A pesticide is generally a chemical that is used to control or eradicate a “pest”. In this context a pest can be an insect (insecticide), a weed (herbicide), a fungus (fungicide), a nematode (nematocide) or a rodent (rodenticide). Pesticides are widely used in food production and in agriculture.
The main exposure of humans to pesticides occur with administration and the consumption of contaminated food or water. The effect of pesticides on human health is extremely varied, and depends on the biochemical mode of action of the pesticide and whether it accumulates in body tissues. The maximum level of exposure allowed for different pesticides is expressed as the Maximum Residue Limit (MRL), the level above which toxic effects may be observed. Exceeding this limit in food or drinking water is generally a trigger for specific actions by the relevant authorities, such as public announcements and the commencement of counter measures. The LD50 of a pesticide is the dose that is lethal to 50% of a test population. The lower the LD50, the more toxic the pesticide. Information on the toxicity of individual pesticides can be obtained from Inchem or the National Pesticide Information Center. Note that several pesticides are also endocrine disruptors.
The World Health Organisation (WHO) has ongoing programmes to identify and assess the toxicity of pesticides, and releases tables annually on recommended MRLs. Different countries develop their own MRL guidelines, often based on that of the WHO. In the US the Environmental Protection Agency (EPA) sets such limits. In South Africa pesticide levels in food is regulated by the Foodstuffs, Cosmetics and Disinfectants Act, 1972 (Act No. 54 Of 1972) Regulations R1542 of 2002, R722 of 2005 and R1047 of 2006. Compliance with prescribed levels is critical in the import and export of agricultural products. A list of MRLs of pesticides can also be downloaded from the South African Department of Health.
Water pollution by pesticides is a major concern. Pesticides generally enter river systems from farmland run-off after agricultural application and contamination is therefore highly seasonal. The pesticide does not simply impact on the use of river water for drinking purposes and for recreation, it can also reduce biodiversity or accumulate in aquatic organisms. Following introduction into rivers, the pesticide may, apart from entering the food-chain, also enter dams, reservoirs and urban drinking water. Depending on the permeability of sediments and the resistance of the pesticide to natural breakdown, it can also enter the ground water. DTT, for instance, has been found in groundwater in the US even though its use was discontinued in the 1970s. Both the EPA and the WHO have published strict guidelines for pesticide levels in drinking water. Although South Africa has enforceable regulations regarding the allowed levels of pesticides in food and in bottled water, legislation addressing pesticides levels in drinking water is currently lacking.
Pesticides residues have also been detected in wine, due to its use in the cultivation of grapes. The WHO has recommended acceptable levels for pesticides in wines. In South Africa the MRLs of pesticides in wine are identical to that of grapes, and is goverened by the Foodstuffs, Cosmetics and Disinfectants Act, 1972 (Act No. 54 Of 1972).
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