consumption factors for the standard edible tissues (3). Only that portion of the ADI
not reserved for milk or eggs is available for calculating a safe concentration of total
residues in the edible tissues.
Safe Concentration¼ADI60 kg=Food Consumption Factor (3)where, Safe Concentration is the safe concentration for total residues in a specified
edible tissue as defined in the model food basket; ADI is the acceptable daily intake;
the Food Consumption Factor is the daily consumption of the specified edible
tissue.
In general, estimates of predicted dietary exposure to residues are unrealistically
high compared to the actual dietary intake, which is often well below the ADI. This
reflects the conservative nature of the assumptions used when estimating the
predicted dietary exposure. These assumptions include the treatment of all animals
at the maximum approved dose rate and duration and slaughter at the withdrawal
period; the presence of residues in the edible tissues and products such as milk and
eggs from all animals at MRL (for the TMDI calculation), or alternatively, at the
median residue concentrations (for the EDI calculation); and the daily consumption
for a lifetime of the model food basket by a 60-kg person.
2.1.4 Risk Characterisation
The risk characterisation step considers the results of the hazard identification,
hazard characterisation, and exposure assessment steps of the risk assessment. In
the context of veterinary drugs, the primary outcome of risk characterisation is a set
of MRLs based on the ADI concept of “no appreciable health risk” and the model
food basket (Arnold 2004 ). The established MRLs are subsequently used as tools by
risk managers for protecting consumers against possible harmful effects. The MRL,
known as a tolerance in the USA, is the maximum concentration of residue
(expressed in mg/kg ormg/kg of food on a fresh weight basis) resulting from the
authorised use of a veterinary drug that is recommended by the Codex Alimentarius
Commission or national authorities to be legally permitted or recognised to be
acceptable in or on food. However, not all substances leave residues that are
considered to be a health risk to the consumer. While MRLs for these substances
are not necessary, alternative regulatory mechanisms such as Annex II of Council
Regulation (EEC) No 2377/90 in the EU and Table 5 of the MRL Standard in
Australia must be complied with. Numerous excipients and some drugs such as
ketoprofen for use in horses, cattle and pigs and ketamine for use in all food
producing species in the EU, and ophthalmic preparations of some veterinary
drugs for use in cattle and sheep in Australia, do not require MRLs. At the other
extreme, the residues of certain substances constitute a health hazard to consumers
irrespective of the concentration present in food. As a result, MRLs for these subs-
tances cannot be established and their use in food-producing animals is not permit-
ted. Examples include chloramphenicol and the nitrofurans. With chloramphenicol,
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