill of the UK Central Science Laboratory made a strong plea at the York Conference for 'cheap, rapid and minimally disruptive sampling techniques for use in the field, coupled with equally cheap rapid and small scale analytical testing on-site.'

The future of residues
The issue of residue variation makes it even more difficult to maintain pesticide residues at safe levels in the food supply. Not only do sampling procedures need to be improved, but additional complications are introduced-what is the likelihood of eating a hot apple, or a hot apple followed by another hot apple? Regulators are now bracing themselves for the expense and complexity of sampling population diets in great detail to enable them to construct probability models of consumption.
    In the 1980s the EU developed the Drinking Water Directive, now seen by some commentators as far-sighted. Regulators in effect decided that levels of pesticide residue in water were too difficult to predict individually, and on the precautionary principle decided that any pesticide in drinking water should be limited to 0.1mg/litre and that drinking water should not contain more than 0.5 mg/l in total of all pesticides.
    The European Commission has decided to submit stringent draft rules on pesticide residues in baby food (see PN42 p.17). In future, baby food will be allowed to contain not more than 0.01 mg of pesticide residues per kg. Is it becoming too difficult to regulate pesticides on an end-of-pipe basis? This goes to the root of the Codex system of proposing 'safe' levels for most pesticides in most foods: the calculations are too complicated and expensive. Instead, a flat maximum level of pesticides in food may be the future.  (PB)

References
1. Pesticide Residues Variability and Acute Dietary Risk Assessment. Conference Proceedings, Pesticides Safety Directorate, York, 1998. 
2. Orchards and Fruit Stores in Great Britain 1996, Pesticide Usage Survey Report 142, MAFF Publications, London, 1998. 
3. Outdoor Vegetable Crops in Great Britain 1995, Pesticide Usage Survey Report 134, MAFF Publications, London, 1997.

[This article first appeared in Pesticides News No. 43, March 1999, page 7]