Nanotoxicology and nano-risk have been attracting increasing attention of toxicologists and regulatory scientists as the production of nanomaterials increases worldwide (Oberdorster et al. in Environ Health Perspect 113:823-839, 2005). In general, nanotoxicology is associated with manufactured nanomaterials. In atmospheric science and environmental health science, however, very small particles that exist transiently at high count concentrations near road intersections and roadsides are called environmental nanoparticles, and most of these have originated from automobiles. Accordingly there are two types of nanoparticle in toxicology and health science-environmental nanoparticles and manufactured or engineered nanoparticles. In this minireview I would like to address the following issues: (1) What is a nanoparticle? (2) Why is the nanoparticle currently a significant health issue? (3) How has "testing manufactured nanoparticles" been discussed worldwide? (4) What problems have scientists encountered in assessing the health hazard of nanoparticles? and (5) What research is required in the future in nanotoxicology?