Arsenic, Mercury and Nickel in Nigerian Seafood: A Critical Appraisal of Exposure Estimation, Speciation and Risk Characterisation

Abstract

Seafood supplies a substantial share of animal protein in Nigeria, and the aquatic systems from which it is harvested receive inputs from petroleum extraction, artisanal gold processing, industrial effluent and untreated urban waste. A large Nigerian research effort has responded by measuring trace elements in fish and shellfish and converting those measurements into hazard quotients and lifetime cancer risks. This review examines what that literature can and cannot support for three elements whose toxicology diverges sharply: arsenic, mercury and nickel. The three are treated together not because they behave alike but because they are routinely pooled into a single additive hazard index, a practice that obscures the differences on which any defensible conclusion depends. Evidence was assembled through direct querying of a bibliographic metadata registry and general scholarly web searching, with every retained source verified against its registered record. The synthesis identifies a recurring structural problem. Nigerian assessments almost universally determine total element concentrations, then apply toxicity reference values derived for specific chemical species: an inorganic arsenic reference dose applied to total arsenic that is predominantly organic in marine organisms, a methylmercury reference dose applied to total mercury, and oral cancer slope factors applied to nickel for which oral carcinogenicity has not been established. These substitutions bias arsenic and mercury estimates upward, while nickel risk statements rest on a quantitative basis that authoritative evaluations do not endorse. Consumption rates, body weights and processing losses are typically imported from unrelated populations, and results are presented as point estimates without uncertainty bounds. The consequence is a literature in which measured concentrations below regulatory limits coexist with hazard indices above unity and cancer risks above one in ten thousand, and in which the direction of the discrepancy is set by analytical and parametric choices rather than by exposure. Confidence is highest for the conclusion that Nigerian seafood mercury concentrations are generally low by international standards and lowest for any quantitative statement about arsenic or nickel risk. Priorities are speciation analysis on Nigerian samples, nationally representative consumption data, bioaccessibility measurement on locally prepared foods, and probabilistic characterisation of the resulting exposures.