Asbestos fibers were exploited commercially beginning in the early 20th century, with 50 million metric tons used between 1920-1970. Inhalation of asbestos fibers is associated with pulmonary fibrosis (asbestosis), lung and laryngeal cancers, and malignant mesothelioma, with a long latent period of 20-40 years. The 15-year cumulative mortality from malignant mesothelioma from 1994-2008 is 175,000 deaths.
The physical and chemical properties of asbestos fibers related to carcinogenicity include fiber dimensions, surface reactivity, and biopersistence following inhalation into the lungs. Long, rigid fibrous materials trigger incomplete or frustrated phagocytosis by target cells in the lungs, resulting in impaired clearance and persistent release of reactive oxygen species and proinflammatory mediators. Direct intraperitoneal injection of carbon nanotubes has also been shown to induce malignant mesothelioma in mice. Carbon nanotubes have shapes and dimensions similar to asbestos fibers; however, their graphenic surface is hydrophobic, while crystalline mineral fibers are hydrophilic. Asbestos fibers and carbon nanotubes are biopersistent; however, carbon nanotubes can be chemically modified to accelerate their degradation by oxidants.
Carbon nanotubes are a rapidly growing worldwide market, and production is projected to exceed 12,000 metric tons in 2016. At this early stage in commercialization, it should be possible to engineer less toxic, biocompatible, and biodegradable carbon nanotubes to minimize potential adverse health impacts.