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December 23, 2008

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Carbon nanotube (CNT) health and safety

How do you regulate in a world of unknowns? Jim Noonan provides an overview of the work carried out in the area of carbon nanotubes, and cautions organisations that work with such substances to keep a close eye on developing research.

In May 2008, the BBC website published a report with the ominous-sounding headline: ‘Asbestos warning’ on nanotubes.1 It referred to the potential risks associated with carbon nanotubes (CNTs) — cylindrical particles effectively comprising a rolled-up sheet, or sheets, of a matrix of carbon60 atoms. However, the headline was considerably tempered by a quote further down the story from Professor Ken Donaldson, one of the authors of the research paper on which the article was based. Quite unequivocally, the professor stated: “We are a long way from saying that any form of carbon nanotubes causes mesothelioma.”

What the research paper had, in fact, identified was inflammation and fibrosing of the lining of the abdomen walls of test mice.2 In an experiment, the mice had been injected with differing types of CNTs and asbestos fibres. There appeared to be a correlation between the aspect ratio of the fibres and the biological effects for both asbestos and CNTs, while a control group injected with nanoscale, carbon black particles failed to display the same symptoms.

So, while the paper appears to demonstrate the potential for CNTs to produce biological effects similar to those of asbestos fibres, the BBC story did not address in any detail the extent to which these results could be extrapolated to human exposure to CNTs.

The occupational health debate surrounding nanoparticles (particles with one or more dimensions less than 100nm, or 100 millionth of a metre) started in earnest in 2004, when the HSE produced a research report considering the occupational-hygiene implications of the new technology,3 and the Royal Society presented to government its report on the opportunities and risks of the technology.4 The HSE view at the time was that the existing regulatory framework is adequately equipped to deal with the emerging materials and processes. It warned, however, that further research was required on the toxicology, kinetics, and metrics of these materials in order to continue to maintain adequate control of any occupational risk they might present.5

Similarities with asbestos

It is certainly inviting to make a comparison between asbestos and CNTs, as both have been heralded for their superlative properties. In the case of asbestos, these include: its inertness and consequent resistance to combustion, insulating properties, low density, high tensile strength, and its ability to be contained in various physical matrices. Individual nanotubes can have a diameter as small as 1nm, while being up to 3mm in length,3 and they possess the same chemical inertness and insolubility as the silica compounds that constitute the asbestos mineral range.

The adverse effects of asbestos are founded on the capacity for its long and thin fibres to deposit in the respiratory tract, particularly the gas-exchange areas of the lung, and for the lungs’ inability to remove them. Various mechanisms, such as oxidative stress, collagen deposition, and iron chelation and translocation may be triggered by the presence of the fibres, and could result in the development of fibrous, inflexible tissue, resulting in pleural plaques, or pleural thickening and asbestosis. Other outcomes include genotoxic damage, leading to lung cancer or mesothelioma.6, 7

When considering the relationship between CNTs and asbestos, it is important to note that these effects vary for the different types of asbestos, and that these differences are related to the properties of the fibres themselves. Professor Donaldson’s study sought to distinguish between the known differences of effect caused by fibre morphology by using both long and short asbestos fibres, and CNTs. His results indicate the need for further research and precaution because, for both materials, the development of lesions, granulomas and the biological indicators for these effects were significant in the mice exposed to the longer, higher aspect-ratio CNTs.

This leaves a set of questions for occupational health and safety researchers, practitioners and regulators to consider:

Where is current research heading?

Who is currently, or potentially, exposed to CNTs?

What prevention and protection is effective, and how should it be put into place? and

How will the regulatory framework deal with CNTs?

A funding imbalance

The Council for Science and Technology (CST) report reviewing UK government progress on nanotechnology highlighted earlier this year that “the balance between research that develops new applications of nanotechnologies, and that which provides the necessary underpinning for its safe and responsible development, must be addressed”.8 This is made more apparent when you consider the Government’s contribution to health and safety nano-research amounts to £3 million, whereas its contribution to nano-development and commercialisation totalled £340m over five years.

