7 Agrochemical Residues in Food
Most people will tell you that their greatest worries about food areresidues of chemicals from farming (agrochemical residues) thatfood might contain,and genetic modification (see Chapter 9).Thisperception of the risk is likely to be wrong.Toxicity studies suggestthat the levels of agrochemical residues that are usually found infood are far below those that will harm the consumer.There is stillconcern in some scientists minds (including mine) that we dontunderstand fully the effects of long-term exposure to residues, orworse, the long-term exposure to cocktails of residues. Regulatorsmight try to convince us that residues are safe, or have negligiblerisk, but they really dont know because the studies necessary tomake these statements simply have not been carried out. Despitethis, the evidence that we have suggests that the risk is low.
The fact that consumers regard chemical residues as thebiggest problem is a good example of risk perception (see Chap-ter 2). In reality the risk is likely to be much lower than the per-ceived risk. This is certainly the case for exposure to singlechemicals.
What Are Agrochemical Residues?
Agrochemicals are chemicals used in farming, including:
Pesticides herbicides, insecticides, fungicides Veterinary medicines Fertilisers (e.g. nitrate)
When these chemicals are used they are applied to crops andanimals that will eventually become our food. If, for example, afungicide is sprayed onto wheat to control rust (a fungal diseasecharacterised by an orange/brown powdery residue on the plantthat looks like iron rust), the fungicide will first fall onto theplant. Its concentration on the outside of the plant will be veryhigh it would likely be very risky to eat the plant at this stage.As time passes, a proportion of the fungicide might be taken upby the plant, while some might be washed off by rain, and somedegraded by air and UV light. The fungicide remaining in andon the plant is termed the fungicide residue. The same applies toother pesticides, vet medicines and fertiliser components.
Back to the fungicide scenario; if the wheat in our exam-ple is harvested and milled into flour, the fungicide residuesmight then be present in the flour. If they are on the husk of thewheat, they are likely not to contaminate white flour because thehusk is removed, but will contaminate whole-wheat (brown)flour because the husk is incorporated into the flour. If the flouris made into bread, the residues will be in the bread. Then whenyou eat a nice whole-meal bread ham sandwich, you will get adose of the fungicide with your lunch.
The concentration of fungicide in the final product (e.g.bread) will be millions of times less than the concentration onthe wheat immediately after the farmer sprays his crop. The rea-son for this is in part due to the decrease in residue concentra-tion due to degradation (e.g. by UV light effects), dilution (notall of the wheat milled into flour will have residues), and by theeffects of cooking.
Many agrochemicals are chemically unstable, so if theyare heated they break down. Residues in raw foods might de-cline considerably on cooking or processing. In our flour sce-nario the bread is cooked at 200C or more, this is likely to breakdown some agrochemical residues. Work in my own lab hasshown that the organophosphorus pesticide (OP), triazophosdecreases when apples containing residues are cooked. Impor-tant questions are: what are the breakdown products? Are theymore or less toxic than the parent agrochemical residues? Wedont have the answers to these questions for most residues butthere is evidence emerging that some agrochemicals are de-
graded to potentially toxic chemicals (e.g. pyrethroid insecti-cides), while with others, the toxicity is lower. But more of thislater.
Regulations Protect us from Residues
Many of the chemicals that farmers use to grow our food arevery toxic. They are designed to kill fungi, bacteria, nematodes,insects, weeds, and a myriad other organisms that contrive toreduce the farmers productivity and therefore profitability. Be-cause of their toxicity agrochemicals are highly regulated. Theregulations are intended to minimise both farmers and con-sumers exposures and the possible harm that might result.
Standards are set to minimise exposure and make tradein foods with unacceptable residues illegal. These safeguardsare an excellent way of protecting the consumer providing thatfarmers and traders comply with them. Compliance is assuredin most countries by regular surveillance of food to make surethat residues are below the levels set in the standard. Theprocess of approving an agrochemical (e.g. a pesticide) for usein all developed countries involves a consideration of the ac-ceptability of residues that will result in food if the chemical isused properly. To most people these safeguards are adequate butsome people simply do not want agrochemical residues in theirfood whether regulators and scientists say that they posea miniscule (if any) risk or not. These people tend to favourorganic food because agrochemical residues should be lowerthan in conventionally produced food. This, of course, is theirchoice, and their rightbut dont forget natural toxinsandthe increasing evidence that some of them might be present inhigher concentrations in organic food (see later in this Chap-ter).
