For a chemical to be bioaccumulated by the organism, it must be biovailable; available for uptake. For passive uptake from water, this means that the chemical must be truly dissolved, and not associated to particles. However, if not available for passive uptake by respiratory surfaces, or by adhering to the surface, it may be available for dietary uptake, by ingestion of particles. The partitioning of a chemical in the dissolved or particulate phase of water is determined by the chemical's physicochemical properties, especially its hydrophobicity, reflected in their Kow. As a general rule, chemicals that bioaccumulate have log Kow higher or equal to 5.0; however, some persistent contaminants with log Kow lower than 5 but higher than 3.5 also show bioaccumulation.
One group of bioaccumulating contaminants is halo-genated organic compounds, in which Kow increases with increasing halogenation degree. Along with increasing halogenation, the molecule grows larger and may no longer be as available (due to steric hinderance) for uptake over biological membranes as are smaller molecules. If absorbed, however, the higher degree of halogenation which increases the hydrophobicity, makes the chemical more difficult to eliminate from the animal. The elimination is more difficult, not only from passive diffusion, but also from enzymatic breakdown, as the higher degree of halogenation leaves fewer positions available to the enzyme to attack.
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