Nanomaterials: Assessing workplace exposure and controls

17-Nov-2015

The UK Health & Safety Executive (HSE) recently commissioned a project to improve understanding of the growing nanomaterials industry in the UK and employees’ potential exposure to materials at the nanoscale. This article provides an overview of the project and a brief summary of the findings

Nanotechnology is moving from the research environment into wider application in the workplace. Across the world there are companies large and small manufacturing or using nanomaterials, even though the scientific community does not, as yet, have a good understanding of whether working with nanomaterials poses a risk to workers’ health.

As a result, the UK Health & Safety Executive (HSE) decided to commission a project that would provide an improved understanding of the UK nanomaterial industry and of worker exposure to engineered nanomaterials. To do this it carried out visits to willing companies manufacturing or using these materials, to assess exposure levels to airborne nanomaterials and to review the effectiveness of the controls used.

The findings, prepared by the Health and Safety Laboratory (HSL) for the HSE, were recently published under the title: Summary of work undertaken to assess workplace exposure and control measures during the manufacture and handling of engineered nanomaterials*, and extracts have been used here.

While the HSE and HSL made attempts to identify and engage with several companies that manufactured or used nanomaterials, only four volunteered to take part in the project, and so the report cautions that the observations represent a limited data set, and need to be understood in this context and not over-generalised.

The following outlines briefly the methodology of the project along with its limitations and the findings and potential implications for future work.

Methodology

A site visit to each company included an assessment of the effectiveness of the local exhaust ventilation (LEV) systems by an experienced HSL occupational hygienist, as well as completion of a contextual questionnaire, and a record of activities undertaken at the time of the sampling and exposure monitoring work.

While on site, a site-assessment protocol for short-term monitoring of airborne nanomaterials using hand-held, real-time, measurement instruments and particle-sampling devices was prepared. The methodology was designed to take into account the needs of small and medium enterprises (SMEs) but was also applied to larger workplaces.

Control assessment: Each site visit included a control assessment by an experienced HSL occupational hygienist who carried out basic evaluations of:

  • Risk assessments as required under the Control of Substances Hazardous to Health (COSHH) Regulations (where available)
  • The effectiveness of the LEV systems (using dust lamps and anemometers)
  • Personal protective equipment (PPE) worn
  • Completion of the questionnaire

Short-term, airborne monitoring: Aerosol measurements were mainly based on the use of hand-held, real-time particle counters – Condensation Particle Counters (CPC) and Optical Particle Counters (OPC) – with the collection of a limited number of samples for chemical and physical off-line analysis by electron microscopy (EM). The combined size range covered by the CPC and OPC instruments was 10–20,000nm (20µm).

Activities at the time of the measurements were documented to better understand the background fluctuation in airborne particle concentration with time. This ‘background’ differs from the engineered nanomaterials released during manufacturing or handling processes and can include ultra fine particles (<100nm diameter) from industrial and environmental pollution of the air.

Real-time particle measurement of particle surface area: In addition to the number of concentration instruments, a portable real-time instrument, the Aerotrak 9000, was used to measure the surface area of the aerosol particles in the range of 10–1,000nm, which could deposit in the gas-exchange (alveolar) regions of the lung.

Comprehensive airborne measurements: This involved the deployment of additional research-type instruments with a superior size classification and resolution and included a Scanning Mobility Particle Sizer (SMPS) and an Electrical Low Particle Impactor (ELPI).

Across all four companies a range of tasks were monitored, including production preprocessing and processing of nanomaterials, bagging, handling (e.g. weighing, moulding and machining), material recovery, emptying of powder collected in LEV system bins, maintenance and cleaning activities. Most of the tasks monitored involved manufacturing processes or handling of powders, with the amount of material handled ranging from hundreds of grams to more than 100kg. It should be noted that none of the tasks or processes monitored in this project involved the manufacture or handling of nanotubes or nanofibres, or the machining of composites containing nanomaterials.

Engineering controls: Most of the activities that related to the handling or production process of nanomaterials in a powder form employed some degree of engineering controls. These ranged from an extracted and enclosed process (one company) to the use of capturing (captor) hoods. During the visits, dust lamps were used to visualise airborne particles generated by the processes observed and any leakage.

Results

From both the occupational hygiene assessment and the measurements, it was found that, in general, appropriate and effective controls were in place for production processes except for a poorly designed LEV at one company. However, from the measurements, it was observed that short-term release of nanomaterials could take place during maintenance work or when emptying powder collected in LEV system bins. When a release during powder-handling activities was identified, the particles, in some circumstances, agglomerated to larger particles.

Personal Protective Equipment: PPE worn at the companies was, on the whole, appropriate for the work carried out. Respiratory Protective Equipment (RPE) offering protection against particles was provided at three of the four companies. RPE used included FFP2 or FFP3 disposable respirators and TH2 powered-hood devices. Where companies considered it was appropriate, RPE was used in addition to LEV or where no local engineering control was in place (e.g. during powder recovery, LEV/process system bin emptying or maintenance work). Of the three companies using RPE only one had carried out face-fit testing of some their staff. The other two companies had not conducted face-fit testing.

Assessment of the short-term monitoring approach: The use of a short-term exposure monitoring approach helped demonstrate that only a fraction of the airborne particles were likely to have originated from the handling of the nanomaterials. Using several portable, hand-held, real-time instruments covering a size range of nanometres (10 or 20) to several micrometres (at least 10µm) was a practical approach to assess short-term emission from processes. However, real-time instruments do not yet provide chemical or morphological information on the particles and hence it was necessary to assess these air samples using EM.

