The changing face of nanomaterials: Risk assessment challenges along the value chain.

Risk assessment (RA) of manufactured nanomaterials (MNM) is essential for regulatory purposes and risk management activities. Similar to RA of "classical" chemicals, MNM RA requires knowledge about exposure as well as of hazard potential and dose response relationships. What makes MNM RA especially challenging is the multitude of materials (which is expected to increase substantially in the future), the complexity of MNM value chains and life cycles, the accompanying possible changes in material properties over time and in contact with various environmental and organismal milieus, and the difficulties to obtain proper exposure data and to consider the proper dose metric. This article discusses these challenges and also critically overviews the current state of the art regarding MNM RA approaches.