Engineered Nanomaterials: Exposures, Hazards, and Risk Prevention Robert a Yokel1*, Robert C Macphail2

Yokel and MacPhail Journal of Occupational Medicine and Toxicology 2011, 6:7 http://www.occup-med.com/content/6/1/7 REVIEW Open Access Engineered nanomaterials: exposures, hazards, and risk prevention Robert A Yokel1*, Robert C MacPhail2 Abstract Nanotechnology presents the possibility of revolutionizing many aspects of our lives. People in many settings (academic, small and large industrial, and the general public in industrialized nations) are either developing or using engineered nanomaterials (ENMs) or ENM-containing products. However, our understanding of the occupational, health and safety aspects of ENMs is still in its formative stage. A survey of the literature indicates the available information is incomplete, many of the early findings have not been independently verified, and some may have been over-interpreted. This review describes ENMs briefly, their application, the ENM workforce, the major routes of human exposure, some examples of uptake and adverse effects, what little has been reported on occupational exposure assessment, and approaches to minimize exposure and health hazards. These latter approaches include engineering controls such as fume hoods and personal protective equipment. Results showing the effectiveness - or lack thereof - of some of these controls are also included. This review is presented in the context of the Risk Assessment/Risk Management framework, as a paradigm to systematically work through issues regarding human health hazards of ENMs. Examples are discussed of current knowledge of nanoscale materials for each component of the Risk Assessment/Risk Management framework. Given the notable lack of information, current recommendations to minimize exposure and hazards are largely based on common sense, knowledge by analogy to ultrafine material toxicity, and general health and safety recommendations. This review may serve as an overview for health and safety personnel, management, and ENM workers to establish and maintain a safe work environment. Small start-up companies and research institutions with limited personnel or expertise in nanotechnology health and safety issues may find this review particularly useful. 1. Introduction necessary occupational health and safety expertise, A. The objectives of this review knowledge, and/or staff. Although there has been considerable work to advance A comprehensive review described the field of nano- nanotechnology and its applications, understanding the toxicology six years ago, including some mechanisms of occupational, health and safety aspects of engineered toxicity, portals of ENM entry, their translocation, and nanomaterials (ENMs) is still in its formative stage. The the state of their risk assessment at the time [1]. More goals of this review are to describe some general fea- recent reviews have focused on the major challenges, tures of ENMs, how a worker might be exposed to key questions, and research needs to assess ENM toxi- ENMs, some potential health effects, and approaches to city and risk [2-7]. This review addresses issues not minimize exposure and toxicity. The target audience extensively covered in prior reviews, including recent includes industrial hygienists, investigators working with exposure-assessment studies, and engineering and perso- these materials, institutes and universities conducting nal protective equipment (PPE) options and their effi- research, and start-up companies that may not have the cacy to minimize ENM exposure. This review also includes accepted but not yet published reports, recently completed studies not yet published, and ongoing work. * Correspondence: [email protected] Our goal was to provide up-to-date information on 1Department of Pharmaceutical Sciences, College of Pharmacy and Graduate ENM exposures, their health hazards, and ways to mini- Center for Toxicology, University of Kentucky, Lexington, KY, 40536-0082, mize risk. USA Full list of author information is available at the end of the article © 2011 Yokel and MacPhail; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Yokel and MacPhail Journal of Occupational Medicine and Toxicology 2011, 6:7 Page 2 of 27 http://www.occup-med.com/content/6/1/7 B. Engineered nanomaterials to 2 and 2 to 50 nm wide, respectively, and can be > 1 Nano is a prefix derived from the Greek word for dwarf. μmlong.TheC60 diameter is ~1 nm. Metal and metal The parts of the U. S. National Nanotechnology Initia- oxide ENMs most commonly studied are cadmium in tive (NNI) definition that are relevant for this review various complexes, gallium arsenide, gold, nickel, plati- define nanoscale materials as having at