Towards sensible toxicity testing for nanomaterials: Proposal for the specification of test design

During the last decade, nanomaterials were extensively tested for potential harmful effects towards humans and environmental organisms. However, a sound hazard assessment was so far hampered by many uncertainties and a low comparability of test results. The reason for the low comparability is a high variation (1) in the type of NM tested with regard to raw material, size and shape and (2) during all steps of the testing procedures before and during the toxicity testing. This calls for tailored, nanomaterial-specific protocols. Here, decision trees can be a versatile tool for structuring the test procedures involved, leading to test protocols not only tailored for a specific type of nanomaterial, but also for the respective test system intended to be used for toxicity testing. There are existing standards on single procedures involving nanomaterials, however, not all relevant procedures are covered by standards. Hence, our approach offers a more detailed way of weighting several plausible alternatives for e.g. sample preparation, in order to decide on the procedure most meaningful for a specific nanomaterial and toxicity test. Decision trees are flexible both in starting point and extend, which makes them universal in use and adoptable to specific research questions. Hence, a framework of several decision trees and flow charts to support testing of NM is proposed here as a basis for further refinement and in-depth elaboration. Decision trees and flow charts were drafted for (1) General procedure - Physical-chemical characterisation, (2) Choice of test media, (3) Decision on test scenario and application of NM to liquid media, (4) Application of NM to the gas phase, (5) Application of NM to soil and sediments, (6) Dose metrics, (S1) Definition of a nanomaterial, and (S2) Dissolution. The applicability of the proposed approach was surveyed by using experimental data retrieved from studies on nanoscale CuO. This survey demonstrated the decision trees and flow charts to be a convenient tool to systematically decide upon test procedures and processes, and hence pose an important step towards harmonization of NM testing.