Safety of Engineered Nanomaterials

Abstract

Background: The emerging field of nanotechnology has led to substantial advances in energy, medicine, and clean technologies. Discussion: This area relies on nanomaterials that have been defined as materials where at least one dimension is less than 1 nM. Nanomaterials are used in many different products, and the numbers of engineered products are increasing due to their unique properties. Due to their small size and their unique properties, there has been concern that exposure to these materials is associated with an increased risk for adverse health effects. These concerns were due to increased risk of cardiovascular disease (CVD), cancer and mortality associated with ambient air pollution that also contains ultrafine particles, as well as the similarity of some nanoparticles to asbestos e.g. carbon nanotubes. However in contrast to ambient air particles, nanoparticles are chemically well defined and not a complex mixture of different compounds. In order to address a public concern about using this new technology and the derived products, the European Commission, at an early stage, asked the question, could the safety of nanoparticles be evaluated using the Technical Guidance Documents used for chemicals, or is a new paradigm needed. The major concern was 1) the metric dose required, i.e., weight basis, particle number or surface area, and 2) if the traditional Organisation for Economic Co-operation and Development (OECD) validated test methods could be used or should be modified. Since then, recommendations have been developed for specific applications, e.g. food, cosmetics, medical products by relevant agencies. Due to their high surface area relative to the weight, it is assumed that the nanomaterials are very reactive, and based upon our knowledge of the toxicology of ambient air ultrafine particles, it is generally assumed that the toxicity of nanoparticles is mediated by reactive oxygen species (ROS), and formation of ROS has been detected following both in vitro and in vivo exposure to various types of nanoparticles (NP). The consequence of this exposure could be induction of inflammatory responses, or damage to cellular macromolecules by reactive oxygen species. Due to its high reactivity the NP could directly react with cellular macromolecules, e.g. enzymes, DNA and thus interfere with the homeostasis of the cells. A tiered approach to screening for nanoparticle toxicity has been proposed based upon the formation of ROS.1 Of special concern is genotoxicity testing, as many NP are not mutagenic in bacterial assays, and the fact that the NP interfere with reagents used in standard assays.2 One of the major problems in nanotoxicology has been the characterization of the nanoparticles, both prior to testing and during the testing protocol, e.g. NP tends to react with components in serum and thus changes the size, and also agglomeration of the nanoparticles. In order to characterize these particles several microscopic techniques are used as well as other physical chemical methods. Thus nanotoxicology requires a cross-disciplinary approach involving material scientists, physicists and molecular toxicologists. The focus of our work has been on the toxicity of silver (Ag) NP particles, as these are used in many different consumer products, mostly related to their antibacterial activity. The toxicity of silver nanoparticles and silver ion has been investigated in both human and animal cells using different parameters for toxicity, e.g. mitochondrial activity (MTT assay), induction of apoptosis, and induction of reactive oxygen species. ROS was demonstrated both by direct measurements and by formation of DNA adducts, e.g. bulky adducts measured by P32 postlabelling, and 8-oxo-dG by mass spectrometry (MS). Potential genotoxicity was assessed by induction of micronuclei detected by flow-cytometry. The observed toxicity was due to both the Ag NP and the release of silver ions, however using gene array assay, significant differences between Ag NP and silver ion heat maps were noted. This indicates different toxicological mechanisms for the ion and the NP.3 In order to compare the toxicity in cells of different organs and to compare the effect in human and animals cells, pairs of mouse and human cell lines, i.e. lung, colon, macrophage were compared, and the mouse cell lines were significantly more susceptibility to the toxicity of Ag NP. Comparison of different types of nanoparticles in the same cells (lung cancer cells A549) suggests that the toxicological mechanism is more related to the specific chemical properties of the nanomaterial, rather than the nanoparticle structure. A comparison of the gene expression profiles of silica NP and Ag NP showed significant differences. In vitro studies are the preferred model for assessment of toxicity and determination of the mechanism of action for nanoparticles. Many of the assays used are very sensitive to culture conditions, cell types, serum and media composition due to interaction with the nanomaterial. Risk assessment normally requires information on hazard characterization and exposure characterization; whereas many studies have been conducted to assess the hazard, limited information is available on exposure to both humans and the environment. A special concern with the use of nanomaterials is the release of the NP into the environment through wear and release from waste. Thus environmental risk assessment is an integral part of the evaluation process. Conclusion: Regulatory procedures are currently being developed to ensure that exposure to engineered nanomaterials or the released particles are not associated with any health risk.