Nanotoxicology: Characterization, Dosing and Health Effects. Monteiro-Riviere NA, Tran CL, eds., New York: Informa Healthcare, 2007 Jul; :153-171
Nanostructured materials including nanoparticles (NP) are generally defined as having at least one dimension smaller than 100 nm (1). Ultrafine particles are similarly defined as having diameters less than 100 nm; the general convention being that NP are manufactured or engineered materials in contrast to incidental or natural ultrafine particles. NPs are of special interest to the health effects researcher. They are not merely smaller forms of particulate matter; they can profoundly differ in their toxicological properties from fine-sized respirable particles, i.e., particles between 0.1 and 2.5 microm in size. For example, fine-sized respirable TiO2 particles are typically inert when studied in vitro and in vivo and are typically used as particle negative controls; while in contrast, nanoparticulate TiO2, when used in animal model inhalation studies, causes lung injury (2). The basis for change in health effects associated with decreasing particle size is not necessarily size per se. There are other physical factors that change very strongly with decreasing particle size: one is particle number per unit mass; another is specific surface area (surface area per unit mass). Because of their submicrometer sizes, NP or ultrafine particles have high specific surface areas. For example, ultrafine diesel exhaust particulate materials (DPM) can have specific surface areas in the range of 100 to 1000 m2/g, in contrast to many fine-sized mineral dusts with values in the 10 m2/g range. In some cases, ultrafine particle surface area has provided an effective metric-relating exposure and response: TiO2 ultrafine particles and carbon black dust were active for tumor induction in the rat with toxicity increasing with dust surface area (1). And surface area in some cases provides a measure of comparability between different ultrafine dusts, e.g., for some clearance or inflammation processes in vivo (3). However, not all respirable particulate materials are equally toxic when concentration or exposure dose are normalized by surface area. The composition or structure of the particle surface can greatly affect toxicity. For fine-sized respirable particles, surface structural properties which are submicrometer or ultrafine in dimension can be determinants of health hazard and disease risk. For instance, unexpected morbidity and mortality in the workforce of a hard metal fabrication plant using a new process were related to subtle surface structural features of the generated fine respirable dusts: ultrathin cobalt coatings on the tungsten carbide particles were strongly catalytic in aqueous media for toxic reactive oxygen species generation (4-6). Anomalous differences in silicosis risk between two worker cohorts in China, identified in a 20,000 worker medical registry, were largely resolved by quantitatively normalizing exposures to respirable silica dust that was free from submicrometer aluminosilicate surface contamination (7-9). Such surface ultrafine structural effects on health hazard, albeit for fine-sized respirable particles, suggest the need for a thorough characterization of surface physicochemical and toxicological properties of new NP respirable materials with their very large specific surface areas. Expression of toxic activities associated with respirable particle surfaces can be significantly modified by the initial interaction of particles depositing in the acinar region of the lung. After inhalation, the first contact of respired particles is with the lung's rich surfactant-coated hypophase lining of the air-tissue interface of the alveoli or respiratory bronchioles. Insoluble particles can adsorb components of that pulmonary hypophase surfactant, resulting in fundamental changes in the particles' biological disposition and expression of toxicity. In vitro toxicology studies can be designed to retain particle surface structure and composition and to model the conditioning of those surfaces upon particle deposition in the deep lung in order to analyze consequent effects on toxicity. Ultrafine or fine respirable particulate materials have been dispersed into phospholipid components of lung surfactant as preparation for in vitro cytotoxicity and genotoxicity studies. Pragmatically, this overcomes laboratory handling problems innate in attempting aqueous system preparations of insoluble hydrophobic particulate materials, by permitting their dispersion in a physiologically reasonable manner. More profoundly, this provides a model of their surface conditioning as would occur upon deposition in the lung; of their in vivo biological availability; and of potential nanostructural synergisms for the expression of surface-associated toxicant activity. Some in vitro studies of phospholipid surfactant-conditioned ultrafine or fine respirable particles are reviewed here to illustrate testing procedures and their limitations for identification of NP toxic activities and potential respiratory hazard.