In addition to the measurements a parallel hygiene assessment was considered to be essential to check the appropriateness of the control measures being used.

Technical limitations of the short-term monitoring approach: In the workplaces visited, measuring emissions of engineered nanomaterials (i.e. in terms of number concentrations) from the processes and tasks observed was difficult for the following reasons:

  • There was substantial spatial and temporal variation to background levels of ultrafine1 particles in the size range below 600nm
  • High background levels of ultrafine particles (above 100,000 particles/cm3 between 20nm and about 1,000nm) were measured using a CPC
  • Industrial and traffic emissions of ultrafine particles below 600nm (e.g. combustion and diesel particles) contributed to this background
  • Other sources of nanoparticles present in the workplace may have been emitted from machinery, for example, diesel engine exhaust emissions from lift trucks operated inside the premises
  • There were low emission concentrations from the production tasks or processes monitored.

The use of an OPC instrument in addition to a CPC instrument was beneficial. In some circumstances, when monitoring powder-handling activities, the OPC was useful as the engineered nanoparticles agglomerated to form larger particles and were detected using the OPC rather than the CPC.

Overall findings

The project revealed that some companies perceived there to be no specific risks with the materials they were handling and therefore had not carried out training specific to working with nanomaterials. The project did, however, find that applied good control practices can be used to reduce exposure to airborne nano-materials. The report suggested it is therefore important that in any study, all control methods used are thoroughly assessed.

The findings indicate that an effective risk management assessment strategy could include a combination of a simple exposure monitoring approach and an occupational hygiene assessment of the process and the controls.

With improvements in technology over the past few years, approaches to the characterisation of airborne nano-materials from samples collected in workplaces have moved towards semi-quantitative rather than qualitative techniques

This strategy could be used to evaluate which tasks give rise to potential emissions before committing to expensive monitoring. However, challenges remain with regard to measuring emissions of, or exposures to, specific nanomaterials where background levels of ultrafine particles are high or fluctuate. The project also found that the company COSHH assessments were not specific to nanomaterials and all of the assessments reviewed could have been improved.

Furthermore, while RPE offering protection against particles was provided at three out of the four companies visited, of the three companies using RPE only one had carried out face-fit testing of some their staff. The other two companies had not conducted face-fit testing to ensure the PPE would work effectively.

From both the occupational hygiene assessment and the measurements, it was found that in general the LEV systems in place were appropriate and effective except for a poorly designed LEV system at one company.

From the measurements, it was observed that short-term release of nanomaterials could take place during maintenance work or when emptying powder collected in LEV system bins.

Benchmark/exposure limits

Proposed benchmark or exposure limits for airborne nanomaterials in the workplace based on numbers have been defined for ‘primary’ or free non-agglomerated/non-aggregated nanoparticles.2 However, the project highlighted the following problems associated with using these benchmark or exposure limits:

  • The OPC can only measure agglomerates and the CPC will measure both primary particles and agglomerates. In the presence of agglomerated nanoparticles (e.g. when a release occurs during the handling of powders containing nanoparticles), these instruments will count agglomerate made of many ‘primary’ nanoparticles as one particle, thus making it impossible to quantify the total primary number of particles.
  • The relatively low emissions, compared with high background levels and fluctuations found at several of the visited sites, limited the use of a simple subtraction of the background particle number concentration from the total particle number concentration to obtain the specific engineered particle number concentrations.

Even though health-related mass concentrations were not measured during this project, personal respirable- and inhalable-mass concentrations could also be evaluated, especially when monitoring powder-handling activities because nanoparticles aggregate and agglomerate.

Further characterisation and quantification of the elements, for example, by X-ray fluorescence or Inductively Coupled Plasma Mass Spectrometry could also be performed for some types of nanomaterials.

Characterisation of nanomaterials

With improvements in technology over the past few years, approaches to the characterisation of airborne nano-materials from samples collected in workplaces have moved towards semi-quantitative rather than qualitative techniques. The HSE says there is a need for more research in this area, particularly semi-quantitative analysis of particles by EM and the harmonisation of reporting results.

Where the situation merits further investigation, a long term monitoring programme (e.g. over several days) could be an option but very few researchers have investigated the potential benefits. Difficulties arising from a fluctuating background of ultrafine particles could be addressed by a long-term monitoring approach and this could smooth any short-term variation of the background number of concentrations.

The report says that in the future, as monitoring instruments are miniaturised, there could be more emphasis on the personal sampling of workers, but the short-term monitoring strategy currently gives a practical and cost-effective approach to monitor emission of airborne nanomaterials. It could be used to evaluate whether tasks or processes lead to potential emissions before committing to expensive monitoring. In some circumstances, use of real-time monitors could allow the immediate detection of leaks or release from processes.

In conclusion, the HSE notes that this project represents a very small sample of the industry and a finite selection of nanomaterials and, as such, gives only a limited view of industry across the UK. However, in view of the increased use of nanomaterials it provides a valuable insight with a view to future work.

References

1. The term ‘ultrafine particles’ describes particles at the nanoscale that are unintentionally produced or naturally occurring; they are usually generated from combustion processes. This can include particles from industrial and environmental pollution of the air.

2. Van Broekhuizen et al (2012). Ann. Occup. Hyg., 56, No 5, pp 515–524, 2012

*The full report RR1068 – Summary of work undertaken to assess workplace exposure and control measures during the manufacture and handling of engineered nanomaterials is available to download from the HSE website at http://www.hse.gov.uk/research/rrhtm/rr1068.htm.

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