Nanomaterials: Feasibility and Challenges for Human Health Risk Assessment
Several of these engineered nanomaterials are made of elemental metals or the dioxide salts of metals and by virtue of their size possess unique physiochemical properties that may present both a benefit and health risk to humans. Some of the unique physicochemical properties include decreased size, increased surface area, increased aspect ratios, increased reactivity, and light scattering properties. The physicochemical properties of these nanomaterials may increase the toxicity of the base chemicals compared to their bulk counterparts or change their behavior in the body, which could lead to increased persistence or reactivity in target organs. Due to the perceived benefits of these materials, engineered nanomaterials have already been incorporated into many consumer products . Many of these consumer products have the potential for human exposure through inhalation, for example, from paints and coatings, spraying of pesticides, air purifiers, and filters . There are considerable data gaps in our understanding of the biological response to such materials. Existing studies of engineered nanomaterials have indicated that biological response is dependent on composition and structure but is also not predictable by conventional toxicological understanding. There have been numerous proposals, frameworks, and guidance on how to conduct risk assessment on the potential risks of engineered nanomaterials, [2-10]. These proposals and frameworks consist of the basic risk assessment methodology but acknowledge the need for additional information needed to assess the risk for engineered nanomaterials due to their unique physicochemical properties. The additional information needed for risk assessment of engineered nanomaterials include particle number, surface area or mass concentration, size distribution, shape, composition, and chemical reactivity. Other concerns addressed in these preliminary frameworks are: (1) high mobility in different media; (2) persistence of the nanomaterial’s characteristics; (3) potential to bioaccumulate; (4) high reactivity; (5) possible interactions with other toxicants;
(6) interaction reactions (especially at the nanoparticle surface over time);
(7) challenges in engineered nanomaterial’s characterization; and (8) distribution of engineered nanomaterials . Traditional risk assessment methodologies require these existing data gaps to be filled. This may result in the necessity for a new methodology for noncancer risk assessment of engineered nanomaterials or the creation of new safety/uncertainty factors to address their unique physicochemical properties. To assess the risk and potential toxicity for nanomaterials, it is important to recognize that a “one size fits all” definition for nanomaterials may not encompass all that is needed to address the potential risk. An effective risk assessment of nanomaterials must address individual physicochemical properties and unique particle properties in order to ultimately come to human health and environmental risk conclusions. It is important to note that this chapter does not attempt to review the full body of literature available on engineered nanomaterials, but rather provides an overview of challenges in using traditional frameworks of risk assessment. 16.1.1 What Is a Nanomaterial?