Persistent Toxic Chemical Compounds

Many toxic chemicals are in use worldwide. A great many of these are persistent in the environment. They have long half lives and are passed up the food chain. Many are soluble in human adipose tissue and are slowly metabolized. The most persistent of these arel53  

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More than 30,000 different chemicals are produced or used in the Great Lakes region of the United States. This area contains 118 different hazardous waste sites that are contaminated with thousands of different chemicals. A study of this region has shown that human populations in it have elevated body burdens of persistent toxic chemical compounds. 

Many of the observed Great Lakes region toxic effects are attributed to persistent toxic chemical compounds. TCDD is associated with human thyroid hormone fluctuation, problems in male reproductive function, and neurological malfunction. Hexachlorobenzene (HCB) and methyl mercury (MeHg) are associated with human thyroid hormone fluctuation, problems in female reproductive function, neurological and neurodevelopmental problems. These effects, summarized in Table 13.1, illustrate the difficulty in attributing a particular toxic effect to a single chemical, particularly when one considers the potential impacts of thousands of other chemicals. 

Table 13.1 Toxic Effects of Persistent Toxic Chemical Compounds [54]...

Insect resistance and environmental pollution due to the repeated application of persistent synthetic chemical insecticides have led to an Increased interest in the discovery of new chemicals with which to control Insect pests. Synthetic insecticides, including chlorinated hydrocarbons, organophosphorus esters, carbamates, and synthetic pyrethroids, will continue to contribute greatly to the increases in the world food production realized over the past few decades. The dollar benefit of these chemicals has been estimated at about 4 per 1 cost (JJ. Nevertheless, the repeated and continuous annual use in the United States of almost 400 million pounds of these chemicals, predominantly in the mass agricultural insecticide market (2), has become problematic. Many key species of insect pests have become resistant to these chemicals, while a number of secondary species now thrive due to the decimation of their natural enemies by these nonspecific neurotoxic insecticides. Additionally, these compounds sometimes persist in the environment as toxic residues, well beyond the time of their Intended use. New chemicals are therefore needed which are not only effective pest... 

It turns out that most of these compounds have similar characteristics that contribute to their toxicity to both humans and other species of plants and animals. First, the compounds are environmentally persistent. Many of the early pesticides, and certainly the metals, do not break down in the environment or do so only very slowly. If persistent chemicals are released continually to the environment, the levels tend to rise ever higher. This means they are available to cause harm to other organisms, often not even the target of the pesticide. Second, the early pesticides were broad acting and toxic to many species, not just the target species. These poisons often killed beneficial insects or plants. Third, many of these compounds would bioaccumulate or concentrate in species as they moved up the food chain. The chlorinated pesticides accumulate in the fat of animals. Animals that consumed other animals accumulated more and more of these pesticides. Most species could not metabolize or break down the compounds. Lead accumulates in bone and methyl mercury in muscle. And finally, because of their persistence in the environment and accumulation in various species, the persistent toxicants spread around the world even to places that never used them. Animals at the top of the food chain, such as polar bears and beluga whales, routinely have fat PCB levels greater that 6 ppm. 

The organophosphorus (OP) and carbamate insecticides are used to control a wide variety of insect pests. The acute toxicity of the OPs and carbamates varies, and many of them have high mammalian toxicity. These compounds react chemically with the active site of acetylcholinesterase, producing a blocked enzyme that cannot degrade acetylcholine. The concentration of acetylcholine then builds up and hyperexcitation occurs. The signs of intoxication include resdessness, tremors, convulsions, and paralysis. Blockage of acetylcholinesterase by OPs is persistent, and recovery of the enzyme takes many hours or even days. The mode of action of the carba-... 

A number of complex chemical compounds (e.g., pesticides, vapors and fumes, automobile exhausts, petroleum distillates, foundry fumes, heterocyclic amines, solid particles) have caused adverse effects to humans and environment. Some persistent contaminants have originated from industrial sources. For example, DDT, mirex, PCBs, dioxin, and others have been traced in the atmosphere as toxic depositions causing concern to human health.3... 

United States because they build up in the environment and can cause harmful health effects, they are still in use in many other countries. Polychlorinated dibenzodioxins and polychlorinated dibenzofurans (dioxins) (Fu et ah, 2003) consist of 210 different compounds that have similar chemical properties (Bhandari and Xia, 2005). This class of compounds is persistent, toxic, and bioaccumulative. They are generated as by-products during incomplete combustion of chlorine containing wastes such as municipal solid waste, sewage sludge, and hospital and hazardous wastes (Bhandari and Xia, 2005). PCBs were widely used in the past and now contaminate many industrial and natural areas. 

Czaja, C., Ludwicki, J.K., Robson, M.G., Goralczyk, K., Struciski, P., Buckley, B. (2001). Concentrations of persistent organo-chlorine compounds in the placenta and milk of the same women. In Persistent, Bioaccumulative, and Toxic Chemicals I Fate and Exposure (R.L. Lipnick, J.L.M. Hermens, K.C. Jones, D.C.G. Muir, eds). ACS Symposium Series Monograph 772, pp. 284-91. American Chemical Society, Washington, DC. 

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