The International Organization for Standardization (ISO) recently announced publication of standard ISO/TS 18827:2017, “Nanotechnologies — Electron spin resonance (ESR) as a method for measuring reactive oxygen species (ROS) generated by metal oxide nanomaterials.”  The introduction states that the use of metal or metal oxide-based nanomaterials has dramatically increased in biomedical and industrial applications.  The scientific basis for the cytotoxicity and genotoxicity of most manufactured nanomaterials are not fully understood, however.  The study of the hazardous effects of metal oxide nanomaterials is still in its initial stages.  The ability to generate ROS is one main source of toxicity of metal oxide nanomaterials.  According to the introduction, overproduction of ROS can induce oxidative stress, resulting in cells failing to maintain normal physiological redox-regulated functions.  This in turn may lead to DNA damage, unregulated cell signaling, change in cell motility, cytotoxicity, apoptosis, and cancer initiation. Critical determinants that can affect the generation of ROS include size, shape, particle surface, surface positive charges, surface-containing groups, particle dissolution, metal ion release from nanometals and nanometal oxides, ultraviolet (UV) light activation, aggregation, mode of interaction with cells, inflammation, and pH of the medium.  Thus, to detect and quantify ROS formation on the surface of metal oxide nanomaterials, the standard suggests the ESR method.  The standard specifies methods of detection by ESR of 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) hydroxyl adduct, 5-tert-butoxycarbonyl-5-methyl-1-pyrroline-N-oxide (BMPO) superoxide adduct, and 2,2,5,5-tetramethyl-3-pyrroline-3-carboxamide (TPC) singlet oxygen adduct formation from metal oxide nanomaterials.  The standard provides a method to assess ROS generation on the metal oxide nanomaterials in a cell free condition.  According to ISO, this method may provide valuable information for the prediction of ROS-mediated cytotoxicity without cytotoxicity assay at physico-chemical evaluation phase.