9
Quantifying
Bioconcentration
INTRODUCTION
Compounds that concentrate in the food chain can result in a much higher dose of
toxic chemical than simply from the initial release of the chemical to the environment. Bioconcentration is the process by which an organism develops an internal concentration of a chemical that is higher than that of its environment. Bioconcentration
is represented by published bioconcentration factors (BCFs), or the ratio of chemical
concentration in the organism to that in surrounding water. Bioconcentration results
when the organism takes in and absorbs the chemical in its tissue at a rate faster than
it is excreted or metabolized. As the organism is consumed by other organisms, the
chemical can concentrate further up the food chain. For example, although mercury
is only present in small amounts in seawater, it is absorbed by algae, generally as
methyl mercury. It is efficiently absorbed but only slowly excreted by organisms
(Croteau et al. 2005). Mercury builds up in the adipose tissue of successive levels
in the food chain. At each step, mercury that is eaten accumulates until that organism is eaten. The higher the level in the food chain, the higher the concentration of
mercury will be in a particular organism. This process explains why predatory fish
such as swordfish and sharks or birds like osprey and eagles have higher concentrations of mercury in their tissue than could be accounted for by direct environmental
exposure alone. For example, herring contains mercury at approximately 0.01 part
per million (ppm), and shark contains mercury at greater than 1 ppm.
BIOCONCENTRATION ADJUSTMENT FACTOR
Bioconcentration provides a path for increasing exposure to a chemical through consumption of seafood. Therefore, the associated analysis of the impact on human
health is dependent on the relative ingestion of a chemical through consumption of seafood as compared to the direct amount of chemical consumed through
breathing (air) and through consumption of water or indirect consumption of soil.
Bioconcentration is specific in that it refers to uptake and accumulation of a substance from water alone, whereas bioaccumulation refers to uptake from all sources
combined (e.g., water, food, air). For the purpose of this analysis, bioconcentration
associated with uptake through the consumption of seafood in water was used as a
surrogate due to the availability of reliable BCF data. Bioaccumulation across other
environmental media (i.e., the chemical uptake in plants via the soil and the air and
the plants are then eaten; by cattle in the soil and air, which then affects meat and
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