Environment, pesticides food chain

The effects of pollution can be direct, such as toxic emissions providing a fatal dose of toxicant to fish, animal life, and even human beings. The effects also can be indirect. Toxic materials which are nonbiodegradable, such as waste from the manufacture of insecticides and pesticides, if released to the environment, are absorbed by bacteria and enter the food chain. These compounds can remain in the environment for long periods of time, slowly being concentrated at each stage in the food chain until ultimately they prove fatal, generally to predators at the top of the food chain such as fish or birds. 

As explained in Section 5.2.3, p,p -DDE is much more persistent in food chains than either p,p -DDT or p,p -DDD, and dnring the 1960s when DDT was still extensively used, it was often the most abundant of the three compounds in birds and mammals found or sampled in the field. Since the widespread banning of DDT, very little of the pesticides has been released into the environment, and p,p -DDE is by far the most abnndant DDT residue found in biota. While discussing the ecological effects of DDT and related compounds, effects on population numbers will be considered before those on popnlation genetics (gene frequencies). 

Both PCDDs and PCDEs are refractory lipophilic pollutants formed by the interaction of chlorophenols. They enter the environment as a consequence of their presence as impurities in pesticides, following certain industrial accidents, in effluents from pulp mills, and because of the incomplete combustion of PCB residues in furnaces. Although present at very low levels in the environment, some of them (e.g., 2,3,7,8-TCDD) are highly toxic and undergo biomagnification in food chains. 

The terrestrial environment suffers from the persistence of metals from industrial processes, as well as herbicides and pesticides which may also be transmitted through the food chain with deleterious effects to both individual species and ecosystems. 

Biologically available organic contaminants may find their way into the food chain and be toxic, but they are also more easily decomposed and are thus removed from the environment. Slowly decomposed biologically available organic compounds, such as pesticides, may have long lifetimes in soil and thus pose a hazard to animals and humans. Biologically unavailable compounds, such as tars, produce undesirable characteristics in soil, such as water... 

Ellgehausen, H Guth, J.A., and Esser, H.O. Factors determining the bioaccnmulation potential of pesticides in the individnal compartments of aquatic food chains. Ecotoxlcol Environ. Saf., 4(2) 134-157,1980. 

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. 

Many organochlorine pesticides (OCPs), which have shown undesirable effects in humans and the environment, have been banned in developed countries. Even so, OCPs deserve particular attention, for they are very stable and can accumulate in food chains (9). Products of animal origin as well as human mother s milk almost always contain residues of organochlorine compounds. The residue content of this milk is 10-30 times higher than that of cow s milk (10,11). 

Organophosphorus pesticides (OPPs) have been used extensively as insecticides, fungicides, herbicides, and animal pesticides for more than four decades. They have been used instead of organochlorine pesticides because their relatively rapid decomposition makes them less persistent in the environment and because their accumulation in the biological food chain is limited. Basic to the proper assessment of the potential health hazards of OPPs is the knowledge of their fate in biological systems and in the environment (12,13). 

The approved uses for chlorinated chemicals have now become so restricted that for some of them at least they are now more likely to contaminate food as a result of their persistence in the environment rather than from their direct use on the food chain. A former pesticide in this category is hexachlorobenzene. There... 

Once a pesticide does its job, the toxicant doesn t immediately disappear but becomes a factor in the environment. Hence, the chemist must be concerned with the ultimate fate of this material in relation to drift, residual nature in soil, residue in run-off water, and effect on food chain. 

PCBs are industrial compounds which were widely used as organic diluents, plasticizers, pesticide extenders, adhesives, dust-reducing agents, cutting oils, flame retardants, heat transfer fluids, dielectric fluids for transformers and capacitors, hydraulic lubricants, sealants and in carbonless copy paper. PCBs have entered the environment via multiple pathways and residues of these compounds have been identified in air, water, wildlife and human adipose tissue, serum and milk.89-91 Like many other aromatic hydrocarbons, PCBs are highly lipophilic and chemically stable and this has contributed to their environmental persistence and bioconcentration in the food chain. PCBs were originally... 

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