On March 15, 2017, the National Institute for Occupational Safety and Health (NIOSH) published a Federal Register notice announcing the availability of a draft Current Intelligence Bulletin (CIB) entitled The NIOSH Occupational Exposure Banding Process: Guidance for the Evaluation of Chemical Hazards for public comment. Under occupational exposure banding, chemicals are assigned to specific categories or bands based on the chemical’s potency and the negative health outcomes associated with exposure to the chemical. An occupational exposure band (OEB) corresponds to a range of exposure concentrations that is expected to be protective to worker health. NIOSH states that it recently developed a process to apply the occupational exposure banding process to a broad spectrum of occupational settings. The NIOSH process uses available, but often limited, toxicological data to determine a potential range of chemical exposure levels that can be used as targets for exposure controls to reduce risk among workers.
Subchapter 3.15 of the draft CIB, “Consideration of Special Categories of Aerosols,” notes that some particles have unique physical characteristics that support modifications to the general occupational exposure banding process. According to the draft CIB, modification is necessary to address the observation that the total mass dose delivered does not always describe well the dose-response behavior for a single chemical across all particulate sizes and forms. Examples of particle categories include nanoscale solid-phase particles. For the purpose of the draft CIB, nanoscale particles are defined as those particles with primary particle diameters less than 100 nanometers. NIOSH recommends that the occupational exposure banding process — when applied to nanoparticles — be modified according to the following guidelines:
- Poorly-soluble nanoscale particles: If the toxicity data include no observed adverse effect levels that were developed specifically for the nanoscale form of the chemical, the NIOSH occupational exposure banding process can be used directly with no modifications. If data are only available for the microscale form of the chemical, the band assignment should be shifted to the next most potent band on the assumption that poorly soluble nanoscale agents will likely be an order of magnitude more toxic than their microscale equivalents.
- Soluble nanoscale particles: Data support a role of increased total particle surface in the increased toxicity associated with poorly-soluble nanoscale particles, as discussed above. Thus, because the retained surface area is lower over time for soluble particles (due to dissolution), increased solubility would decrease the potency of particles if the adverse effects are due to the retained particle surface dose. On the other hand, higher solubility could result in increased potency (compared to poorly soluble particles) if the toxic effects are due to released ions. In the Agence nationale de sécurité sanitaire and International Standards Organization control banding schemes, soluble particles are addressed with regard to the toxicity of the solute, without consideration of nanoparticle-specific toxicity. The draft CIB states that acceptance of these general conclusions requires caution, however, because of limited data on which to evaluate their effectiveness. According to the draft CIB, it is recommended that in the absence of data to the contrary, all nanoscale particles should be treated in the same manner without regard to solubility. NIOSH recommends shifting the banding assignment to the next most potent band if data are only available for the microscale form of the agent.
- Nanoscale fibers (or tubes): Since the toxicity of nanoscale fibers and nanoscale tubes may differ significantly from other forms of the compound, the occupational exposure banding process may not fully and accurately capture the toxicity of these chemicals. Therefore, Tier 1 and Tier 2 should not be used. Instead, a Tier 3 assessment is required as described for other fibers.
The draft CIB states that these general recommendations are considered precautionary in nature. According to the draft CIB, limitations in the available scientific information include uncertainty in the mechanisms of potential potency differences in toxicity of nanoscale vs. microscale particles of various chemical composition, surface properties, shape, and degree of agglomeration. The draft CIB notes that the number of chemicals with adequate data for such size-based toxicity comparisons is small, preventing firm conclusions at this time about relative potencies among various particle types and sizes. NIOSH is currently evaluating the state of the science for deriving occupational exposure limits or 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.
The public meeting will be held May 23, 2017, in Cincinnati, Ohio. Registration to attend the meeting in person or via remote participation is due April 21, 2017, and April 7, 2017, for non-U.S. citizens who wish to attend in person. Comments are due June 13, 2017.