This is important in the light of Professor Donaldson’s study. If CNT product development, production and distribution increase exponentially in the absence of adequate toxicological research, and there is a real health risk from CNTs, then there is a possibility of scientifically-based control and regulation evolving at a slower pace than significant exposure. However, a sense of scale should be maintained — worldwide asbestos annual production is currently over 2 million tonnes,9 while current estimated CNT production is in the order of a few thousand tonnes.10

Gaps in the data

Data on production and numbers of workers exposed to CNTs is lacking in detail. The HSE estimated in 2004 that about 2000 people in universities and product-development laboratories were exposed to manufactured nanoparticles, but calculating what proportion of these would be working with CNTs is difficult.

Owing to a similar paucity of information on toxicity, it is also difficult to define exposure limits. This is coupled with problems of metrology. There is general consensus that it is not simply nanoparticles’ immediate bulk chemistry, or mass fraction, that determines their toxicity but their surface area and particle number.3, 4, 11 As particles become smaller a much higher proportion of their constituent atoms are “exposed” than in larger particles, and this is thought to affect their surface reactivity and bonding behaviour. Hence, future research may prove it necessary to measure the relative surface area, instead of fibre count, if it emerges that any potential toxicity is more closely linked to the former. The current view is that all parameters should be measured and taken into account when considering exposure measurement, or setting limits.12

Measurement problems

To facilitate workplace measurement, different technologies need to be developed into affordable and useable devices. Current asbestos fibre-counting techniques, for example, use phase-contrast microscopy, which enables fibre detection to about 0.2 micron in diameter.13 However, counting CNTs with a diameter of about 0.001 of a micron (1nm), and possibly also assessing their surface area, would require different techniques either in the laboratory after sample collection, or within the place of exposure.14

HSL research in 2006 reviewed the effectiveness of equipment in measuring mass, concentration, surface area, and count in nano-aerosols.12 No CNTs were used in these tests, but a number of aerosol shapes was deployed. This paper concluded that, for nanoparticles in general, separate parameters of mass, count, size, and surface area should continue to be used in combination for the assessment of toxicity and exposure, and that work should continue to develop devices that could integrate these measurements.

With regard to control, nanoparticles should respond to enclosure and extraction systems suitable for gases and vapours, subject to the usual provisos regarding design, operation, and maintenance. CNTs also have a tendency to agglomerate into larger particles, so gravimetric behaviour will be a factor to be considered in the calculation of air flows and capture distances.15

Research on filtration seems to have focused on less fibrous nanoparticles than CNTs. However, CNTs’ aspect ratio and capacity for agglomeration should mean that appropriately designed and specified filters will be effective. What remains unclear is the propensity for CNTs or other nanoparticles to ingress into respiratory protective equipment via the face seal.

Meaningful guidance

With the absence of hard toxicology data and, as yet, problematic measurement, those managing workers exposed to CNTs in the workplace have had little solid data and guidance to assist them, and have needed to resort to precautionary principles. This situation is also changing, and there is now meaningful guidance emerging from various bodies, including BSI, NIOSH, and the HSE.

In 2007, the BSI published two documents, giving practical guidance.14, 16 One of these dealt with general control strategies, and the other with technical aspects of exposure measurement. The former gives a useful classification scheme and has attempted to provide a guidance exposure limit for each (see table).

This document is clear, concise, and underpinned by the fundamental principles embodied in the COSHH Regulations. It predicates its guidance with the principle that any substance in its nano-form should be assumed to be more hazardous to health than its non-nano parent material, unless it is known to be otherwise.

The exposure limits given are due to be presented to the Working Group on the Assessment of Toxic Chemicals (WATCH) for review. Perhaps this will be the embryonic stage of a real exposure standard, though it will be some time before metrology equipment and techniques will be widely accessible. Practitioners dealing with exposure will need to seek advice on measurement, although it is worth noting that the BSI document provides a useful table of the relevant techniques and equipment, and their pros and cons.

