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The term bioaccumulation is used to refer to storage of a pollutant at levels higher than found in the environment.
Many chemicals including dioxins, polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs) and organ metallic forms of metals bio-accumulate in animal fat (Hill, 1997) and are difficult to degrade.
Lead and fluoride bio-accumulate in bones. Chemicals bound to proteins and cadmium can bio-accumulate in the liver, kidney and other tissues. The route of almost 90% of human exposure to PAHs is from food consumption, especially the leafy vegetables and unrefined grains.
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Breathed into the lung as fine particulates, PAHs can cause respiratory problems and a number of them are carcinogens. Cadmium bio-accumulates in the kidney and the amount of this toxic metal stored in this organ increases with age. Cadmium can also accumulate to high levels in liver Organ chlorine pesticides have low solubility in water; they are highly soluble in lipid materials, including animal fat, in which they bio-accumulate to high levels.
The term bioaccumulation is also used to describe increase in quantity of a chemical through the food chain. For example, in a food chain— phytoplankton zooplankton -> small fish -> large fish eating birds – each component of the food chain receives some chemical from the environment (bio-concentration) but the amount of chemical becomes several times more through the food chain and organisms at the top of the food chain are worst hit as they contain the maximum quantity of the harmful chemical. Through this route pesticides sprayed on crops accumulate in vegetables food grains, fish and human tissues (Fig. 8.1).
Bioaccumulation is more marked with some chemicals than others. For example, organ chlorine pesticide, DDT is stored in the body fat for a much longer time than methoxychlor. The half-lives of these insecticides in rats are 6-12 months and 12 weeks respectively (Lu and Kacew, 2002).
