Toward Defining an Absolute Sustainability Boundary for Nanomaterials

Our Earth systems are threatened by human activities, yet we still lack a comprehensive understanding of the magnitude of this impact. To understand our impact, the planetary boundary (PB) concept was introduced in 2009 and aims to define the environmental, global limits within which humanity can safely operate. While it builds on the best available information, it often relies on inherently uncertain estimates. The PB for novel entities, which describes any anthropogenic substances or modified organisms that are new in their nature or quantity with the potential to disrupt Earth system processes, is no exception hereof. 
Engineered nanomaterials (ENMs) represent a growing class of novel entities with diverse applications across industries, and while they are driving advancements in agriculture, medicine, and manufacturing, their increasing use raises concerns about their impact on human and ecosystem health. The quantification of the PB relating to ENMs has been particularly challenging due to the complexity and diversity of novel entities, their global spread combined with spatialised effects depending on local ecosystems, and large data gaps on monitoring and toxicity data. While estimated to be in the zone of exceedance, a more robust quantification of the boundary and a better-defined limit is needed to instate appropriate and efficient mitigation and adaptation measures.
The aim of this study was to explore the existing definition of the PB for ENMs and explore alternative sustainability assessments of ENMs. Through a systematic literature review, we found that approaches for quantifying a PB for ENMs are largely lacking. By further reviewing available methods used to assess the sustainability of ENMs and the PB framework for other environmental impacts, we provide conceptual guidance for defining a PB for ENMs. This work thus lays the foundation for quantifying the PB for ENMs, accounting for their diverse nature and the multi-dimensionality of novel entities.