Evaluating the various toxicities of mNPs is a very active research area [7, 8], made difficult by the number of variables [types of mNP and organism; NP properties (shape, size, crystal form, composition, concentration); types of 'cap' or suspension stabilizers/dispersants used], and by the difficultly of separating the toxicological effects of the NPs from those of the associated dissolved phases [4, 7]. Not surprisingly, some apparently contradictory results exist, though these may be resolved in future. Nevertheless, some general conclusions can be made.
Many studies have shown mNPs to have toxic effects, and in some cases this has been demonstrated to be in excess of that produced by exposure to the dissolved phase of the same element [7, 2]. 'Organisms' examined include bacteria, algae, invertebrates, fish cells/organs, and various mammalian cells and organs, including human cells. mNP types examined include CNTs, metals (e.g. Ag and Au), and metal oxides (including those of Ti, Ag, Zn, and Fe). When seen, significant effects typically occur at low aqueous concentrations, of the order of ppm to tens of ppm. Size is not the only important property - shape, for example, has also been found to be of importance in some systems, at least via airborne exposure .The toxicity of mNPs can arise through a range of mechanisms including : the production of reactive oxygen species (ROS) leading to membrane, protein, and DNA damage and oxidative stress; oxidative stress affecting enzyme activity and mitochondrial performance; protein denaturation and degradation; uptake by neuronal tissue leading to brain damage; and disturbance of phagocytic ('cell engulfing') function leading to reduction in efficiency in removal of infectious agents. NPs are potentially very mobile within organisms (cf. viruses, which are typically 10s nm in diameter and hence NPs in their own right), being able to pass through membranes including by phagocytosis. Thus, another possibility is that mNPs can act as carriers for other contaminants . In some instances, interaction with other contaminants may change the toxicity of the NPs .
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