Dioxins in pesticides

Presence of dioxins in pesticides in general Woolson et al. (1972) examined 129 samples of 17 different pesticides derived from chlorophenols for polychlorinated dibenzo-p-dioxins. The tetra derivatives were found primarily in 2,4,5-T samples. Twenty of 42 samples contained more than 0.5 ppm. The presence of TCDD in some pesticides is easy to understand due to the chemical reactions used for its synthesis. The herbicide 2,4,5-T is produced by condensing 2,4,5-trichlorophenol with chlo-roacetic acid in the presence of NaOH at 105°C to yield the sodium salt of 2,4,5-T. Trichlorophenol may be made by the action of alkali on 1,2,4,5-tet-rachlorobenzene produced by chlorination of the trichlorobenzenes resulting from dehydrochlorination of benzene hexachloride (BHC). Today most countries will not approve a pesticide without data on dioxin content. 

The biological activity of several halogenated herbicides in water is destroyed by ultraviolet irradiation (18). Irradiation seems to be a promising method for decontaminating small quantities of pesticides. The chemical similarity between the chlorinated dioxins and other chlo-rinted aromatic compounds suggested that if there were parallels in their photochemical behavior, sunlight might destroy dioxins in the environment. 

The detection of a potent dioxin impurity in a major herbicide has focused attention on the nature of chlorinated impurities in pesticides, and in a larger sense, impurities in all chlorinated industrial compounds used extensively in man s environment. The present 2,4,5-T controversy is overshadowed by the dioxin problem. Major disagreement still exists on their relative contributions to the teratogenic effects observed in chicks and the validity of interpretation of high dosage rates used to achieve these effects. We have avoided any assessment of the health-related aspects of dioxins but have dealt almost exclusively with dioxins as an environmental entity. 

Miellet [80] and Lopez-Avila et al. [81] have reviewed the applications of Soxhlet extraction to the determination of pesticides in soil. This technique has been applied extensively to the extraction of polycyclic aromatic hydrocarbons, volatile organic compounds, pesticides, herbicides and polychlorodibenzo-p-dioxins in soils. Details of the extraction procedures and the analytical finish employed are reviewed in Table 1.1. 

Dioxins (PCDDs) occur as contaminants in many agricultural pesticides and can occur in the environment as a result of pesticide usage, although many other industrial sources and natural sources have been identified. The National Dioxins Program (2004) has revealed that major sources of dioxins in the environment are uncontrolled combustion sources such as bush fires and accidental fires which contribute 70% of the total to the air and 80% to the soil whereas waste disposal and land filling contribute 75% of the total to water. 

Assessment of the risk related to the presence of dangerous chemicals in the environment is both important and difficult. However, accurate assessment of the impact of a chemical is a complex task, for there are many factors involved, and chemical substances are in many cases estimated with a large uncertainty. Further, for most of the currently used industrial chemicals there is no knowledge of their environmental and ecotoxicological properties, and even worse, some chemicals are not commercialized but are present in the environment nevertheless as degradation products or impurities of industrial substances. The more widely studied chemicals are well-known contaminants such as dioxin, and pesticides. Pesticides have been addressed within specific regulations (e.g., European Directive 91/414), so this makes them a good example on how to address the problem with defined protocols. 

In other cases, stationary phases have been tailored to achieve specific separations. In one case, a new stationary phase was designed to achieve the separation of a particular mixture of volatile priority water pollutants whose separation has posed a real problem (6, 7). A serendipitous finding was that this new stationary phase, DB-1301, also promises to be very useful for the separation of some chlorinated pesticides. Nor have we reached the end of this road an Immobilized form of 2330, with utility for those Interested In dioxins, In positional Isomers of the fatty acids, and In other challenging separations, will soon be available. Fused silica columns with bonded particulate materials, reminiscent of the old PLOT-type columns, are also available. The primary utility of the PLOT-type columns currently available Is for fixed gas analysis, but newer types on the horizon will permit a choice of adsorptive-type separations, partition-type separations, or a combination of both. 

One class of compounds does deserve special attention—those chemicals that are very fat soluble. These include the older chlorinated pesticides, such as DDT, and some environmental contaminants, such as the PCBs (the polychlorinated biphenyls) and dioxins. In these cases, low levels of intact chemicals, if absorbed, may encounter the body s final defense mechanism. This defense is to store the compounds in fat and milk, a paradoxical strategy that the body uses to sequester away chemicals into a storage site (or depot) where they can do no harm to the rest of the body, ft s conceptually the prison system of the body. The absorption and distribution into fat greatly reduces the concentration of chemicals at other body sites and thus serves to blunt the impact of the exposure. Similarly, this simple method of diluting the absorbed chemical often keeps the concentration below an effect level and gives the overworked liver time to try to destroy them. 

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