Q: How is nanoparticle exposure measured?
The traditional exposure assessment is focused on wide size fractions, e.g., all particles below 10 μm (PM10) or 2.5 μm (PM2.5) in ambient measurements, or below 4 μm (respirable fraction) in workplace exposure assessment. These measurements determine the total mass concentration of the respective particle size fraction. The mass concentration scales with the third power of the particle diameter and therefore weights larger particles much more strongly than smaller ones. As an example, a single 10 μm particle has the same mass as one thousand 1 μm particles, one million 100 nm particles, or one billion 10 nm particles. Although nanoparticles typically occur in much higher numbers than micron-sized particles, it is obvious that they usually only contribute a very small fraction to the total mass concentration. To obtain a better representation of the presence of nanoparticles, metrics other than the mass concentration are therefore required.
Instrumentation to assess exposure to nanoparticles can be differentiated into stationary, portable, and personal equipment. The instruments can be differentiated into (quasi-)real-time instruments that deliver the results with high time resolution and particle samplers that collect particles for subsequent chemical and/or morphological analyses. In order to measure the true personal exposure sampling needs to be done in the breathing zone, i.e., within a 30 cm hemisphere around the mouth and nose of the individual [Standard EN1540:2011]. While for “classical” chemical compounds or inhalable/respirable dust, the question of exposure metric to be used is often solved beforehand (by eg looking at the respective occupational exposure limit). For nanoparticles, the metric question (e g mass-, surface-, or number concentration) is not resolved due to lack of toxicological data (Brouwer 2014). If possible (based on instrumentation limitations) as much as possible should be measured.
Exposure assessment of engineered nanoparticles faces a specific problem, as there is virtually always a quite substantial background concentration of ultrafine particles, mainly resulting from various combustion purposes, present [Kuhlbusch 2004 and 2011]. The simplest way to deal with this problem would be to directly and selectively measure engineered nanoparticles, but so far no general method for this exists. Different ways to adjust for the background have been suggested, e.g. by Kuhlbush (2009 and 2011) and Tsai (2011),
Often a so-called 3-tiered approach is recommended to assess exposure. This approach suggest that at first, the workplace is evaluated making use of available documents, like material safety data sheets, production process descriptions, physical properties of the materials in use or produced, etc. If this would result in the possibility of nanoparticle exposure, a second step would have to be performed. This would make use of direct-reading equipment (“monitoring”), using personal devices with either number concentration or surface area concentration as a metric, and possibly some accompanying sampling of particles on a suitable substrate for subsequent qualitative analysis. Depending on the nature of the engineered nanoparticle in question, a variety of methods could be used for that purpose, like electron microscopy in connection with qualitative spectroscopic analysis or ICP/MS. If the obtained information on the nature/quantity of the risk still is insufficient, more and better techniques for assessment are needed. This tier 3 assessment would make use of much more elaborate equipment like particle size distribution measurements, sampling for different mass-based analytical techniques, etc.
Q: How do you work with something that is potentially hazardous?
A: When the risks of a material are not yet established, one should use precaution. That means that you act as if the material in question is toxic. Rational risk management is to first assess, analyze, and evaluate the risks, and then – based on what you know – take appropriate precautions, such as using certain protective equipment. If the risk assessment results in uncertainty, meaning that it is not clear how dangerous something is, you should act based on the “worst” option and take adequate precaution.
Kuhlbusch TAJ, Neumann D, Fissan H (2004) Number size distribution, mass concentration and particle composition of PM1, PM2.5, and PM10 in bag filling areas of carbon black production. J Occup Environ Hyg 1:660–671
Kuhlbusch TA, Asbach C, Fissan H, Göhler D, Stintz M (2011) Nanoparticle exposure at nanotechnology workplaces: a review. Part Fibre Toxicol 8:22
Kuhlbusch TAJ, Fissan H, Asbach C (2009) Nanotechnologies and environmental risks – measurement technologies and strategies. In: Linkov I, Steevens J (eds) Nanomaterials: risks and benefits. Springer, Berlin, pp 233–243
Tsai C-J, Huang C-Y, Chen S-C et al (2011) Exposure assessment of nano-sized and respirable particles at different workplaces. J Nanoparticle Res 13:4161–4172