A variety of pulmonary hazard studies in rats have demonstrated that exposures to ultrafine or nanoparticles (generally defined as particles in the size range < 100 nm) produce more intensive inflammatory responses when compared with bulk-sized particle-types of similar chemical composition. However, this common perception of greater nanoparticle toxicity is based on a limited number of studies, conducted primarily with titanium dioxide and carbon black particle-types. Apart from variables such as particle size and surface area, it is conceivable that several additional physicochemical particle characteristics could play more significant roles in facilitating the development of nanoparticle-related toxicity; particularly when considering particle surface-cell interactions. These include but are not limited to: (i) Surface reactivity of particle-types; (ii) surface coatings; (iii) aggregation/disaggregation potential; and (iv) the method of nanoparticle synthesis. We present results of pulmonary bioassay hazard/safety studies with quartz particles of varying sizes/surface areas. These demonstrated that intratracheal instillation exposures to fine-sized, Min-U-Sil quartz particles (0.5 µm [particle size] – 5 m2/g [surface area]) produced (persistent) enhanced pulmonary toxicity (inflammation, cytotoxicity, cell proliferation and/or histopathology) in rats when compared to nanoscale quartz particles (50 nm–31 m2/g), but not when compared to smaller nanoscale quartz sizes (e.g., 12 nm–91 m2/g). The toxicity results correlated with red blood cell hemolytic potency as a measure of particle surface reactivity. In a second pulmonary bioassay study in rats, pulmonary hazard effects were measured following exposures to three different ultrafine (nano) TiO2 particle-types, each with similar particle size distributions. The various TiO2 particles differed in their crystal structures and surface reactivity endpoints as measured by the Vitamin C yellowing assay. Moreover, the surface activity characteristics correlated with potency of hazard biomarkers as described above, in these dose/response, time-course studies. It is concluded that particle surface reactivity, rather than particle size/surface area endpoints correlated best with lung inflammatory potency following exposures to particles.