When to monitor
There is currently no consensus on which is the most appropriate metric or method to measure nanomaterial in the workplace. However, what is critical is that the background level of nanomaterials is established before any production or processing of the nanomaterial is started. This is because there is a natural background level of nanomaterials in the air, the amount of which will depend on the location.
One of the general principles of risk management includes taking measures to prevent or minimize exposure of workers to nanomaterials and their releases into the environment. Monitoring to assess whether potential exposure occurs and the engineering controls are adequate, is important.
Limitations to monitoring
The monitoring of airborne nanomaterials in the workplace is challenging due to the lack of portable and personal instruments selectively sensitive to the engineered nanomaterials and the presence of a background of non-engineered nanomaterials which can fluctuate. Background is defined as airborne particles present in the workplace and differ from those engineered nanomaterials / nanomaterials released during manufacturing, use or handling. It will include "ultrafines" originating from different sources including urban pollution.
Limitation of Instrumentation
There are limitations to this method, which are not fully understood and research is being carried out in this field. However, the protocol described below is "current best practice". It can be used to detect the presence of engineered nanomaterials in combination with the assessment of engineering control measures in order to mitigate exposure to nanomaterials in the workplace.
The sampling method described below is a pragmatic approach. It uses hand-held Condensation Particle Counters (CPC) and Optical Particle Counters (OPC) combined with the collection of a limited number of samples for Transmission Electron Microscopy (TEM) analysis. Hand-held CPCs and OPCs measure particle number concentrations in the size range from 10nm to 1um and 0.5 um to about 15um or greater respectively. They are portable, easy to use and cost effective. These are fast response instruments and are more likely to detect fugitive or transient nanoparticle release.
This sampling method is based on the Nanoparticle Emission Assessment Technique (NEAT) approach developed by the NIOSH. It requires at least a Condensation Particle Counter (CPC) (CPC1), an Optical Particle Counter (OPC) and a sampler for subsequent electron microscopy analysis. All these instruments should be placed close to the task. Two other CPCs (CPC2 positioned away from the task / process and CPC3 a mobile CPC) can also be used.
Real time measurements
Measurements using CPC1 and the OPC should be carried out before, during and after the activity under study takes place. CPC1 and the OPC are stationary and positioned close to the worker's task (within an approximate 1m radius of the worker's head) taking care that they do not hinder or interfere with the workers' normal duties. Non-activity periods (before and after the activity period) should be monitored for at least 15 minutes if possible.
Measurements using CPC2 should be carried out before, during and after the activity under study takes place. CPC2 is stationary and should be located at a distance from the activity, such that it measures airborne particle concentrations that are representative of the background concentration near the activity. A distance of at least 2 m is suggested. The non-activity periods (before and after the activity period) should be monitored for at least 15 minutes if possible.
OPC should be used to briefly monitor concentrations away from the task/process before and after the activity takes place.
Be aware that any other extraneous sources of non-engineered nanomaterials such as: passing lorries/fork lift trucks, electric motors, smoke-generating systems, welding/soldering activities, open doors and windows can influence particle concentration readings greatly. CPC3 should be used to investigate any other potential sources of non-engineered nanomaterials and if possible these should be isolated or stopped during the monitoring period. CPC3 could also be used with the telescopic probe attachment to monitor particle number concentration inside containment/fume cupboards during activity periods.
Smoke tubes, for the testing of fume-cupboards or local exhaust ventilation (LEV) efficiency, should not be used during the monitoring of the activities. It has been shown during previous studies that these can be a source of very high concentrations of airborne non-engineered nanomaterials.
All instruments should be calibrated at least every year and regularly checked to ensure consistent operation especially their performance to each other if several of the same instruments are used.
Collection of samples for electron microscopy analysis
A number of sampling techniques for the collection of airborne particles and subsequent electron microscopy analysis are available and include:
- Filtration onto filters or carbon films supported on transmission electron microscopy (TEM) grids using conventional sampling pump. TEM grids with a holey carbon film can be attached to filters. Filters can be pre- coated with gold for subsequent scanning electron microscopy (SEM) analysis.
- Precipitation using thermal or electrostatic precipitators.
A sampler should be positioned next to CPC1 and CPC2 (optional). Samples collected inside containments/fume-cupboards are also very useful for comparison with samples collected outside containments/fume-cupboards.
Record and contextual information
The times at which the monitors and samplers were started and stopped together with the sampler flow rates should be recorded. It is also critical that detailed contextual information of all activities before, during and after the task/process takes place are recorded as an increase in particle number concentrations from the real-time instruments may be unrelated to the task / process.
Interpretation of results
Particle number concentration should be plotted and arithmetic means, minimum and maximum concentrations before, during and after the task / process should be calculated. A difficult question to answer is if an increase in particle number concentration means there has been a corresponding emission of engineered nano-objects from the task/process. For that, the "task /process" particle number concentration must be higher than the "background" particle number concentration and this increase has to be statistically significant. However some critical judgement should also be applied. The background may greatly fluctuate or it can gradually increase or decrease with time. The contextual information is important in this decision-making as well as knowing whether other sources of non-engineered nano-objects are present. Finally, a positive off-line identification of the engineered nano-object confirms an emission from the task / process.