The active cellular efflux of toxicants is an efficient biological defence mode present in all organisms. By blocking this so-called multixenobiotic resistance transport - a process also referred to as chemosensitisation -, cellular bioaccumulation and the sensitivity of organisms towards environmental pollutants can increase. So-far, a wide range of compounds, including pesticides, pharmaceuticals, fragrances, and surfactants, have been identified as chemosensitisers. Although significant on a cellular level, the environmental impact of chemosensitisation on the organism level is not yet understood.

Critically evaluating existing data, this paper identifies research needs to support our tentative conclusion that chemosensitisation may well enhance the risks of chemical exposure to aquatic organisms. Our conclusion is based on studies investigating the impact of individual chemicals and complex environmental mixtures on aquatic wildlife and a chemosensitiser mixture toxicity model which, however, is subject to great uncertainty due to substantial knowledge gaps. Those uncertainties include the inconsistent reporting of effect data, the lack of representative environmental contaminants tested for chemosensitisation, and the publishing of highly unreliable nominal exposure concentrations.

In order to confirm the tentative conclusion of this paper, we require the significant and systematic investigation of a broader set of chemicals and environmental samples with a harmonised set of bioassays and rigorously controlled freely dissolved effect concentrations.