Persistent inorganic and carbon nanoparticles are increasingly engineered for applications and may also be present in conventional materials such as carbon black. Furthermore, they may originate from conventional non particulate materials by processes such as wear and tear. Persistent inorganic and carbon nanoparticles can be hazardous to humans. Relatively much research regards the hazards of inhaled nanoparticles. These may give rise to respiratory disease and to negative effects on other organs, including the cardiovascular system. Determinants of risk of inhaled nanoparticles include: number, size, surface characteristics, shape, structure, and the formation of assemblages. These determinants should preferentially be considered in exposure metrics. A major molecular mechanism underlying the inhalation hazard of nanoparticles is the generation of reactive oxygen species, but other mechanisms such as interactions with proteins and DNA may also contribute. Health hazards may also be linked to ingestion of persistent inorganic and carbon nanoparticles, dermal exposure and exposure of the eye. Standards for workplace exposure to persistent inorganic and carbon are currently emerging and there are options for hazard reduction by elimination and substitution of hazardous nanoparticles and by engineering controls.