Secondary poisoning refers to the toxicity of a chemical to an organism via its food. In other words, the concentration of a chemical in a particular organism may not be toxic to the organism itself, but it may be toxic to another organism that feeds upon it. According to the Technical Guidance Document (TGB) (ECB 2003), the assessment of secondary poisoning is performed to address this potential concern associated with “toxic effects in the higher members of the food chain which result from ingestion of organisms from lower trophic levels that contain accumulated substances.” The focus on “higher members of the food chain” seems to imply that higher trophic level organisms are more sensitive than lower trophic levels, or that the tissue concentrations of the accumulated substance may increase progressively with trophic status (i.e., that the accumulated substance biomagnifies).

In the case of nickel (Ni), an earlier assessment completed by DEPA (2004) concluded that Ni bioaccumulation (i.e., uptake into the organism) in algae, crustaceans, and fish is sufficiently negligible that the secondary poisoning potential of Ni via these dietary pathways is not of concern. However, the relatively high bioaccumulation potential of Ni in some marine mollusks (including bivalves, such as clams) resulted in the development of a secondary poisoning assessment for Ni in mollusk-based marine food chains. In addition, a secondary poisoning assessment of Ni from earthworms and other invertebrates to predators in terrestrial (i.e., soil-based) food chains was evaluated. This fact sheet summarizes the methods and results of these secondary poisoning assessments.