**2. Nanoparticles, types, and characteristics**

Nanoparticles (or UFPs) are those that measure between 1 to 100 nm in all dimensions of their surface; they can be composed of different components, either natural or synthesized for some purpose. As previously mentioned, the occurrence of nanoparticles in the environment can be the result of anthropogenic activities and in these cases, they are usually part of the pollutants to which the population of urban areas is exposed. In contrast, anthropogenic (synthetic) nanoparticles have very diverse applications in areas such as medicine, engineering, agriculture, cosmetics, clothing and more [3, 17, 18]. The synthesis of nanoparticles can be done physically, chemically, or biologically, the latter is gaining popularity because there is no risk of generating toxic materials during the process unlike chemical synthesis and various microorganisms can be used for their syntheses such as fungi or bacteria [17]. Different types of synthesized nanoparticles can be described according to their physicochemical characteristics and chemical composition: carbon-based nanoparticles, ceramic, metallic, semiconductor, polymeric, or lipid-based materials, among others.
