Nano-silver is used in an increasing number of products. Some of the applications have resulted in the concern of governments and the public, since little is known about the potential risks of nano-silver. In this review, an inventory is made to identify knowledge gaps that have to be filled before risks for both man and the environment can be assessed as reliable as for ‘non-nanosized’ chemicals. It is hypothesized that the toxic effects of nano-silver are due to a combination of the specific properties of silver nanoparticles and the generation of ions from them. The main topic for future research is validation of our ‘0-hypothesis’ that toxic effects of nano-silver are proportional to the activity of free silver ions released by the nanoparticles. Furthermore, it must be determined whether – or to what extent – nano-silver particles will enter the body. The outcomes of these tests will determine the requirements for further toxicity testing.

This study uniquely describes all steps of the risk assessment process for the use of one specific nanomaterial (nanosilica) in food products. The aim was to identify gaps in essential knowledge and the difficulties and uncertainties associated with each of these steps. Several food products with added silica (E551) were analyzed for the presence, particle size and concentration of nanosilica particles, using experimental analytical data, and the intake of nanosilica via food was estimated. As no information is available on the absorption of nanosilica from the gastrointestinal tract, two scenarios for risk assessment were considered. The first scenario assumes that the silica is absorbed as dissolved silica, while the second scenario assumes that nanosilica particles themselves are absorbed from the gastrointestinal tract. For the first scenario no adverse effects are expected to occur. For the second scenario there are too many uncertainties to allow proper risk assessment. Therefore, it is recommended to prioritize research on how nanosilica is absorbed from the gastrointestinal tract.