Pesticides, classes chlorinated hydrocarbon

The most important insecticides can be grouped in three structure classes—chlorinated hydrocarbons, organophosphorus compounds, and carbamates. Chlorinated hydrocarbons such as DDT are referred to as persistent pesticides because they persist in the environment for years after their use. 

The relative immobility of the chlorodioxins is expected, based on the very low solubility of these compounds in water (0.6 / g/liter). In contrast, the herbicide, 2,4,5-T, is relatively mobile in sandy soils, but movement decreases as soil organic matter increases. What does this information tell us, and how does it compare with other organic compounds A mobility scale has been devised for a large number of pesticides (3). Higher mobility numbers reflect increased compound mobility in soils. The dioxins would be in Class 1—i.e., they are immobile in soils and would compare with several chlorinated hydrocarbon insecticides. 

Smith, A.G., 1991. Chlorinated hydrocarbon insecticides. In Hayes Jr., W.J., Laws Jr., E.R. (Eds.), Handbook of Pesticide Toxicology Volume 2, Classes of Pesticides. Academic press, Inc, Toronto, pp. 731-916. 

These findings are highly significant. The fact that domestic usage of the chlorinated hydrocarbon insecticides has reached a peak and is on the decline can only mean that the maximum amounts of this class of pesticide chemicals in the environment have already been reached and can only decline in the future. It stands to reason, therefore, that the residue levels of these compounds have reached a peak in the fatty tissues of humans, wildlife, and domestic animals that have had access to residues of these pesticide chemicals in the environment and are probably declining. From the standpoint of public health, there is not one shred of evidence that the traces of these compounds in the body fat have any detrimental effect. Of course, it cannot be stated absolutely that some effect will not be discovered in the future. However, by the same token, it cannot be stated absolutely that this effect will not be beneficial ... 

Because of the prowess of the analytical chemist, it is now possible to find residues of the chlorinated hydrocarbon insecticides in the environment at concentrations of a few parts per trillion. He is not so adept at finding nanogram quantities of some of the other classes of compounds for many pesticides, the metabolites are not identified, so he doesn t know what to look for. There is little question, however, that many of the compounds that now escape detection are present in the environment, and it is only a matter of time until the analytical chemist finds them. 

The report by Haas and Guardia (18) pertains to their efforts to apply immunological methods for the assay of pesticide residues, and expresses their aim to test the suitability of methods for field analytical purposes. Haas and Guardia also used DDT and malathion to represent two of the most important classes of insecticides, chlorinated hydrocarbons and organophosphorus compounds. They first attempted to prepare insecticide-protein antigens in which enzymes were used as the protein carrier. Antiserum of rabbits injected with DDA-carbonic anhydrase or malathion-chymotrypsin failed to show the presence of the respective antibodies. 

DDT is not the only chlorinated hydrocarbon used as a pesticide. Other compounds in this class include dieldrin, aldrin, heptachlor, and chlordan. The use of these compounds has also been banned or restricted in the United States. The U.S. government has decided the harm they cause to the environment is more important than the benefits they provide to farmers and other users. 

Indeed, the promise of IA has led to workable enzyme-linked immunosorbent assays (ELISAs) for a variety of pesticides, including thiolcarbamates, triazoles and triazines, substituted ureas and sulfonylureas, bipyridilium compounds, and other groups of chemicals(2) (Table I). Additionally, radioimmunoassay (RIA) methods exist for chlorinated hydrocarbons, organophosphates, phenoxy adds and other chemical classes. 

Smith A.G. (1991) Chlorinated hydrocarbon insecticides. In The Handbook of Pesticide Toxicology Vol. 2 Classes of Pesticides. San Diego Academic Press. 

Of the several classes of synthetic insecticides, the chlorinated hydrocarbon (CH) and organophosphate (OP) insecticides have the greatest psychiatric significance [Pesticides and Neurological Diseases 1982 Ecobichon 1996). The CH insecticides, also called organochlorine insecticides, include three chemical classes di-chlorodiphenyltrichloroethane (DDT), cyclodienes (aldrin, dieldrin, heptachlor, chlordane, endosulfan), and chlorinated benzene and cyclohexanes (lindane) (Ecobichon 1996). Their ban in the United States and Europe resulted from their high chemical stability and lipid solubility that allowed environmental persistence and magnification in the food chain (Ecobichon 1996 Kaloyanova and El Batawi 1991). 

Biotransformation of certain chlorinated hydrocarbon insecticides results in their conversion to metabolites which are less polar than the parent chemical. Heptachlor and aldrin are converted to the more lipophilic compounds heptachlor epoxide and dieldrin, respectively, whereas DDT is converted to DDE. The primary residue of DDT, which persists to the present day in animals and humans after exposure over a decade ago, is DDE. Following biotransformation, these compounds distribute to tissues which are higher in neutral lipid content than are the major organs of metabolism and excretion, the liver and kidney. These lipid-rich tissues are relatively, deficient in the so-called mixed-function oxidase (MFO) enzyme systems necessary for biotransformation of the halogenated hydrocarbons to more polar and thus more easily excreted compounds. As a result, these lipophilic chemicals remain unchanged in adipose tissue with only limited amounts returning to the circulation for possible metabolism and excretion. Paradoxically, aldrin and heptachlor metabolism results in an increased rather than reduced body load. This is opposite of the pattern seen for most other pesticide classes. 

The classic prototype chlorinated hydrocarbon pesticide is DDT (dichloro-diphenyltrichloroethane), a chemical which earned its discoverer, Dr. Paul Muller, a Nobel Prize in 1948. This honor was an event that partly reflected the tremendous beneficial impact that this class of compounds had on disease prevention and food production. Although this compound is not used any longer in the United States (see Chapter 3), it is still widely used throughout the world where the risk/benefit ratio favors continued pesticide use. This is because of its efficacy in agriculture and its ability to kill insect vectors of important human diseases such as malaria. It is thus monitored in the FDA food surveillance programs. 

These classes of pesticides are among the most widely used and studied insecticides. They are defined based on their mechanism of activity, which is to inhibit the cholinesterase enzyme found in nerve junctions (synapses). Their major advantage over the chlorinated hydrocarbons insecticides are their much shorter biological and environmental half-lives, which are measured in hours and days rather than months or years. However, the trade-off is that they... 

Public concern over the possible effects of chemicals on man and his environment has largely focussed on a small number of classes of compounds of these, chlorinated hydrocarbons are perhaps the best known. A cursory examination of the scientific literature could easily lead to the impression that they consist entirely of chlorinated pesticides and polychlorinated biphenyls but in terms of total quantity produced, these are less important than many others. This Chapter discusses another class, the aliphatic and C2 Halocarbons, which includes most of the halogenated products of the Organic Chemicals industry, which are of major commercial importance. (Table 1) it excludes all those containing Fluorine. 

Chlorinated Aromatic Hydrocarbon Environmental Toxins. As a result of human endeavor, toxic compounds containing chlorinated benzene rings have been widely distributed in the environment. The pesticide DDT and the class of chemicals called dioxins provide examples of chlorinated aromatic hydrocarbons and structurally related compounds that are very hydrophobic and poorly biodegraded (Fig. 5.28). As a consequence of their persistence and lipophilicity, these chemicals are concentrated in the adipose tissue of fish, fish-eating birds, and carnivorous mammals, including humans. 

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