Abstract
Even though nanomaterials are used as constituents of many manufactured articles, there are no unified criteria for evaluating their safety or toxicity. Even the same nanomaterial can cause different sensitivities in different cell types. In addition, differences in physiological conditions can cause varied sensitivity of the same cell type to a specific nanomaterial. Generally, nanomaterials form aggregates or agglomerates in the culture medium, resulting in a range of size variations. Therefore, the response of each cell differs depending on the amount of nanomaterial it absorbs. Furthermore, it is also necessary to consider the effects when a nanomaterial releases metal ions. The most complicated scenario for the in vitro evaluation of a nanomaterial is that it affects growth, apoptosis and necrosis collectively. Nanomaterial toxicity arises from generation of reactive oxygen species (ROS) inside the cell. However, many types of cells possess ROS elimination systems. In cells that have the ability to completely remove ROS, cytotoxicity is normally not observed even after exposure to a toxic nanomaterial. However, for cells that are not able to cope with ROS, cytotoxicity occurs. A number of factors determine which specific molecular species of intracellular ROS quenchers or scavengers act inside cells, including cell type, cellular physiological state and the type and amount of nanomaterial present. In this review, we discuss the biological interpretation of nanomaterial toxicity evaluation data.
