The National Institute for Occupational Safety and Health (NIOSH) announced on July 10, 2019, the availability of a new Technical Report, The NIOSH Occupational Exposure Banding Process for Chemical Risk Management. NIOSH describes occupational exposure banding as “a voluntary process that assigns each chemical to a category based on its toxicity and any negative health outcomes associated with exposure to that chemical.” The Technical Report “provides a process with easy procedures and clear rules for assignment and can be used in a broad spectrum of workplace settings.” Section 3.14, “Consideration of Special Categories of Aerosols,” includes recommendations for liquid aerosols; fibers, including nanofibers; and nanoparticles:
- Fibers: The Report notes that fibers and other high-aspect-ratio particles “have unique aerodynamic features that are dependent on their geometry (dimensions) and that influence their deposition in the respiratory tract.” In addition, the physical shape and size of fibers can directly influence their toxicological properties and the nature of their interactions with target cells. The Report states: “These complexities require using a Tier 3 assessment for fibers, and the [occupational exposure band (OEB)] Tier 1 and Tier 2 criteria are not recommended. Some hazard banding frameworks for nanomaterials recommend assigning the most stringent band for bio-persistent, rigid nanofibers.”
- Nanoscale solid-phase particles: The Report describes empirical data and mechanistic hypotheses that have been used to support application of the hazard banding procedures within control banding schemes for engineered nanoparticles (g., as applied in various national standards). Using the same rationale, NIOSH recommends that the occupational exposure banding process be modified as follows when applied to nanoparticles:
- Poorly-soluble nanoscale particles: If the toxicity data include no observed adverse effect levels (NOAEL) that were developed specifically for the nanoscale form of the chemical substance, then the NIOSH occupational exposure banding process can be used with no modifications;
- If data are available for only the microscale form of the chemical substance, then the band assignment should be shifted to the next more stringent band, on the assumption that poorly soluble nanoscale substances will likely be more toxic than their microscale equivalents (g., by an order of magnitude). The Report notes that some other banding schemes also recommend a more stringent band (to reduce exposure by an order of magnitude) when data are available on only the microscale form of the substance; and
- Soluble nanoscale particles: The Report states that data support an association between increased total particle surface area and increased toxicity for poorly soluble nanoscale particles. As particle solubility increases, there may be less need for the OEB to account for enhanced toxicity due to the nanoparticle-specific characteristics. The Report notes that in the French Agency for Food, Environmental and Occupational Health and Safety (ANSES) and International Organization for Standardization (ISO) control banding schemes, soluble particles are addressed with regard to the toxicity of the solute, without consideration of nanoparticle-specific toxicity. Given the uncertainties in the relationship of solubility to particle toxicity, however, NIOSH “recommends that in the absence of data to the contrary, all nanoscale particles should be treated in the same manner without regard to solubility.” If data are available only for the microscale form of the agent, NIOSH recommends shifting the banding assignment to the next more stringent band.
The Report cautions that because the toxicity of nanoscale fibers and nanoscale tubes may differ substantially from other forms of the compound, the occupational exposure banding process described “may not fully and accurately capture the toxicity of these chemical substances.” NIOSH states that Tier 1 and Tier 2 should not be used and instead a Tier 3 assessment is required as described for other fibers. According to the Report, NIOSH is currently evaluating the state of the science for deriving occupational exposure limits (OEL) and OEBs for nanomaterials and is also examining the process and data for developing hazard categories for nanomaterials based on biological mode of action and physical-chemical properties.