Risk Characterization

The risk characterization combines the results of the PEC, PNEC, and relative bioavailability (i.e., RAF) evaluations. As discussed, the risk characterization was conducted in tiers, with the first tier being the most simplified and subsequent tiers being based on increasing levels of complexity. For each tier, a PEC-to-PNEC ratio, or RCR, was calculated as follows:

RCR = PEC × RAF/PNEC

The RAF was 1 (i.e., 100%) for birds in all tiers and in Tiers 1 and 2 for mammals.

Marine

Birds

In Tier 1, the oystercatcher PECs based on a 100% C. edule diet were compared to the generic PNECs of 5.0 and 6.7 mg kg-1. The RCRs suggest a secondary poisoning risk based on the PNEC of 5.0 mg kg-1 but not based on the PNEC of 6.7 mg kg-1. In subsequent tiers, the RCR is reduced to 0.4 when the PNEC is adjusted for the ingestion rate of oystercatcher species and when the PEC is reduced to 0.2 assuming a mixed bivalve diet, as the oystercatcher will not feed exclusively on C. edule. Under this latter more realistic tier, no secondary Ni poisoning risk is estimated (Table 4).

Mammals

In Tier 1, the harbor seal PECs based on either a 100% bivalve diet or a 100% fish diet were compared to the generic PNEC of 0.73 mg kg-1 assuming an RAF of 1. The RCRs are 0.3 and 0.4 for bivalve and fish diets, suggesting no secondary poisoning risk. In subsequent tiers, the RCRs are further reduced when the PNEC is adjusted for the harbor seal food ingestion rate and body weight, and again reduced further to well below 1 if the RAF of 2.5% is incorporated (Table 5).

Tier Species PNEC (mg kg-1) RAF Diet Assumption PEC (mg kg-1) RCR
1 Generic 5.0/6.7 1 C. edule 5.3 0.8-1.1
2 Oystercatcher1 12.3 1 100% C. edule 5.3 0.4
3 Oystercatcher1 12.3 1 50% C. edule, 50% other bivalves 2.8 0.2
  1. PNEC adjusted for the food ingestion rate-to-body weight ratio of the oystercatcher.

Table 4: RCRs for the mollusk (cockle)-eating bird like the oystercatcher

Tier Species PNEC (mg kg-1) RAF Diet Assumption PEC (mg kg-1) RCR
1 Generic 0.73 1 100% mollusks 0.25 0.3
100% fish 0.27 0.4
2 Harbor seal1 4.6 1 100% mollusks 0.25 0.05
100% fish 0.27 0.06
3 Harbor seal1 4.6 0.025 100% mollusks 0.25 0.001
100% fish 0.27 0.001
  1. PNEC adjusted for the food ingestion rate-to-body weight ratio of the harbor seal.

Table 5:  RCRs for fish/octopus/squid-eating mammal such as the harbor seal

Freshwater

Birds

In Tier 1, the PECs for a mollusk-eating bird that are representative of the different freshwater eco-regions were compared to the generic PNECs of 5.0 and 6.7 mg kg-1. Regardless of the PNEC used, the RCRs are all less than 0.5 (Table 6), suggesting no secondary poisoning risk in the regional freshwater exposure scenarios. In Tier 2, the PNEC of 12.3 mg kg-1, adjusted for the food ingestion rate and body weight of mollusk-eating birds was used, which resulted in RCRs well below 0.2. This result supports the conclusion of no secondary poisoning risk in the freshwater eco-regions for the bird freshwater food chain.

Mammals

In Tier 1, the European otter PECs that are representative of the different freshwater eco-regions were compared to the generic PNEC of 0.73 mg kg-1 assuming an RAF of 1. The RCRs range from 1.4 to 2.9 in three of the freshwater eco-regions and are <1 in the remaining three eco-region scenarios (Table 7). In Tier 2, which uses a PNEC adjusted for the food ingestion rate of the European otter, all RCRs are <1, and all RCRs are less than ≤0.02 when the RAF of 2.5% is also incorporated (Table 7).  

Tier Species PNEC (mg kg-1) RAF Diet Assumption PEC (mg kg-1) RCR
1 Generic 5.0/6.7 1 freshwater eco-regions1 0.19-2.1 0.02-0.43
2 Mollusk-eating bird2 12.3 1 freshwater eco-regions1 0.19–2.1 0.02–0.17
  1. See Fact Sheet 4 for a description of the freshwater eco-regions.
  2. PNEC adjusted for the food ingestion rate-to-body weight ratio of a mollusk-eating bird.

