Engineered nanomaterials in the environment: Are they safe?

Engineered nanomaterials (NMs) are increasingly released into the environment with the rapid development of nanotechnology. In this review, we present the current understanding on the safety of NMs in the environment from the existing literature. For the four typical NMs selected (i.e., TiO2, Ag, CuO, graphene), available data on both environmental concentrations and ecotoxicity (e.g., predicted no effect concentrations [PNECs]) were collected from the published literature. All the four typical NMs did not pose a high risk by comparing environmental concentrations and PNECs. Toxicity of NMs as affected by environmental factors was further discussed. It can be concluded that nanotoxicity could be further lowered under environmental conditions for most scenarios. Generally, the environmental risk of NMs is of low concern currently, while the risk in specific scenarios such as surface water and soil near the point sources (e.g., wastewater effluent, sewage sludge) should be paid more attention. Moreover, the NM-induced toxicity could be changed by various environmental factors such as sunlight irradiation, natural organic matter, and mineral particles. In view of the uncertainties on environmental concentrations and ecotoxicity, the challenges on comprehensive understanding on the environmental risks of NMs, and the future research perspectives were addressed and presented.