Perhaps the most overlooked issue when examining potential nano-related environmental, health, and safety concerns is whether there is any true likelihood of exposure in reasonably foreseeable use scenarios. While there should continue to be extensive toxicity testing for certain nanoscale materials, the most interesting research (from my perspective) relates to potential workplace and/or condumer exposure in realistic settings. We examine two studies along these lines below.
C. Su-Jung et al., "Control of Airborne Nanoparticles Releases During Compounding of Polymer Nanocomposites," 3 Nano: Brief Reports and Reviews 4, 301 – 309 (2008).
This study was conducted by researchers at the National Science Foundation-funded Center for High-Rate Nanomanufacturing at the University of Massachusetts at Lowell. The scientists examined potential nanoparticle release related to the twin-screw extruder compounding of polymer nanocomposites. The test was conducted because "commercial compounding (mixing) of nanocomposites is typically achieved by feeding the nanoparticles and polymer into a twin-screw extruder, the airborne particles associated with nanoparticles reinforcing agents are of particular concern, as they can readily enter the body through inhalation."
The nanoparticles in question were nano aluminum oxide particles acquired from Nanophase Technologies in commercially available form. The particles were spherical in shape and ranged from 27 to 53 nm in diameter. They were also specifically "engineered to form agglomerates with a nominal size of 200 nm."
Regarding the test itself, the scientists fed 2.3kg of polymer pellets and 0.16 kg of nano-alumina particles into a twin-screw extruder for processing and then measured potential nanoparticle release through two measurement techniques: (i) TSI Fast Mobility Particle Spectrometer for real time measurement; and (ii) personal air sampling using a special filter media designed to catch nanoparticles.
The study concluded that "[t]he twin-screw extrusion process for compounding polymer nanocomposites tends to break up nanoparticle aggregates and mechanically disperse particles thoroughly during the extrusion process." The study also found that "[nano]particle diffusion was enhanced by . . . poorly-performing local and general exhaust systems."
Interestingly, for part of the test the scientists applied a nominal engineering control by covering the open top of the extruder feeding tube throat with aluminum foil which they found "dramatically reduced" nanoparticle measurements. They also found that consistently cleaning the lab after each use "reduced laboratory background nanoparticle concentration."
D. Bello et al., "Exposure to nanoscale particles and fibres during machining of hybrid advanced composite containing carbon nanotubes," 11 J. Nanopart Res 231 – 249 (2009).
The researchers in this study investigated whether and to what extent airborne nanoparticles were generated by wet and dry cutting of two hybrid carbon nanotube composites. The dry cutting method employed a diamond coated band saw. The wet cutting was performed using a diamond grit rotary cutting wheel with water lubricating the cutting surfaces during the process. Because the scientists were interested in potential "worst case" scenarios, no vacuum or emission controls were used in tests.
The researchers found that wet cutting did not produce airborne nanoparticle emissions above background levels, but that dry cutting "generated statistically significant quantities of nanoscale and fine particles as compared to background (p<0.05), regardless of the composite type, . . . as expected."
Interestingly, the study also found that "CNTs, either individual or in bundles, were not observed in extensive microscopy of collected samples" for either wet or dry tests.
We will continue to track down and summarize these types of potential exposure studies. Right now, they are few and far between.