Assessment of the Carcinogenicity of Carbon Nanotubes in the Respiratory System

SIMPLE SUMMARY: Malignant mesothelioma is an aggressive cancer of the membranes covering the lung and chest cavity (pleura) or the abdomen (peritoneum), mainly linked to asbestos exposure. Asbestos is a proven human carcinogen but its use is far from being universally banned and the forecasts on the incidence of mesothelioma over the next several years are far from optimistic. Carbon nanotubes are a promising type of nano-materials used in the field of nanotechnology for a wide range of applications. However, the similarities between asbestos and CNTs have raised many concerns about their danger and are still the subject of intense research. Keeping in mind that the asbestos tragedy could have been prevented, the aim of this study is to review the recent scientific evidence on CNTs carcinogenicity. ABSTRACT: In 2014, the International Agency for Research on Cancer (IARC) classified the first type of carbon nanotubes (CNTs) as possibly carcinogenic to humans, while in the case of other CNTs, it was not possible to ascertain their toxicity due to lack of evidence. Moreover, the physicochemical heterogeneity of this group of substances hamper any generalization on their toxicity. Here, we review the recent relevant toxicity studies produced after the IARC meeting in 2014 on an homogeneous group of CNTs, highlighting the molecular alterations that are relevant for the onset of mesothelioma. Methods: The literature was searched on PubMed and Web of Science for the period 2015–2020, using different combinations keywords. Only data on normal cells of the respiratory system after exposure to fully characterized CNTs for their physico-chemical characteristics were included. Recent studies indicate that CNTs induce a sustained inflammatory response, oxidative stress, fibrosis and histological alterations. The development of mesothelial hyperplasia, mesothelioma, and lungs tumors have been also described in vivo. The data support a strong inflammatory potential of CNTs, similar to that of asbestos, and provide evidence that CNTs exposure led to molecular alterations known to have a key role in mesothelioma onset. These evidences call for an urgent improvement of studies on exposed human populations and adequate systems for monitoring the health of workers exposed to this putative carcinogen.