specific hypotheses related to material properties and specific outcomes (for example, mobility in the environment or toxic responses). The lack of widespread access to such materials and the lack of agreement as to which materials to consider as standards slows progress toward linking properties of ENMs with their effects, makes comparisons among studies difficult, and limits the utility of data collected for informatics (see section “Barriers to Informatics”).

To characterize correlations between nanomaterial properties and the key interactions or end points in humans and the environment, several tools are needed, including adequately characterized materials that have different properties, appropriate assays for examining interactions or end points, and experimental data of sufficient breadth and depth for assessing correlations between nanomaterial properties and the behavior of the materials. Materials needed for developing those correlations are in four general categories, which are described below. Each type must be characterized sufficiently for test results to be reproducible and for correlations between observed effects and material structure and composition to be established and ultimately used to predict effects of new materials on the basis of knowledge of their structure and composition.

Research or Commercial Samples

These samples may be available from R&D teams or from materials that are near commercialization or in commerce. Many EHS studies have been conducted with such materials because of their availability and because people or the environment may be exposed to these materials. The material definition and characterization metrics needed for nanomaterial research and commercial use are typically different from those needed to study material-effect correlations, and the former materials often do not have the definition, purity, or characterization needed for research purposes. It is important to study the biologic and ecologic effects of the commercial materials, as such materials (and their impurities) have the greatest potential compared to other types of materials to be released into the environment (Alvarez et al. 2009; Gottschalk and Nowack 2011). However, there are limitations to the use of commercial materials in the development of predictive models. The materials are generally insufficiently characterized; when they are studied in isolation, the polydispersity and lot-to-lot variation in their properties make them unsuitable for developing data that can be used for prediction. For greater utility in prediction, material characterization that is specific to EHS research should be conducted in addition to that carried out by material researchers or producers (Bouwmeester et al. 2011).

Reference Materials

Reference materials are developed for hypothesis-driven research or for use as benchmarks to compare results among various tests or assays or among laboratories. They are designed and characterized so that material characteristics

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