Biopersistence and potential adverse health impacts of fibrous nanomaterials: what have we learned from asbestos?

Abstract

Human diseases associated with exposure to asbestos fibers include pleural fibrosis and plaques, pulmonary fibrosis (asbestosis), lung cancer, and diffuse malignant mesothelioma. The critical determinants of fiber bioactivity and toxicity include not only fiber dimensions, but also shape, surface reactivity, crystallinity, chemical composition, and presence of transition metals. Depending on their size and dimensions, inhaled fibers can penetrate the respiratory tract to the distal airways and into the alveolar spaces. Fibers can be cleared by several mechanisms, including the mucociliary escalator, engulfment, and removal by macrophages, or through splitting and chemical modification. Biopersistence of long asbestos fibers can lead to inflammation, granuloma formation, fibrosis, and cancer. Exposure to synthetic carbon nanomaterials, including carbon nanofibers and carbon nanotubes (CNTs), is considered a potential health hazard because of their physical similarities with asbestos fibers. Respiratory exposure to CNTs can produce an inflammatory response, diffuse interstitial fibrosis, and formation of fibrotic granulomas similar to that observed in asbestos-exposed animals and humans. Given the known cytotoxic and carcinogenic properties of asbestos fibers, toxicity of fibrous nanomaterials is a topic of intense study. The mechanisms of nanomaterial toxicity remain to be fully elucidated, but recent evidence suggests points of similarity with asbestos fibers, including a role for generation of reactive oxygen species, oxidative stress, and genotoxicity. Considering the rapid increase in production and use of fibrous nanomaterials, it is imperative to gain a thorough understanding of their biologic activity to avoid the human health catastrophe that has resulted from widespread use of asbestos fibers.

FIGURE 1

Structure of asbestos fibers by transmission electron microscopy (TEM): (a) serpentine and (b–f) amphiboles. (a) International Union Against Cancer (UICC) asbestos chrysotile ‘A’ standard, (b) UICC asbestos crocidolite standard, Death Valley, California, (c) UICC asbestos anthophyllite standard, (d) winchite-richterite asbestos, Libby, Montana, (e) tremolite asbestos and (f) UICC asbestos amosite standard. Chrysolite is the only member of the serpentine group. Because of the mismatch in the spacing between the magnesium ions and the silica ions, chrysotile curls into a thin-rolled, flexible sheet while amphibole fibers are more rigid. Scale bar = 10 μm. (Reprinted with permission from Denver Microbeam Laboratory at the U.S. Geological Survey).

FIGURE 2

Structure of some carbon fibrous nanomaterials by TEM: (a–e) carbon nanofibers (CNFs) and (f–h) Carbon nanotubes (CNTs); (a) Fishbone solid CNFs, (b) Platelet spiral, (c) Stacked-cup, (d) Platelet-symmetry, (e) Platelet, (f) single-wall CNTs, (g) double-walled CNTs, and (h) multiwalled CNTs. Scale bar= 10 nm. TEM sources: (a) Chen, 2005, (b) Bandaru, 2007, (c) Vera-Agullo, 2007, (d) Jian, 2006, (e) Zheng, 2006, (f) www.rsc.org, (g) www.msm.cam.ac.uk, and (h) www.nccr-nano.org. Inset sources: (a–c, e, f, and h) Martin-Gullon, 2006 and (d) Jian, 2006.

FIGURE 3

Relationship between aerodynamic diameters of particles and lung deposition. (Reprinted with permission from Ref . Copyright 2005 National Institute of Environmental Health Sciences).