Biopersistent fiber-induced inflammation and carcinogenesis: Lessons learned from asbestos toward safety of fibrous nanomaterials

Nano-sized durable fibrous materials such as carbon nanotubes have raised safety concerns similar to those raised by asbestos. However, the mechanism by which particulates with ultrafine structure cause inflammation and ultimately cancer (e.g. malignant mesothelioma and lung cancer) is largely unknown. This is partially because the particulates are not uniform and they vary in a plethora of factors. Such variances include length, diameter, surface area, density, shape, contaminant metals (including iron) and crystallinity. Each of these factors is involved in particulate toxicity both in vitro and in vivo. Thus, the elicited biological responses are incredibly complicated. Various kinds of fibers were evaluated with different cells, animals and methods. The aim of this review is to concisely summarize previous reports from the standpoint that activation of macrophages and mesothelial injury are the two major mechanisms of inflammation and possibly cancer. Importantly, these two mechanisms appear to be interacting with each other. However, there is a lack of data on the interplay of macrophage and mesothelium especially in vivo. Since fibrous nanomaterials present potential applications in various fields, it is necessary to develop standard evaluation methods to minimize risks for human health.