Both the BSI guidance and advice from NIOSH15 point strongly to the need for

some kind of health surveillance to be deployed for workers exposed to these particles. This issue highlights the problem of regulation in a world of unknowns.

For example, the issue of health surveillance under regulation 11 of COSHH is open to question in the case of CNTs. While CNTs might qualify as a hazardous substance owing to their dustiness, and thereby require suitable control, employers are required to provide health surveillance where there is an identifiable disease arising from exposure; exposure is of a type likely to give rise to the disease; and there are recognised methods for detecting the disease.

Presently, these criteria are not met, and a cynical employer, or an ill-advised one, may choose to do nothing. Consequently, both the BSI and NIOSH emphasise the need to keep good records of substances used, those exposed to them, the nature of operations, and the extent of likely exposure, so that such information can be integrated into health management as toxicological and diagnostic understanding develop.

Summary/B>

The need for good-quality toxicology information on CNTs is at the forefront in setting an effective framework for the management of occupational exposure risk.

On the positive side, understanding of measurement techniques is developing rapidly, and less bulky and complex equipment for measuring exposure parameters is in the pipeline.

However, if there is a toxic effect of CNTs, it will still be some time before its severity and nature is fully known. So, until then, practitioners advising employers in this field will need to keep a close eye on the developing research and debate to ensure that the best possible prevention and controls are in place.

References

1. http://news.bbc.co.uk/1/hi/sci/tech/7408705.stm

2. Donaldson, K. et al (2008): ‘Carbon nanotubes introduced into the abdominal cavity of mice show asbestos like pathogenicity in a pilot study’ in Nature Nanotechnology, 3;423 — 428

3. Aitken RJ, Creely KS, Tran CL (2004): Nanoparticles: An occupational hygiene review. Research Report 274. HSE Books

4. The Royal Society and Royal Academy of Engineering (2004): Nanoscience and nanotechnologies: opportunities and uncertainties

5. HSE (2006): Review of the adequacy of current regulatory regimes to secure effective regulation of nanoparticles created by nanotechnology

6. Public Health Service Agency for Toxic Substances and Disease Registry (2001): Toxicological Profile For Asbestos. US Department of Health and Human Services

7. Health Protection Agency (2007): Asbestos-toxicological overview

8. Council on Science and Technology (CST) (2007): Nanosciences and Nanotechnologies: A review of the Government’s progress on its policy commitments

9. Index Mundi 2008: Asbestos world production by country, viewed online 18 September 2008, www.indexmundi.com/en/commodities/minerals/asbestos/asbestos_t7.html

10. Kshitij Aditeya Singh (2008): Carbon nanotubes, applications and markets 2008, viewed online 12 September 2008, www.Nanomagazine.Co.Uk/Readarticle.Php?Id=7

11. Oberdorster G, Oberdorster E, Oberdorster J (2005): ‘Nanotoxicology: An emerging discipline evolving from studies of ultrafine particles’, in Environmental Health Perspectives, Volume 113, 7

12. Health and Safety Laboratory (2006): The assessment of different metrics of the concentration of nano (ultrafine) particles in existing and new industries, HSE

13. US Department of Labor Occupational Safety & Health Administration (OSHA) (2007): Sampling and analytical methods from asbestos in air, www.osha.gov/dts/sltc/methods/inorganic/id160/id160.html

14. British Standards Institute (BSI) (2007): PD ISO/TR 27628:2007 Workplace atmospheres — ultrafine, nanoparticle and nano-structured aerosols — Inhalation exposure characterisation and assessment

15. Centers for Disease Control: Approaches to safe nanotechnology: An information exchange with NIOSH 2008, www.cdc.gov/niosh/topics/nanotech/safenano/control.html

16. British Standards Institute (BSI) (2007): PD 6699-2:2007 Nanotechnologies —

Part 2: Guide to safe handling and disposal of manufactured nanomaterials

Jim Noonan is a regional health and safety manager for HM Prison Service.

 

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