All developed countries have government-driven processes forapproving agrochemicals for use (i.e. licensing). The systemsare similar, Ill use the UKs pesticide approval system as an ex-ample. The Pesticide Safety Directorate (PSD an agency of thegovernments Department for the Environment, Food and Rur-al Affairs [DEFRA]) is responsible for approving pesticides. Itoperates via an independent advisory committee the Adviso-ry Committee on Pesticides (ACP), which has members takenfrom the great and good of the pesticides world. There are ex-perts in human toxicology, agronomy, risk assessment, etc. Thisis an excellent (I might be biased though I was a member un-til I moved to New Zealand) committee that very seriously con-siders a new pesticide from every angle before making a deci-sion. Most importantly, the Committee is independent and isnot influenced by government or industry. The PSD and DEFRAhave no say in the decision whether or not to approve, theymerely provide the administration to act on the ACPs delibera-tions. Some countries (e.g. New Zealand) dont have an inde-pendent advisory committee, but rely upon a government de-partment to make approval decisions.
The ACP reviews the pesticide on the basis of its efficacyand safety.
Efficacy does it work? Is it useful? BENEFITSafety Is it toxic to consumers, RISK
farmers and the environment.
They make their decision on a risk:benefit basis. It is acknowl-edged that we must accept risk, but that it must be outweighedby the benefit of the (in this case) pesticide. The risk side of theequation requires that the company applying for approval of anew pesticide, new formulation, or new use carry out significantexperimental work in compliance with strict guidelines to as-sess toxicity (e.g. extensive animal toxicity studies, for bothlong- and short-term effects). On the benefit side of the equa-tion is the usefulness of the pesticide, is it better (e.g. less toxic)than currently used pesticides?
If approval is granted, it will come with conditions.For example in order to minimise the consumers exposure to residues there will be a specified period between applica-tion and harvest (based on farm studies submitted as part of the approvals package by the agrochemicals company). Thiswill be measured against standards (e.g. an acceptable residuelevel).
Once a pesticide has been approved for use, the manu-facturer can market it in line with the approved claims andcrop/animal combinations (e.g. it might have approved use onwheat for control of rust). This means that it cannot be used forany other purpose without further regulatory approval. Once itis in use it is important to make certain that the approved use isthe only use, and that the use regimen (e.g. withholding time)is obeyed.
Regulatory Standards MRL
The MRL (Maximum Residue Level) is a regulatory standard; itis the concentration of an agrochemical in food that might re-sult from the proper use of the chemical in agriculture. Properuse is defined in a set of regulations called Good AgriculturalPractice (GAP). The MRL is not a measure of toxicity, but rathera trading standard. Despite this MRLs are scrutinised by toxi-cologists to make sure that if you eat a food containing residuesat or below the MRL you should come to no harm. MRLs areagreed internationally via Codex Alimentarius.
Basically if a food contains a residue below the MRL itcan be sold, and is unlikely to cause the consumer any ill effects.
This is the time taken between application (or dosing if its amedicine) of the agrochemical to reach the MRL.It is set by Reg-ulators as a condition of approval. If it is obeyed it is very un-likely indeed that an MRL will be exceeded, and therefore that aconsumer will suffer ill effects.
Most developed countries carry out extensive surveillanceschemes to police the use of agrochemicals. Continuing our UKpesticide scenario, the Brits have an independent committee(the Pesticide Residues Committee PRC; formerly the Work-ing Party on Pesticide Residues WPPR) that co-ordinates anddeliberates on surveillance for pesticide residues in food. Theycommission work to investigate pesticide levels in foods (e.g.OP residues in bread), and decide whether standards are beingbreached. If so, they attempt to trace back the food to its pr