Comparison between asbestos- and nanomaterial-induced respiratory disease: Key issues and data gaps for risk assessment

Although studied for decades, questions still remain about the characteristics of fibers that lead to adverse respiratory health effects. These include asbestos fibers, as well as non-mineral fibers (e.g., glass wool fibers, carbon nanotubes). Individual fiber characteristics may play a role in specific adverse health outcomes, and/or these outcomes may be the result of interactions of multiple fiber characteristics. Currently there is no consensus regarding the role of various fiber types in specific health outcomes. Research has focused mainly on the physicochemical properties such as length, width and durability in fiber toxicity, and more recent studies have focused on surface area and reactivity. Although it is clear that multiple fiber characteristics are involved in the induction of various toxicity pathways, the connection between specific fiber characteristics, signaling pathways and fiber-induced health endpoints is not known. Recent in vivo laboratory animal studies have also suggested similar adverse health response to specific classes of nanomaterials, particularly carbon nanotubes. However, little data is available from human populations exposed to these nanomaterials. Important considerations need to be made when examining results from in vivo laboratory animal studies, including the route of exposure, dose and their relevance to human health assessments. Key data gaps remain before an informed risk assessment of carbon nanotubes can be performed. Further, comparisons between the risks associated with exposure to various fiber types (e.g., asbestos, nanomaterials) must be made with caution until a better understanding of the complexities of fiber characteristics, exposure scenarios and biological response information is available.