Summary of major themes of discussion (not conclusions. Sequencing development of understanding of exposure and toxic effects of exposure. The Delphi forum explored the relationship between development of exposure methods and development of toxicology methods needed in order to implement Safe by Design. When asked directly whether exposure methods were needed more than toxicology methods, the responses overwhelmingly favoured exposure. However, comments by participants in the second round of the Delphi forum illustrated a finer point about the need to sequence knowledge generation appropriately to support risk characterization as new uses develop rather than preferring exposure methods over toxicology methods development. The potential for risks associated with some properties of the nanomaterials that make it into exposure pathways from new uses may only be discernible when we understand the forms that are actually released, and thereby the properties to which exposure occurs. Therefore, developing toxicology or hazard based decision rules for SbD on current data for MNMs before we understand the actual exposures would be premature. For example, based on the comments, it seems likely that a high percentage of participants would have said “yes” if the Delphi forum had asked a question such as “should we provide an approach to sequence exposure and toxicology methods development so that it addresses the particular decision needs of products that are coming to market”. In some ways this line of reasoning is a case by case argument wherein the point is made that research to create a toxicological assay system to be applied uniformly to all nanomaterials will waste resources if it is created before we understand which characteristics will survive to exposure pathways in a particular case. We may need specific sets of methods for different types of uses and types of MNM, and until we have
