Testing Strategies to Establish the Safety of Nanomaterials: Conclusions of an ECETOC Workshop

Abstract

The European Centre for Ecotoxicology and Toxicology of Chemicals (ECETOC) convened a workshop in Barcelona, Spain, in November 2005 to develop testing strategies to establish the safety of nanomaterials. It brought together about 70 scientific and clinical experts from industry, academia, government agencies, research institutes, and nongovernmental organizations. The primary questions to be addressed were the following: What can we do today, and what do we need tomorrow? The three major themes of the workshop were: (1) the need for enhanced efforts in nanomaterial characterization; (2) methodologies for assessments of airborne and internal exposures to nanomaterials; and (3) evaluation of the hazard potential--primarily focusing on pulmonary or dermal routes of exposures. Some of the summary conclusions of the workshop included the following: For the development of nanoparticle characterization, the working definition of nanoparticles was defined as < 100 nm in one dimension or < 1000 nm to include aggregates and agglomerates. Moreover, it was concluded that although many physical factors can influence toxicity, including nanoparticle composition, it is dissolution, surface area and characteristics, size, size distribution, and shape that largely determine the functional, toxicological and environmental impact of nanomaterials. In addition, most of the information on potential systemic effects has thus far been derived from combustion-generated particles. With respect to the assessment of external exposures and metrics appropriate for nanoparticles, the general view of the meeting was that currently it is not possible or desirable to select one form of dose metric (i.e., mass, surface area, or particle number) as the most appropriate measure source. However, it was clear that the surface area metric was likely to be of interest and requires further development. In addition, there is a clear and immediate need to develop instruments which are smaller, more portable, and less expensive than the currently available state of the art instrumentation. With regard to a general testing approach for human health hazard evaluation of nanoparticles, a first step to determine potency may include a prioritization-related in vitro screening strategy to assess the possible reactivity, biomarkers of inflammation and cellular uptake of nanoparticles; however this process should be validated using in vivo techniques. A Tier 1 in vivo testing strategy could include a short-term inhalation or intratracheal instillation of nanoparticles as the route of exposure in the lungs of rats or mice. The endpoints that should be assessed include indices of lung inflammation, cytotoxicity, and cell proliferation, as well as histopathology of the respiratory tract and the major extrapulmonary organs. For Tier 2 in vivo testing for hazard identification, a longer term inhalation study is recommended, and this would include more substantive mechanistic endpoints such as determination of particle deposition, translocation, and disposition within the body. Additional studies could be designed with specific animal models to mimic sensitive populations. With regard to dermal exposures, currently there is little evidence that nanoparticles at a size exceeding 100 nm penetrate through the skin barrier into the living tissue (i.e., dermal compartment). The penetration of nanoparticles at a size less than 100 nm should be a topic of further investigation. Moreover, considering the impacts of dermal exposures and corresponding hazard potential of nanoparticles, it must be taken into consideration that the dermal uptake of nanoparticles will be an order of magnitude smaller than the uptake via the inhalation or oral routes of exposure. For the evaluation of the health risk of nanoparticles, it has to be determined whether they are harmful to living cells and whether, under real conditions, they penetrate through the skin barrier into the living tissue. For the evaluation of the penetration processes, in principle, three methods are available. Using the method of differential stripping, the penetration kinetics of nanoparticles in the stratum corneum and the hair follicles can be evaluated. This analysis can be carried out in vivo. Diffusion cell experiments are an efficient method for in vitro penetration studies. Also, laser scanning microscopy is well suited to test penetration kinetics, although requiring fluorescent-labeled nanoparticles. Emerging topics such as (1) environmental safety testing, (2) applications of nanoparticles for medical purposes, and (3) pathways of inhaled nanoparticles to the central nervous system were also briefly addressed during this workshop. However, it has become clear that these topics should be the subjects of separate workshops and they are not further addressed in this report.