Challenges of Using Models with Nanoscale Materials
There are several models that span multiple levels of complexity and are designed to estimate exposure at several points in the exposure paradigm. The effectiveness of these models at predicting human exposure will depend on the parameters and assumptions of each model. For models that are based on assumptions specific to the chemical such as the physical and chemical properties, and interactions in humans and the environment based on these properties, much substance-specific data may be required.
Data Sets for Modeling
The availability of ambient data is clearly critical to modeling exposure, and there are a number of resources within EPA for this type of data. In some cases such as for pesticides, the exposure can be anticipated based on the quantity of the substance that is proposed to be applied and the anticipated residue on a food item as an example. Sometimes there are data collected under statutory obligations, such as data collected for the Toxics Release Inventory (TRI) under the Emergency Planning and Community Right to Know Act (EPCRA). For contaminants in drinking water, the data may be reported to the Safe Drinking Water Information System (SDWIS). Generating data for nanomaterials necessitates the identification of nanomaterials as separate and different from other chemicals of identical nomenclature, and their classification as toxic substances, or in a manner that adds nanomaterials to the list of reportable releases/contaminants.
Though not fully representative of population exposure, workplace data have frequently provided the foundation for understanding exposure and toxicity for many chemicals in industrial production. A recent study in the United States, in which ambient air concentrations and glove deposit levels were measured, identified a concern for exposure during handling of nanotubes (Maynard et al., 2004). In the work environment, data on workplace exposure is frequently collected under the purview of Occupational Safety and Health Administration (OSHA)mandated programs to assess worker exposure and assure compliance with workplace regulations and worker protection. Employers, however, are not required to report these data. In addition, OSHA standards are typically airborne exposure levels that are based on health or economic criteria or both, and typically only defined exceedences of these standards are documented. To understand nanotechnology risks in the workplace, the National Institute of Occupational Safety and Health (NIOSH) is advancing initiatives to investigate amongst other issues, nanoparticle exposure and ways of controlling exposure in the workplace (NIOSH, 2004