Setting the limits for engineered nanoparticles in European surface waters - are current approaches appropriate?

The current widespread and diverse use of engineered nanoparticles (ENPs) in consumer products is expected to result in direct and indirect emissions to the aquatic environment. In Europe, the Water Framework Directive (WFD) is responsible for maintaining a good chemical and ecological status of surface waters. This review provides an insight to the likely scenarios if an engineered nanoparticle should be considered a "priority substance" in the WFD. Through a brief literature review of the aquatic toxicity of carbon- and metal-containing ENPs and in light of principles to establish environmental quality standards, we conclude that it is impossible to set limit values for ENPs in surface waters now and in the foreseeable future. This is due to the extensive lack of knowledge not only in relation to unknown toxic effects, degradability, and bioaccumulation of ENPs in the aquatic environment, but also due to the questionable validity of test systems and methods to establish environmental quality standards (EQS). From this, we also suggest some paths to follow to compensate for these knowledge gaps which include environmental monitoring and a closer look at the use of ENPs in consumer products.