least one dimen- num, silver, aluminum oxide (alumina), cerium dioxide sion in the range of 1 to 100 nanometers (nm), with (ceria), silicon dioxide (silica), titanium dioxide (TiO2, properties that are often unique due to their dimen- titania), and zinc oxide. The size of ENMs is in the sions, and that are intentionally manufactured [8]. There same range as major cellular machines and their compo- are many definitions of nanoscale materials, which gen- nents, such as enzymes, making it likely that they will erally encompass the same bounds on ENM size [9,10]. easily interact with biochemical functions [22]. This is in contrast to naturally occurring and uninten- Some ENMs contain contaminants, such as residual tionally-produced materials on the same scale, which are metal catalysts used in the synthesis of CNTs. ENM referred to as ultrafine particles. The term ultrafine has toxicity has been attributed to these residual metals, as been used by the aerosol research and occupational and discussed in II, B, 1. ENM exposure effects in the environmental health communities to describe airborne lung. The physico-chemical properties of ENMs, when particles smaller than 100 nm in diameter [11]. Ultrafine tested prior to their use, are often different from those particles are not intentionally produced. They are the stated by the supplier [23,24]. A major cause of changes products of combustion and vaporization processes such in the physico-chemical properties of ENMs over time as welding, smelting, fuel combustion, fires, and volca- and in various media is agglomeration, discussed in II, noes [1,12,13]. In this review, intentionally-manufac- A, 2. The physico-chemical properties of ENMs that tured nanoscale materials will be referred to as ENMs. impact their uptake. When ENMs are not sufficiently They are usually produced by bottom-up processes, characterized to identify their composition or properties such as physical and chemical vapor deposition, liquid it makes the prediction of toxicity, when added to the phase synthesis, and self-assembly [5,14]. insufficient understanding of their biological effects, The health and environmental effects of ENMs are not even more difficult [25]. well understood, leading some to caution development of this technology [15-19]. Some understanding of ENM D. Some uses of ENMs and the projected market and effects can be derived, however, by analogy from ultra- workforce fine particles, which have been shown to produce There is considerable interest in developing ENMs inflammation, exacerbation of asthma, genotoxicity, and because their properties differ in fundamental and valu- carcinogenesis following inhalation. The following sec- able ways from those of individual atoms, molecules, tions describe ENMs, and some of their uses and uncer- and bulk matter. Nanoscale products and materials are tainties, providing the context of this review. increasingly being used in optoelectronic, electronic (e. g., computer hard drives), magnetic, medical imaging, C. Common ENM size, composition, and quality drug delivery, cosmetic and sunscreen, catalytic, stain Figure 1 relates ENM size to other chemical and biolo- resistant fabric, dental bonding, corrosion-resistance, gical materials. There are a staggering number of ENM and coating applications [26]. Major future applications compositions and shapes. Over 5000 patents have been are expected to be in motor vehicles, electronics, perso- issued for carbon nanotubes (CNTs) and > 50,000 vari- nal care products and cosmetics, and household and eties of CNTs have been produced [20]. The sheer num- home improvement. These applications capitalize on ber of ENMs contributes to the lack of our adequate their electromagnetic, catalytic, pharmacokinetic, and understanding of ENM health and safety. They are pri- physico-chemical properties, including strength, stiff- marily composed of carbon or metal/metal oxide, as ness, weight reduction, stability, anti-fogging, and illustrated by the representative manufactured nanoma- scratch resistance. Current products contain various terials selected for testing by the Organisation for Eco- ENMs including nanotubes, metal oxides, and quantum nomic Co-operation and Development (OECD) [21]. dots (semiconductors developed as bright, photostable Carbon-based ENMs include single-walled and multi- fluorescent dyes and imaging agents). Nanowerk identi- walled carbon nanotubes (SWCNTs and MWCNTs), fied ~2500 commercial nanomaterials, including ~27% graphene (a single sheet of carbon atoms in a hexagonal metal oxides, 24% CNTs, 18% elements, 7% quantum structure), spherical fullerenes (closed cage structures dots, and 5% fullerenes

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