Table 6: RCRs for a mollusk-eating bird

Tier Species PNEC (mg kg-1) RAF Diet Assumption PEC (mg kg-1) RCR
1 Generic 0.73 1 freshwater eco-regions1 0.19–2.1 0.26–2.9
2 European otter2 2.3 1 freshwater eco-regions1 0.19–2.1 0.08–0.93
3 European otter2 2.3 0.025 freshwater eco-regions1 0.19–2.1 0.001–0.02
  1. See Fact Sheet 4 for a description of the freshwater eco-regions.
  2. PNEC adjusted for the food ingestion rate-to-body weight ratio of a European otter.

Table 7: RCRs for fish-eating mammal like the European otter

Soil

Birds

In Tier 1, regional PECs for a predominantly worm-eating bird were compared to the generic PNECs of 5.0 and 6.7 mg kg-1. All regional RCRs were <1, except for an RCR of 2.4 in the clay soil (see Fact Sheet 5 for additional information on the representative eco-region soils) based on the PNEC of 5.0 mg kg-1 (Table 8). In Tier 2, using the PNEC adjusted for the food inges-tion rate of the worm-eating bird slightly lowers the regional RCRs to ≤1.4 assuming a 100% earthworm diet, and to ≤0.8 assuming a 50% earthworm/50% isopod diet.

Mammals

In Tier 1, regional PECs for the shrew were compared to the generic PNEC of 0.73 mg kg-1 assuming an RAF of 1. Several soil eco-region RCRs were >1 (Table 9). In Tier 2, the PNEC of 0.12 mg kg-1, adjusted for the food ingestion rate of the shrew was used, which resulted in higher RCRs because the shrew-adjusted PNEC was lower than the generic PNEC (Table 9). Tier 3 included incorporation of the RAF to account for difference in Ni bioavailability between the PECs and PNECs. For Tier 3, in which a 100% earthworm diet was assumed, two of the soil eco-regions resulted in RCRs >1. The Ni concentrations in these soils were 26 mg kg-1 (peaty soil) and 81 mg kg-1 (clay soil). In Tier 4, it was assumed that the shrew diet contains 30% earthworms and 70% isopods. The RCRs were ≤1.4 when the RAFs of 0.036 and 0.025 for the earthworm and isopod, respectively, were included (Table 9). In just a single regional scenario, the clay soil with a Ni concentration of 81 mg kg-1 resulted in a RCR >1.  

Tier Species PNEC (mg kg-1) RAF Soil Type Diet Assumption PEC (mg kg-1) RCR
1 Generic 5.0/6.7 1 eco-region soils1 100% earthworm 0.30–12.0 0.02–2.4
2 Worm-eating bird2 8.5 1 eco-region soils1 100% earthworm 0.30–12.0 0.02–1.4
3 Worm-eating bird2 8.5 1 eco-region soils1 50% earthworm, 50% isopod 0.087–7.0 0.01–0.8
  1. See Fact Sheet 5 for a description of the eco-region soils. The background nickel concentrations in the eco-region soils ranged from 1 to 81 mg kg-1 dry weight.
  2. PNEC adjusted for the food ingestion rate-to-body weight ratio of a worm-eating bird.

Table 8:  RCRs for a worm-eating bird such as the European starling

Tier Species PNEC (mg kg-1) RAF Soil Type Diet Assumption PEC (mg kg-1) RCR
1 Generic 0.73 1 eco-region soils1 100% earthworm 0.15–12.0 0.2–16.0
2 Shrew2 0.12 1 eco-region soils1 100% earthworm 0.15–12.0 1.3–100.0
3 Shrew2 0.12 0.0363 eco-region soils1 100% earthworm 0.15–12.0 0.09–3.6
4 Shrew2 0.12 0.0363, 0.0254 eco-region soils1 30% earthworm, 70% isopod 0.061–4.9 0.02–1.4
  1. See Fact Sheet 5 for a description of the eco-region soils. The background nickel concentrations in the eco-region soils ranged from 1 to 81 mg kg-1 dry weight.
  2. PNEC adjusted for the food ingestion rate-to-body weight ratio of the shrew.
  3. Earthworm RAF.
  4. Isopod RAF.

Table 9:  RCRs for a worm-eating mammals like the common shrew