In the January edition of the Journal of Occupational and Environmental Hygiene (pps 1-13), there is an interesting study of the potential for nanoparticle release during the shredding of plastics reinforces with nano materials. The study is interesting not only in its conclusions, but in the differences in measurement between the various instruments and procedures used to assess the exposure.

Nano materials are added to plastic car parts in order to maintain or improve thermal stability and stiffness while reducing weight. Since up to 84% of cars are recycled, including the plastic components, it was theorized that as cars containing these new materials were shredded during the recycling process the workers involved could be exposed to released nano materials. Since nanoparticles have been shown to have a stronger impact on respiratory disease than an equivalent mass of a larger particle size, the potential exists for a hazardous exposure during this type of operation.

The research team assessed the potential for exposure by shredding specially made components. Some of the components contained one of two commonly used nanomaterials while others contained only the polymer material. The dust produced during shredding was collected and analyzed. Surprisingly, more nano-sized particles were produced when the plain materials were shredded than when the composites containing the materials were processed and no nano materials were observed in the shredder dust via electron microscopy. No mechanism for these finding were suggested, but it probably relates to the bonding strength of the nano material with the plastic when the composite is formed.

Not only are these results interesting, but so are the instruments the team used to measure the particles, and the way the function of each instrument produced a set of data, that if interpreted on its own would cause a different overall set of conclusions. The data had to be interpreted from a macro standpoint taking into account the biases caused by the measurement mechanism of each instrument in order to come up with a cohesive conclusion.

Several of the instruments used, the P-Trak, Dust Trak, and gravimetric filters with a PM 2.5 inlet did not measure particle concentrations during shredding that were different from background. A “Fast Mobility Particle Sizer” (FMPS) Spectrometer however measured particle concentrations substantially higher during the shredding of all three types of material. This is because of the small mean particle size produced. In all three tests the vast majority of particles were less than 20nm in diameter. Instruments like the P-Trak are less sensitive to particles in this size range and thus underestimated the concentrations during shredding. The research team expected larger sized nano particles to be produced because of the dimensional sizes of the two materials used in the composite. The fact that the particle size generated was smaller than expected and that the concentration was higher shredding the non-composite materials indicates that the nanoparticles are being produced by a different mechanism than the simple release of the nanomaterials used in the composites. It also illustrates that the ability to measure particle size over a wide range is essential in order to observe the entire picture of what occurs during the operation being measured. At least to me, this study indicates that the FMPS might have significant advantages and might be the instrument of choice in measuring nanoparticle generation.

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