Contaminants of emerging concern and aquatic organisms: the need to consider hormetic responses in effect evaluations

Contaminants of emerging concern are widespread in the world’s waters, raising concerns regarding their effects on living organisms. To evaluate the effects of and predict risks associated with such chemicals, dose-response studies are needed, while the nature of the dose-response relationship is critical for the outcomes of such evaluations. Here, we summarize the literature reporting hormetic responses of aquatic organisms to contaminants of emerging concern. Hormesis is a biphasic dose response encompassing stimulatory responses to low doses and inhibitory responses to high doses. We demonstrate that it occurs widely in numerous aquatic organisms exposed to a wide array of contaminants, including nano/microplastics, suggesting potential effects at doses/concentrations that are considerably lower than the traditional toxicological threshold, which cannot be identified or predicted unless hormesis is considered in the study design. To tackle the effects and associated risks of nano/microplastics and other contaminants on aquatic organisms, hormesis should therefore be taken into account early in the design of studies as well as in relevant risk assessments.