Carbon nanotubes represent a new form of carbon that has several unique properties, including great tensile strength, high conductivity (in some states), high surface area, unique electronic properties, and potentially high molecular adsorption capacity. A number of laboratories are generating small quantities of this material, and commercial interest in the substance is motivating the rapid development of large-scale production facilities. However, little is known of the potential toxicity of the material, or how it should most appropriately be handled to minimize exposure. In a collaborative effort the National Institute for Occupational Safety and Health (NIOSH), National Aeronautics and Space Administration (NASA), Rice University, and Carbon Nanotechnologies Inc. (CNI) are investigating the nature of the aerosol released when unrefined nanotubes are handled-that is, the material that is aerosolized during the production process, prior to its being purified. Two separate studies have been undertaken on single-walled carbon nanotubes (SWCNT)-a specific form of the material comprised of carbon tubes around 1.5 nanometers (nm) in diameter and up to a millimeter or more in length, and having a single layer of carbon atoms that form the tube wall. Two sources of the material were considered: (1) laser ablation, which leads to a relatively compact powder, and (2) nanotubes formed using the High-Pressure Carbon Monoxide process (HiPCO®), leading to an expanded material that has very low bulk density. Primarily, the propensity for the material to form an aerosol while being agitated was investigated in the laboratory. Aerosol size distribution between 4 nm and 20 µm was measured when the material was agitated in a number of ways. At high agitation levels, a bimodal or trimodal aerosol was formed below 10 µm, depending on the material source, and HiPCO®-generated material led to the release of particles smaller than 10 nm in diameter. However, it was unclear whether these particles represented nanotubes, catalyst particles (used in the manufacturing process), or compact carbonaceous particles. Generation rates were typically two orders of magnitude below those for a similar volume of fumed alumina-chosen because it represents another material that has a very low bulk density and is formed from nanometer-sized primary particles. The second study investigated dermal and airborne exposures to nanotubes when unrefined material was handled. Material from the laser ablation and HiPCO® processes was removed from production vessels and handled in a clean air enclosure, while aerosol number and mass concentration were continuously monitored. Personal and area filter samples were also taken. Interpretation of the data is somewhat tenuous because instrument response to nanotube particles is not known. However, the results did not indicate appreciable increases in mass concentration when the nanotubes were handled, and number concentration actually decreased during the handling period. When a vacuum cleaner was used to clean up nanotube material, the aerosol concentration increased dramatically, possibly due to inefficient or incorrectly installed backup filters. It was not clear whether the particles detected were nanotube-based or arose from the vacuum motor, although it would seem prudent to exercise caution when cleaning up the material in this way. Large numbers of airborne nanotube clumps were visible to the naked eye, although the visible aerosol and mass concentration readings were not correlated. Analysis of filter samples was confounded by low collection rates, although an estimate of the overall airborne concentration of nanotubes was below 100 µg/m3. As these samples were dominated by a few large particles on the filters, we speculate that the respirable mass concentration was significantly lower. The levels of nanotube material detected on gloves after the material was handled ranged from 0.4 mg to 7 mg. Overall, both studies confirmed that at low agitation levels very little nanotube material becomes airborne, and that the individual nanotubes and associated catalyst particles are difficult to separate into isolated particles. However, caution is advised in handling the material until more is known about its toxicity.
NIOSH, 4676 Columbia Parkway, Cincinnati, OH 45226