Lack of significant risk from everyday dietary nickel: Rethinking the application of oral toxicity reference values

Toxicity reference value (TRV) is a general term for an exposure limit below which health impairment is not expected. TRVs are used to develop environmental quality standards and for other risk-based activities. Nickel (Ni) TRVs developed by several influential regulatory agencies have used toxicity data from rats orally exposed to nickel sulfate hexahydrate (NSHH). These TRVs converted doses from an NSHH-basis to an 'as-nickel' basis, under the belief that controls received no Ni, which is not the case. The extrapolation from a point-of-departure to a human-relevant limit has resulted in TRVs that are, in some instances, less than the estimated normal daily dietary nickel intake observed in certain human life stages. As a result, Ni that is present naturally in food has been incorrectly believed to be contributing to human health risk. The now common risk assessment practice of applying these Ni TRVs on an 'inclusive of baseline diet' basis, contrasts with the Institute of Medicine (IOM) perspective that there is no evidence of adverse effects associated with a normal diet: rather, human health risk is associated with excess nickel intake as soluble nickel salts. TRVs based on soluble Ni salts are to be distinguished from the normal Ni-containing diet.