Environmental Engineering Reference
In-Depth Information
based mainly on ingestion of contaminants: intake of drinking water, direct intake
of soil, intake of foodstuffs. Generally, inhalation is neglected. This is because the
contaminants for which assessment of risks from consumption of animal products
is relevant are persistent and bio-accumulating contaminants, which generally have
low volatility.
The bioaccumulation of a contaminant in animal tissues depends on the rates of
absorption, degradation and elimination within the animals. For the organic contam-
inants, the potential for bioaccumulation increases with the hydrophobicity of the
contaminant. Lipophilic contaminants tend to accumulate in fat tissue. The uptake
of contaminant by animals causes an increase of the quantity of contaminant in the
animal tissues up to a steady-state level, at which the contaminant uptake is balanced
by degradation and elimination. The lower the degradation and elimination rate, the
longer it takes to reach the steady-state and the higher the steady-state concentration
will be.
In most Human Health Risk Assessment studies, the contaminant concentra-
tions in animal tissues are considered to be at steady-state. A motive for this is
that generally the aim is to assess chronic exposure and the intake by animals is
assumed to be constant. Under these conditions the steady state concentration does
not underestimate the concentration in animal tissues. In this case, concentrations
in animal products (in general, meat, milk, eggs) are calculated by multiplying the
daily intake or the animal food concentration by a constant factor called biotrans-
fer or bioconcentration coefficient, respectively. Regression models based on the
lipophilicity of contaminant have also been developed to estimate the values of
biotransfer coefficient (see Section
11.3.3.2
).
However, calculations can also be performed using a dynamic approach. Such an
approach is useful under the following conditions:
•
to assess concentrations in tissues before the steady-state is reached;
•
to estimate the time required for the concentration to revert to a previous level
after a period of high exposure or
•
in the case of contaminant intake changing with time.
In the simple approach described below, animals are represented by a single
compartment and the animal tissue concentration is calculated by a mass-balance
between input (uptake) and output (excretion and degradation). For lipophilic con-
taminants, the production of eggs (for poultry) and milk (for dairy cows) are
assumed to be the main elimination pathways.
11.3.1.2 Calculation of Human Exposure
The calculation of human exposure considers the average consumption of animal
products from the site over the relevant exposure period. Exposure is calculated
by multiplying the concentration of the contaminant in each product by the daily
ingestion rate of that product (daily intake in mg contaminant per day), which is
then divided by the body weight (daily intake in mg/kg
body weight
and day).
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