The Potential of PELS Based on Particle Size

Date: Dec 22, 2005

The emergence of nanotechnology has resulted in an increasing focus on the potential effects of particle size on the toxicological effects of airborne chemical exposures. This increased focus is not limited to nanoparticles, which are intentionally engineered to nanoscale size, but also includes ultrafine particles, which are incidentally generated at nanoscale size. A recently-issued draft NIOSH bulletin addressing ultrafine particles of titanium dioxide (TiO2) would establish mass-based recommended exposure limits (NIOSH RELs) tied to particle size, and suggests a possible shift toward the development of mass-based permissible exposure limits (PELs) tied to particle size.

TiO2 is described by NIOSH as a poorly soluble, low toxicity (PSLT) dust that is used extensively in many commercial products, including paint, cosmetics, plastics, paper, and food as an anti-caking or whitening agent. Other ultrafine particles receiving attention are welding fumes and diesel combustion products.

The draft NIOSH bulletin would establish RELs of 1.5 mg/m3 for fine particles (greater than 0.1 micrometers in diameter) of TiO2 and 0.1 mg/m3 for ultrafine particles (0.1 micrometers or less in diameter) as time-weighted averages for up to 10 hours per day during a 40-hour work week. The basis for this differentiation is explained by NIOSH as follows:

TiO2 and other PSLT particles show a consistent dose-response relationship for pulmonary responses in rats, including persistent pulmonary inflammation and lung tumors-when dose is expressed as particle surface area. The higher mass-based potency of ultrafine TiO2 compared to fine TiO2 is associated with the greater surface area of ultrafine particles for a given mass. The NIOSH RELs for fine and ultrafine TiO2 reflect this mass-based difference in potency.

On October 1, 2005, NIOSH issued a draft document for peer review titled "Approaches to Safe Nanotechnology, An Information Exchange with NIOSH." That document includes the following passage:

Experimental studies in rats have shown that at equivalent mass doses, insoluble ultrafine particles (smaller than 100 nm) are more potent than large particles of similar composition in causing pulmonary inflammation and lung tumors. Whether these effects would occur in exposed workers is not known. If engineered nanoparticles involve the same characteristics that seem to be associated with ultrafine particles, they may raise the same concerns. The greater hazard may relate to the larger number and total surface area of nanoparticles compared with that of the larger particles at the same mass concentration.

Based on our work in this area, and with the ASTM E56 Nanotechnology Committee and the US TAG representing the United States in the ISO/TC 229, Nanotechnologies initiative, we are concerned about the potential for the EU precautionary principle to filter its way into this area. Furthermore, given the status of the OSHA PELs process, there is the potential that NIOSH documents will encourage EPA involvement through TSCA.

In light of the referenced NIOSH documents, it is worth noting that some of the weakest evidence of carcinogenicity for hexavalent chromium (CrVI) presented by OSHA in connection with its pending CrVI rulemaking involved welding fume exposures, despite the presence of the ultrafine particles.

NIOSH has scheduled a public meeting on the draft TiO2 bulletin in Cincinnati on February 27, 2006, and is accepting written comments on the draft until March 31, 2006.

Finally, Cal-OSHA has scheduled a meeting on December 12, 2005 to discuss its overall approach to the development of PELs. In light of these developments, we urge those with a stake in these issues to get involved.