Chapter 18: Safe Use of Nanomaterials
This chapter discusses the unique properties of nanomaterials, solid superatomic materials with at least one dimension in the range of one to 100 nanometers. Subsequent sections discuss the potential safety and health concerns from nanomaterials (based on cell culture and animal studies), the routes of exposure, and guidance on how to prevent exposures to nanomaterials.
The National Institute for Occupational Safety and Health (NIOSH) has a strategic plan and research agenda to address the health and safety of nanomaterials. Consult their Nanotechnology Page for more information.
How do nanomaterials compare in size to other objects regarded as “small”? Dust mites have a diameter of approximately 200 µm. Human hairs have a diameter of 60-120 µm. Thus, both are 1000 or more times larger than nanoscale. The smallest known bacterial species, such as the genusMycoplasma, have a diameter of approximately 300 nm (.3 µm), which is still greater than nanoscale. Some smaller viruses (e.g. Parvoviruses, diameter ~25 nm) exist at nanoscale, but most viruses are larger. Typical double-stranded DNA has a diameter of ~2.5 nm.
Nanomaterials are among the smallest materials that can exist, because the smallest unit of elemental matter that retains the properties of the element (the atom) is not much smaller than nanoscale. Due to the uncertain position of the electron cloud around the central nucleus, scientists can only estimate the diameter of atoms. However, most estimates of atomic diameter range from .05 to .25 nm (0.5 to 2.5 Angstroms).
Nanomaterials divide roughly into two main categories: ambient (or “natural”) nanoparticles, and engineered/manufactured nanomaterials. The rest of this Chapter will use the term “nanoparticles” when referring specifically to nanoscale natural (non-engineered) substances. The term “nanomaterials” will be used as a blanket term for all nanoscale substances. In most scientific uses, the terms are interchangeable.
Ambient nanoparticles are also known as “ultrafine” particles in standard industrial hygiene terminology. Sources include diesel engine exhaust, welding fumes, and other combustion processes. Most grinding and crushing processes are incapable of producing nanoparticles, unless fine bead mills are used. Ultrafine/nanoparticles have a larger surface area per unit volume than an equal volume of same composition larger particles. This can lead to different physical, chemical, and biological response properties.
Other natural nanoparticles include smaller viruses and rickettsia, and intracellular proteins, nucleic acids, and organelles.
Engineered or manufactured nanomaterials are deliberately created and used for a structural/functional purpose. Engineered nanomaterials can include both homogeneous materials and heterogeneous structures with specific applications in computing, medicine, and other disciplines. The next section will examine several examples of engineered nanomaterials.
CNTs can be up to several millimeters long, and possess tensile strength more than twenty times greater than carbon steel. CNTs also are efficient conductors of heat, excellent electron emitters, and can assemble into strong ropes of increasing diameter through VanDerWaal’s forces. All of these are highly desirable material properties. Currently, CNTs are used in diverse applications such as lightweight carbon fiber bicycle pieces, water desalination filters, concrete strengthening, and solar cells. Their field emission properties have been harnessed to produce scanning X-ray imaging systems5. As the production and use of carbon nanotubes in laboratory research environments increases, the potential for exposure to CNTs also increases. The next section will cover known and suspected health effects from CNTs and other nanomaterials.
1Wang J, Liu G, Engelhard MH, and Lin Y. Sensitive Immunoassay of a Biomarker Tumor Necrosis Factor-a Based on Poly(guanine)-Functionalized Silica Nanoparticle Label. Analy Chem 78(19): 6974-6979 (2006).
2Roy I, Mitra S, Maitra A, and Mozumdar S. Calcium Phosphate Nanoparticles as Novel Non-Viral Vectors for Targeted Gene Delivery. Intl J of Pharmaceutics 250(1): 25-33 (2003).
3Euliss LE, DuPont JA, Gratton S, and DeSimone J. Imparting Size, Shape, and Composition Control of Materials for Nanomedicine. Chem Soc Reviews 35: 1095-1104 (2006).
4Datta R and Jaitawat SS. Nanotechnology – The New Frontier of Medicine. Med J Armed Forces India 62(3): 263-268 (2006).
5Zhang J, Yang G, Rajaram R, Guan E, Lee Y, LaLush D, Chang S, Lu JP, and Zhou O. A Stationary Scanning X-ray Imaging System based on Carbon Nanotube Field Emitters. Med Physics 33(6): 2159 (2006).
CNTs can, like asbestos, reach the gas exchange (alveolar) regions of the deep lung, and trigger inflammation and oxidative stress. Studies in mice have shown that single-walled CNTs can produce pulmonary granulomas, whereas equivalent mass doses of ultrafine carbon black did not6,7. This implies that the shape and surface chemistry properties of CNTs impart an increase in pulmonary toxicity.
Respiratory studies for other nanomaterials are also informative. Extensive research on diesel exhaust particulates has led to their characterization by the International Agency for Research on Cancer as a reasonably anticipated human lung carcinogen. For titanium dioxide (a substance used at nanoparticle size in cosmetics, sunscreens, and self-cleaning windows), 25 nm diameter particles produced more potent lung damage than 250 nm diameter particles8. Since both sizes of particles are capable of reaching the deep lung, there must be another factor such as surface area to volume ratio, solubility, or agglomeration creating the toxicity difference. Nanoparticles that agglomerate tend to deposit in the nasopharyngeal region or the upper airways.
