A Basics of the Risk Assessment Approach

The basic structure of this risk assessment followed the TGD (ECB 2003) and included the development of dietary predicted exposure concentrations (PECoral) and dietary predicted no effect concentrations (PNECoral). Because only PECoral and PNECoral values are discussed in this fact sheet, for simplicity these will hereafter be simply referred to as PECs and PNECs. Marine, freshwater, and terrestrial habitats were evaluated, and both mammalian and bird food chains were addressed for each of these habitats. The PEC in this assessment was the estimated Ni concentration in the diet of the representative predatory birds or mammals in each of the food chains evaluated, while the PNEC was the Ni concentration in bird and mammal diets at which no adverse effects are expected. The ratios of the PECs to corresponding PNECs were then calculated, which were termed Risk Characterization Ratios, or RCRs. Assuming that the data used to derive the PEC and PNEC are adequately conservative, an RCR <1 indicates that there is a negligible potential risk, while an RCR >1 indicates that there is a potential risk and a more refined assessment is necessary.

This refinement is based on a tiered approach, where conservative assumptions (i.e., assumptions that are likely to overestimate potential risk) were used in the first tier and increasingly more realistic assumptions were used in successive tiers (Figure 2). If the secondary poisoning risk estimates from the lower tiers indicate a low potential risk then it can be confidently concluded that the risk is negligible. However, the assumptions used in the lower tiers also tend to be unrealistically conservative and it is necessary to refine the approach with each increasing tier. 

Figure 2 

 Figure 2:  Flow chart for tiered risk characterization approach