The toxicology of ion-shedding zinc oxide nanoparticles

Zinc oxide nanoparticles (ZnO NPs) are nanomaterials that are widely used in many fields. ZnO NPs are ion-shedding particles, and zinc ions produce important and potent effects that differ from those of other metal or metal oxide NPs. Several studies have reported the toxicological effects of ZnO NPs administered via several different routes, including orally, dermally, by pulmonary absorption, intraperitoneally, and intravenously. Some potential routes for human exposure have produced various toxic effects in animal models. Moreover, several in vitro studies using a range of cell lines have reported the mechanisms underlying ZnO NP toxicity. Zinc ions play a very important role in ZnO NP toxicity, although the effects of the particulate form cannot be excluded. A crucial determinant of toxicity is the solubility of ZnO NPs, which is influenced by various factors, including the pH of the environment in tissues, cells, and organelles. In addition to the inflammatory responses and oxidative stress known to be induced by ZnO NPs, these NPs also exhibit some positive anti-inflammatory, anti-diabetic, and pro-coagulant effects at sub-toxic doses; these effects are probably induced by zinc ions, which are an essential element in cell homeostasis. It is highly likely that there are additional distinct mechanisms at sub-toxic doses and concentrations, which may be concealed or altered by the toxic effects observed at higher levels of ZnO NPs. Furthermore, many signaling pathway molecules associated with necrosis and apoptosis can be activated, leading to cell death. This review presents the status of ZnO NP toxicology and highlights areas requiring further investigation.