Heavy metals in fish: bioaccumulation, toxicity, and human health risk

The problem of heavy metal pollution of aquatic ecosystems represents one of the rising global environmental and public health threats. Unlike many organic contaminants, heavy metals are non-degradable and remain in ecosystems for an indefinite period of time, accumulating in the tissues of fish and bioaccumulating within food chains by orders of magnitude higher than ambient water concentrations. This paper reviews the major mechanisms and pathways of bioaccumulation and biomagnification of five priority heavy metals, arsenic, mercury, cadmium, chromium, and lead spanning all aspects of the contamination cycle, from their anthropogenic and geogenic origin to the specifics of biological uptake in fish tissues, quantifiable by means of the bioconcentration factor (BCF) and bioaccumulation factor (BAF) indicators; from the effects of toxicity in fish, spanning molecular, histopathological, physiological, reproductive, and behavioural levels of organisation, to the corresponding effects in humans, based on epidemiological data from fish-consuming populations; and from available toxicological risk assessments of human health based on target hazard quotient (THQ), hazard index (HI), and cancer risk (CR) methods to the comparison of different methods of remediation, which are considered according to the efficiency and scalability. Knowledge gaps include a lack of standardised BCF/BAF data, the frequent reliance on total rather than speciated metal concentrations, which for arsenic can overstate the toxicologically relevant exposure by roughly an order of magnitude, poor epidemiological dose-response evidence in fish-consuming populations, and the unavailability of field data on nanotechnological cleanup.