Maximum residue limits (MRLs) are statutory limits set on individual active ingredient and foodstuff combinations. They are based on residue levels which result when the pesticide is used according to the instructions on the label and in accordance with good agricultural practice (GAP). MRLs may be used to ensure that the pesticides are only being used in accordance with GAP. Many countries have codes of good operating practice with training for farmers and operators to ensure that pesticides are used at optimal levels. Some countries rely on the Codex Alimentarius Committee on Pesticide Residues to establish MRLs, while others set their own. (Codex Alimentarius is an international body which has over 120 countries as members and their standards are increasingly being accepted as the basis of world trade in foodstuffs.)
In the USA the FDA used to set tolerances for pesticide/foodstuff combinations but under the 1996 Act it sets a level for each pesticide in all foods based on the principle of a reasonable certainty of no harm. This is defined as a lifetime cancer risk of less than 1 in a million. There is also a requirement that residue tolerances must be specifically determined as being safe for children.
Within the EU, individual member states have historically set their own MRLs which differ from state to state. Directive 76/895 established a common MRL setting regime and a series of subsequent directives has fixed the levels for a series of pesticides in fruit, vegetables, cereal products, and products of animal origin. There is an ongoing program to harmonize the levels throughout the Union.
Most industrialized countries have pesticide surveillance programs which cover both home-produced and imported commodities and these report annually. The EU has an annual specific coordinated program to check compliance in nominated combinations of pesticide and foodstuff. MRLs require sophisticated equipment for their determination because the levels are so low and the minimum detectable limit depends on the foodstuff. For example the tolerance for aldrin and dieldrin (two orga-nochlorines) in the USA is between 0.05 and 0.1mgkg_1 (parts per million), depending on the foodstuff. There are over 600 different active ingredients available commercially. Because there are so many, laboratories around the world have developed sophisticated rapid analytical techniques to allow them to screen pesticides by class so that retailers, food manufacturers, and governments can carry out analyses as a matter of routine.
The MAFF 7th Report of the UK Working Party on Pesticide Residues in 1996 showed 68% of samples had no detectable residue, 31% had residues below the MRL, and <1% were over the MRL. Similar results were obtained by the FDA who report results with relation to the tolerance to the pesticide/commodity combination. In 1995, of over 9000 samples analyzed, 64% had no detectable residues, 34% had residues below the tolerance, <1% had residues over the tolerance and <1% had residues for which there is no tolerance in that particular pesticide/commodity combination.
In all cases where MRLs or tolerances are exceeded follow-up action is taken. For home-produced materials, this involves investigation of the grower and prosecution if necessary. For imports, exceeding the level causes the consignment to be refused entry.
Maximum levels of pesticides are also set for drinking water. Pesticides get into water from spraying, runoff, percolation or from treatment of fish in aquaculture. Good practice is increasingly being developed to minimize the levels in raw water and treatment works are developing systems to reduce incoming levels to levels acceptable for drinking water.
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