Studies such as those described above are particularly important in the case of plants and/or animals which may enter the human food chain. An initial and still primary concern has been for agricultural systems such as the classic case of radioactive fallout being taken up and concentrated through the grass-cow-milk pathway leading to human intake of radioactive elements such as strontium-90 which, as a chemical analog of calcium, tends to be taken up and concentrated in bone where it shows long biological and ecological half-lives. More recently, however, additional concern has come to focus on the uptake and concentration of radioactive contaminants by free-living fish and wildlife species which may also be eaten by humans. Although generally not as common in human food chains as agricultural products, there are many cases of sportsmen and individuals in rural areas of developed countries, as well as entire societies in underdeveloped parts of the world, for whom fish and wildlife food products may form an important part of their annual diet. In these cases, radioactive contaminant concentrations in edible parts of such game may come to represent significant health hazards. In this regard, a phenomenon known as the 'Jenkins effect' has shown that free-living fish and wildlife invariably show significantly higher levels of radioactive contaminant concentrations than do domestic livestock from the same area even though they are exposed to the same levels of fallout contamination. This phenomenon is the result of nonra-dioactive fertilizers being widely used to supplement soil nutrient levels in crop fields and pastures used by domestic livestock. These fertilizer supplements serve to dilute the radioactively contaminated analogs of these same elements in the radioactive fallout, thus decreasing the amount of radionuclide uptake by the crop plants and/or grazing domestic livestock.
A dramatic demonstration of this phenomenon occurred during the global distribution of radioactive fallout following the Chernobyl nuclear accident. This resulted in extremely high levels of radioactive contamination in reindeer grazing unfertilized tundra ecosystems at northern latitudes, while contamination levels of domestic livestock grazing fertilized pastures in more temperate regions were generally less affected and posed a lesser risk to human consumers.
In studying these situations with fish and wildlife, radionuclide uptake and concentration in natural food chains frequently fails to conform to the expected pattern of contaminant-level biomagnification or trophic-level concentration in consumers feeding at higher trophic levels. Although top carnivores frequently show the highest levels of heavy metals or organic contaminants, in the case of radionuclide contamination highest levels are often showed by herbivores feeding lower on the food chain.
Radiocesium: A radioactive contaminant of concern
Following a nuclear weapons detonation or nuclear industrial accident, a variety of radioisotopes may be introduced into natural food webs, including those leading to human food products. However, the radionuclide of greatest concern is often the gamma-emitting isotope radiocesium (cesium-137). Characteristics of radiocesium which make it of particular concern as an environmental contaminant include the following: (1) As a gamma-emitter, its emissions are highly penetrating of biological material and can cause extensive chemical/genetic damage at greater distances from the isotope itself than would be the case of alpha- or beta-emitters whose radioactive emissions are less penetrating. (2) Radiocesium has a relatively long physical half-life of over 30 years which means that significant levels of this isotope could remain in the environment itself for well over a century unless the isotope is eliminated from the system by biological/ ecological processes. (3) Within the body, radiocesium is selectively concentrated in skeletal muscle, which is the one tissue which is most likely to be consumed by humans eating meat from fish, game, or domestic livestock. On the other hand, the first two of the above factors also make radio-cesium an excellent in situ tracer for use in studies designed to study basic ecological processes as explained above.
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