Nanomaterials

In document New Horizons (Page 117-121)

This section is authored by Alistair Boxall, University of York

Nanomaterials (NMs) are generally regarded as materials that have one or more dimensions of less than 100 nanometres in size. At this size range, the materials have very different properties from their equivalent ‘bulk’ material and consequently NMs are now being used in a wide range of products including cosmetics, paints and coatings, medicines and medical devices, water treatment technologies and

agrochemicals.25 Release of NMs into the natural environment is inevitable. Emission pathways include: entry to air from vehicle exhausts; entry to surface waters from down-the-drain chemicals that are released to the sewerage system or from runoff from highways and buildings; entry to soils through direct application of

agrochemicals and the applications of sewage sludge to land as a fertiliser.26 NMs may also occur naturally in the environment or be formed from the breakdown of larger man-made particles such as plastics and polymers.27

The analysis of NMs in environmental matrices is challenging – due to their size - so much of the work done to quantify concentrations of these materials in the

environment has involved the use of models. Predictions from these modelling exercises suggest that highest concentrations of NMs in surface waters will be in the tens of microgrammes per litre range in surface waters, tens of mg kg-1 range in soils and 100s of ng m-3 in the air compartment.28

Consumers will be exposed to residues of NMs in the environment through breathing contaminated air, the consumption of contaminated soil or drinking water, or through skin contact with contaminated soil or water. NMs can also be accumulated by plants, fish and shellfish29,30 so exposure from consumption of contaminated food items may also occur. While it is inevitable that human exposure to residues of NMs in the environment is occurring, there is less direct evidence of this. The only

experimental evidence of such exposures comes from studies that used magnetic analyses and electron microscopy to demonstrate the presence of magnetite nanoparticles in the human brain.31 They proposed that the most likely source of these particles was from airborne particulate matter pollution.

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Modelling studies have attempted to quantify the importance of environmental exposure to a particular NM compared to exposure in occupational and product-use settings. For example, Tiede et al. (2015)32 explored the potential exposure of consumers from drinking water. They concluded that for the majority of types of nanoparticles that were studied, human exposure via drinking water was less important than exposure via other routes. The exceptions were some NPs from clothing materials, paints and coatings and cleaning products containing Ag, Al, TiO2, Fe2O3 engineered NPs and carbon-based materials. A similar study by

Nowack et al., (2013)33 concluded that environmental exposure to materials used in agricultural production, drinking water treatment, groundwater remediation and in medical textiles is more significant than occupational exposure or exposure during use of a product by consumers.

Evidence for potential effects of NMs on human health generally comes from in vitro studies and in vivo studies using model test organisms. For example, silver

nanoparticles have been shown to reduce lung function, produce inflammatory lesions in the lungs of rats and also to accumulate in the brain.34 At the cellular level, the particles reduce mitochondrial function and increase membrane leakage and alter levels of glutathione.34 Whether or not environmental exposures can result in these types of effects is however, uncertain.

It is inevitable that the English population will be exposed to NMs via the natural environment. The degree of exposure will vary depending on the particle and product type and, in a few instances, environmental exposure will be more important for health than other exposure scenarios (i.e. in occupational settings or during product use). NMs do have the potential to cause toxicological effects but whether exposure concentrations are high enough to reach toxic levels is still unclear. As the

nanotechnology sector is rapidly growing, and exposure levels will continue to

increase in time, there is a real need to begin to better align environmental exposure studies with toxicological studies in order to better characterise the risk of these materials.

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Box 1 Health concerns over Carbon Nanotubes

Dr Craig A. Poland and Dr Rodger Duffin, MRC Centre for Inflammation Research, University of Edinburgh

Carbon nanotubes (CNTs) are classified as a ‘nano-object’ as they have two

dimensions within the nano-range (1-100nm) but can have a length many millimetres long. Due to exceptional structural and electrical properties, interest has increased in the commercial use of CNTs within various industries - mostly relating to use within electronics and composites. However, concerns have been raised as to the possible health effects arising from exposure to CNTs owing to their similarity to certain pathogenic fibres, most notably asbestos.

These concerns have led to a significant body of work addressing the respiratory toxicity of CNTs utilising different models. Typical lung responses noted in numerous studies include inflammation, formation of granulomas (typical of a foreign body reaction), fibrosis and lung cancer.i A significant concern has been whether or not CNTs could reach the pleural cavity and cause mesothelioma, a hallmark cancer of asbestos exposure with a long latency period (>30yrs). Several studies have shown that lung exposure can lead to deposition of CNTs in the sub-pleural region,

transition from the lung into the pleural cavity and length-dependent accumulation. The retention of CNTs in the pleural cavity has been shown to cause inflammation, fibrosis and mesothelioma.ii

It is important to note that not all CNTs display the same level of pathogenicity and results are conflicting. This, in part, is because CNTs are produced in a vast array of different shapes and sizes which impacts on toxicity meaning there is a spectrum of toxicity associated with CNT exposure. Broadly, those very short and/or highly curled CNTs, forming a compact structure (<4µm), show a much lower toxicity than those

which are longer (>10µm) with a straighter, fiber-like morphology. The association

between shape/ length has also been shown for other nanofibres with materials such as titanium dioxide showing greater toxicity with increased length.iii In addition,

concern has been raised over platelet-shaped nanoparticles such as graphene- based nanomaterials leading HSE in recent guidanceiv to consider nanoplatelets alongside CNTs.

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Irrespective of hazard status, the risk of human health effects is very much dependent on exposure. Exposure is most likely to occur in the occupational

environment during the production of CNTs as well as the incorporation of CNTs into products further down the manufacture chain (for example addition to a composite resin). Another area of possible exposure is during recycling of CNT containing products. A limited number of studies have shown CNTs (and other nano-carbons) can be produced from anthropogenic sources such as diesel exhaustv, leading to possible exposure of the public from the general environment. However, CNTs produced from anthropogenic sources are not the same as those shown to cause respiratory toxicity in animal models (for example they are shorter, more compact – generally types thought to be less harmful). The possible impact of anthropogenic CNTs on health is yet to be fully elucidated. Another source of exposure is through interaction with CNT containing products yet exposure to free CNTs is unlikely due to being sealed within products (such as electronic circuitry or embedded within a composite resin).

Numerous exposure limits have been proposed for CNTs based on either mass or fibre number metrics yet there are currently no statutory limits for CNTs or other engineered nano-fibres. Evaluation of CNTs by the International Agency for Research on Cancer led to the classification of a specific multi-walled CNT (MWCNT-7) as a Class 2b carcinogen with all other forms of CNT as Class 3 (Unclassifiable).vi

References

i Poulsen et al. (2016) Nanotoxicology;10(9):1263-75, Porter et al. (2010) Toxicology;269(2-

3):136-47, Kasai et al. (2016) Part Fibre Toxicol; 13(1):53.

ii Rittinghausen et al. (2014) Part Fibre Toxicol; 11:59 iii

Porter et al. (2013) Toxicol Sci.;131(1):179-93

*v

Health & Safety Executive. (2013) Publication HSG272

V

Jung et al. J Air Waste Manag Assoc. 2013 Oct;63(10):1199-204.

v*

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In document New Horizons (Page 117-121)