ABSTRACT
IntroductionNanotechnology is the manufacturing and application of materials and devices at the nanoscale (1-100 nm) by using unique characteristics of nanoparticles (NP), which are different than those of larger particles. Nanoparticles are defined as particles greater than 1 nm and smaller than 100 nm in two or three dimensions (ISO, 2007). At this size scale, particles exhibit physicochemically unique optical, magnetic, and electrical characteristics that find many uses in technology (Kreyling et al., 2006). The rapid growth of nanotechnology in recent years encompasses a range of industries, including pharmaceuticals, materials, medicine, agriculture, electronics, and energy (Lane & Kalil, 2005). New products have emerged from the laboratories and into the worldwide commercial market estimated to be as large as $1 trillion
by 2015 (Roco, 2007). On the basis of the promise of the technology to lead to new jobs and economic growth, there has been significant investment in this technology by governments and industry worldwide (Lux Research, 2007; Roco, 2007). As consumer products that contain nanomaterials become more commonplace, it is only a matter of time before a significant proportion of the population will use or come into contact with products containing nanomaterials. However, the earliest and potentially the most significant exposures and risks will likely be in the occupational arena. An estimated two million new workers will be exposed to engineered nanomaterials in occupational environments over the next 15 years (Roco, 2003). There are several industry sectors and processes where worker exposures to nanomaterials have the potential to be significant if not properly contained, including chemical and pharmaceutical companies, construction and manufacturing (e.g., powder handling & cement), and electronics and communications. Despite the large investments in nanotechnology, corresponding investments in environmental, health, and safety aspects of this technology and its processes and products have not been as high. Much is still unknown or poorly known regarding the health risks of nanomaterials. For example, the mass concentration has traditionally been the metric for exposure assessment of airborne particles and is the basis for regulation even for nanomaterials such as the recently proposed NIOSH Recommended Exposure Limit (REL) for carbon nanotubes (CNTs) of 7 µg/m3. However, alternative metrics such as surface area and number concentration have been proposed for characterizing nanoparticle exposures. Thus, uncertainty pervades even basic issues. Given the predicted far-reaching influence of nanotechnology over our lives, the dynamic nature of many engineered nanomaterials, and the lack of knowledge of human health and ecological risks relating to this technology, a life cycle approach to considering the risks relating to nanomaterials is prudent. The potential impacts at every stage of a material’s life cycle-from production to transport to use to end-oflife treatment and disposal/recycling need to be studied (Klöpffer et al., 2007). Such analyses may affect the design of products based on these technologies. Life cycle analysis includes a consideration of the material and energy flows involved in the manufacture and commerce of nanotechnology-based products, important environmental
impacts, burdens, and weak spots, thereby identifying the relative contributions from each life cycle stage. Most studies to date have focused on inventory analysis and material and energy flows (Lave et al., 1995). Lloyd and Lave (2003) studied the economic and environmental implications of using nanocomposites in automobiles, while Lloyd et al. (2005) studied the relative benefits of using nanotechnology to stabilize platinum metal particles in automotive catalyst technologies. However, none of these studies considered occupational and environmental risks in their calculations. Risk assessment is an analytical approach, which helps determine whether the level of a hazard and its probability of occurrence are acceptable or if the risk needs to be managed. Life cycle risk analysis integrates these two ideas to address issues such as the likelihood and magnitude of risk contribution from each stage of the product’s life cycle, the availability of data for risk assessments, and the options for managing the risk. Where potential impact depends on physical form as well as chemistry, changes in physicochemistry-along with availability or exposure potential-across a material’s life cycle can have a profound impact on risk within different contexts. Within this complex challenge, much attention has been placed on exposure potential as a first order determinant of potential risk. Kohler et al. (2008) studied the potential release of carbon nanotubes throughout the life cycle of two different products as case studies-lithium ion secondary batteries and synthetic textiles. They found that release of CNTs can occur in not only the production phase but also the usage and disposal phases. For example, textile production, where CNTs are used as additives, has several scenarios that can lead to potential exposures-blending of polymers with CNTs that can involve dry powders or liquid phase dispersion and fabric finishing and tailoring that may involve wet or dry abrasion of fiber fragments. In the use phase, degradation of the matrix in composite textiles can result in CNT release, because CNTs are much more stable and do not degrade in the same manner and time as polymers. Textile industrial waste from fabric manufacture is recovered and reused often. In fiber-to-fiber recycling, either the material is physically shredded or the textiles are cut into small pieces and granulated to form pellets. All these mechanical and thermal processes provide opportunities for CNT release. Disposal could be through landfilling or incineration. While municipal solid waste incinerators most likely will completely incinerate CNT/polymer composites, uncontrolled incineration in
open fires that occur frequently in developing countries will lead to emissions of CNTs as it is an incomplete combustion process. Thus, the risk presented by just-generated carbon nanotubes, for instance, may be markedly different from the risk presented by processed/purified nanotubes, which represent not only an altered physicochemistry but also a different exposure potential. Likewise, once these carbon nanotubes have been incorporated into a product-for example, a fabric composite-the exposure potential and the physicochemical nature of any material that is released are profoundly different from that of the starting material. As the resulting product is used and eventually disposed or recycled, the hazard and exposure potential differ yet again. Thus, the risk profile of a nanomaterial over its life cycle is complex, even if that material is relatively stable. However, when nanomaterials undergo transformations through their life cycle through processes, such as agglomeration, dissolution, surface adsorption/desorption, chemical reaction, or other interactions with close-proximity materials, the challenges of evaluating and addressing risk become more difficult. We have little quantitative information about the exposure potential in each of the phases of the life cycle described above. The populations being exposed in each of these phases also differ as, presumably, do their susceptibilities. In summary, a life cycle risk assessment requires the estimation of risk at every step of the life cycle of the product and, therefore, an assessment of the hazard and exposure at every step of the life cycle. The following sections describe the risk assessment process, the roadblocks to carrying this out for nanomaterials, and the potential ways to move ahead. Traditional Risk AssessmentRisk assessment is a complex process that involves the integration of hazard, exposure, and health effect information to characterize the potential for risk to human health (Kandlikar et al., 2007). This requires information across a range of domains, including source characterization, fate and transport, modeling, exposure assessment, and dose-response characteristics. Such methods typically utilize quantitative predictions of health impacts and explicitly model and incorporate uncertainties.
