Nanofibers, nanoplates, nanowires, quantum dots and other related terms have been defined based on this ISO definition. Similarly, The International Organization for Standardization (ISO) has described NMs as a “material with any external nanoscale dimension or having internal nanoscale surface structure”. The US Food and Drug Administration (USFDA) also refers to NMs as “materials that have at least one dimension in the range of approximately 1 to 100 nm and exhibit dimension-dependent phenomena”. According to the Environmental Protection Agency (EPA), “NMs can exhibit unique properties dissimilar than the equivalent chemical compound in a larger dimension”. Different organizations have a difference in opinion in defining NMs. However, a single internationally accepted definition for NMs does not exist. Today, there are several pieces of legislation in the European Union (EU) and USA with specific references to NMs. In principle, NMs are described as materials with length of 1–1000 nm in at least one dimension however, they are commonly defined to be of diameter in the range of 1 to 100 nm. A nanometer (nm) is an International System of Units (Système international d'unités, SI) unit that represents 10 −9 meter in length. NPs and NSMs have gained prominence in technological advancements due to their tunable physicochemical characteristics such as melting point, wettability, electrical and thermal conductivity, catalytic activity, light absorption and scattering resulting in enhanced performance over their bulk counterparts. Nanoparticles (NPs) and nanostructured materials (NSMs) represent an active area of research and a techno-economic sector with full expansion in many application domains. Additionally, the types of toxic reactions associated with NPs and NSMs and the regulations implemented by different countries to reduce the associated risks are also discussed. The review presents an overview of the history and classifications of NMs and gives an overview of the various sources of NPs and NSMs, from natural to synthetic, and their toxic effects towards mammalian cells and tissue. The aim of this review is to compare synthetic (engineered) and naturally occurring nanoparticles (NPs) and nanostructured materials (NSMs) to identify their nanoscale properties and to define the specific knowledge gaps related to the risk assessment of NPs and NSMs in the environment. Due to increased growth of production of NMs and their industrial applications, issues relating to toxicity are inevitable. The ability to predict the unique properties of NMs increases the value of each classification. NMs are categorized depending on their size, composition, shape, and origin. Nanomaterials (NMs) have gained prominence in technological advancements due to their tunable physical, chemical and biological properties with enhanced performance over their bulk counterparts.
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