Discrete nanoparticles that reach the deep lung might be small enough to penetrate through alveolar epithelial cells, enter the capillaries, and translocate to other organs9. For discrete nanoparticles that remain in the nasopharyngeal region, translocation to the brain via axonal transport through the olfactory nerve has been shown in rats10.
In summary, animal studies indicate the following potential concerns from exposures to nanomaterials through the inhalation route:
- CNTs might possess asbestos-like properties
- For all nanomaterials, equivalent mass doses of the same materials might exhibit higher toxicity at nanoscale size
- Previously unobserved translocation routes (via alveoli to bloodstream, via olfactory nerve to brain) might exist
6Shvedova et al. Unusual Inflammatory and Fibrogenic Pulmonary Responses to Single-Walled Carbon Nanotubes in Mice. Am J Physiology – Lung, Cell, & Mol Physiol 289: L698-L708 (2005).
7Lam CW, James JT, McCluskey R, Arepalli S, Hunter RL. Pulmonary Toxicity of Single-Wall Carbon Nanotubes in Mice 7 and 90 Days after Intratrachial Installation. Toxicol Sci 77: 126-134 (2004).
8Oberdörster, G. Phil. Trans. Roy. Soc. London Series A 358(1775):2719-2740 (2000).
9Oberdörster et al. Extrapulmonary translocation of ultrafine carbon particles following whole-body inhalation exposure of rats. J Toxicol Environ Health 65 Part A(20): 1531-1543 (2002).
10Oberdörster et al. Translocation of inhaled ultrafine particles to the brain. Inhal Toxicol 16(6-7): 437-445 (2004).
11The Royal Society, The Royal Academy of Engineering. Nanoscience and Nanotechnologies. London, UK: The Royal Society and the Royal Academy of Engineering: http://www.nanotec.org.uk/finalReport.htm (2004).
12Ryman-Rasmussen JP, Riviere JE, Monteiro-Riviere NA. Penetration of Intact Skin by Quantum Dots with Diverse Physicochemicals Properties. Toxicol Sci 91(1): 159-165 (2006).
13 Monteiro-Riviere et al. Multi-walled carbon nanotube interactions with human epidermal keratinocytes. Toxicol Lett 155(3): 377-384 (2005).
The greater activity of nanoscale materials forms the basis for research into nanoenergetics. For example, nanoscale thermite powders composed of aluminum and molybdenum trioxide ignite more than 300 times faster than corresponding micrometer-scale material15.
14Health and Safety Executive. Horizon scannon information sheet on nanotechnology. Sudbury, Suffolk, United Kingdom: Health and Safety Executive. (2004).
15Granier JJ and Pantoya ML. Laser Ignition of Nanocomposite Thermites.Combustion Flame 138:373-382 (2004).
The most significant exposure route in laboratory settings would be inhalation, and this is the route with the most toxicity data. Respirable exposures would be most likely during the creation or handling of nanomaterials in aerosol, powder, or colloidal suspension. Nanomaterials in composites are not as likely to result in respirable exposures unless they are cut, ground, or degraded. As with all potential exposures, the best place to start is the OSHA “hierarchy of controls”, which goes from engineering controls to work practice controls to personal protective equipment. Engineering controls always come first, since they have the potential to remove the exposure from the work area. Do not consider using personal protective equipment until you have considered all engineering and work practice controls.
To avoid re-entrainment of nanomaterials, EHS recommends High-Efficiency Particulate Air (HEPA) systems in conjunction with local exhaust ventilation. Filtration systems must be able to capture at least 99.97% of monodispersed 300 nm aerosols in order to qualify for HEPA rating. The 300 nm diameter is the most penetrating particle size. Particles smaller than 300 nm (including the nanoscale of 1-100 nm) are actually captured more effectively due to diffusion or electrostatic attraction, and particles larger than 300 nm are captured by impaction and interception. Thus, HEPA filters should effectively capture nanomaterials, however, the filter must sit properly in the housing, or nanomaterials will bypass it and take the path of least resistance.
Filtering facepiece respirators such as the N-95 (Figure 18.4b) are capable of filtering nanomaterials, but are prone to gaps and inward leakage. These respirators are not personal protective equipment from exposure to nanoaerosols.
There is currently very little data to indicate whether skin protective equipment such as gloves, Tyvek® sleeves and suits, etc., can protect from nanomaterials. Most available data is from bloodborne pathogen protective equipment, which is challenge-tested with a 27 nm bacteriophage per ANSI (the American National Standards Institute).
Currently, it is unknown whether the skin is a significant route of exposure to nanomaterials. Until more data becomes available, you should use gloves (double gloving for extensive skin contact) and sleeves to prevent skin contact, and change gloves frequently. When handling nanomaterials in solution you should wear gloves that are chemically resistant to the solution or solvent nanomaterials are suspended in.
The EHS website has further resources available regarding Nanotechnology Safety. Please contact EHS if you have any questions, or if you want to request an evaluation of your work conditions.