Building blocks of modern materials, carbon nanotubes are valued for their strength and special electrical properties. However, growing concerns have surfaced about how harmful they might be when inhaled into the lungs. As more of these materials are produced globally, scientists are racing to figure out how they affect human health. Special attention is being paid to how their shape and stiffness might influence the risk of lung diseases, including cancer.
The research group headed by Professor Hiroyuki Tsuda from Nagoya City University took a close look at both stiff and flexible versions of these tiny tubes. Their research, published in the journal Nanomaterials, reviews a range of earlier animal experiments that explored how these materials interact with lung tissue. Their review offers new evidence that challenges past decisions about how risky these materials are to health. The group noted, “MWCNT-7 acts similarly to non-nanoparticle long fibers and induces mesothelium carcinogenicity.”
Previously, the international health organization IARC had listed a type of carbon nanotube called Mitsui multi-walled carbon nanotube number 7 as potentially harmful to humans, but that there was not not enough information to determine the effect of other types of carbon nanotubes on human health. Multi-walled carbon nanotubes are tiny cylindrical fibers made from layers of carbon atoms, and carbon nanotubes with more layers exhibit increased stiffness. Consequently, their lengths and stiffness suggest that they may be carcinogenic, similar to long rigid asbestos fibers, well-known human carcinogens. This also suggests that thinner flexible carbon nanotubes may not be carcinogenic. However, newer long-term studies using instillation into the rat lung show that both stiff and flexible types can lead to lung cancer. This challenges earlier studies that were short-term or used injection into the peritoneal cavity. Notably, even small exposures to the stiff type have led to the development of mesothelioma, a severe cancer affecting the lining of the pleural cavity. While the bendable types seem less dangerous, they too can cause damage to the lung over time if inhaled regularly.
Perhaps the most important takeaway is that, as shown in the figure, the shape and physical makeup of these particles result in different effects in the pleural cavity and the lung. The stiffer and thicker nanotubes can more easily puncture and injure cells and cause tissue damage and inflammation—much like asbestos. Mitsui multi-walled carbon nanotube number 7, which consists of multiple concentric tubes, acts similarly to non-nanoparticle long fibers and triggers cancer-causing activity in the mesothelium. This highlights how the shape of the nanotube plays a big role in how dangerous it can be in the pleural cavity. As Professor Tsuda explained, “Rigid MWCNTs are not readily phagocytosed, remain in the pleural cavity, and induce chronic inflammation and genotoxicity.” In contrast, inflammation in the lung is associated with activation of macrophages and production of inflammatory cytokines, and both thicker, stiffer carbon nanotubes and thinner, flexible carbon nanotubes interact with macrophages in the lung and induce inflammation and tissue damage. If the carbon nanotubes are not removed from the lung they cause cycles of inflammation and tissue damage, which can result in carcinogenesis.
The researchers examined several studies where the carbon nanotubes were introduced either through injection, instillation, or breathing. Injection means delivering the material directly into the body using a syringe, and instillation exposure mimics inhalation through the air. While the injection tests helped identify basic dangers, it was the long-term breathing and instillation exposure studies that revealed the strongest link to cancer under more realistic working conditions. In these tests, animals instilled with the stiff nanotubes and animals instilled with flexible nanotubes developed tumors in the lung, supporting concerns about long-term respiratory damage.
Framing their results within a broader context, the scientists are calling for health officials to take a fresh look at the current assessment of the toxicity of carbon nanotubes. Reassessment refers to a careful re-evaluation of existing health classifications. They conclude that current findings support the reassessment of the cancer-causing classification of multi-walled carbon nanotubes. They argue that all forms—whether thick or thin—should be judged by how they behave in long-term exposure, not just by older short-term tests. This change would be especially important for industries that use these materials, helping to protect workers and shape better safety policies. Professor Tsuda emphasized, “These results support the reassessment of the carcinogenicity classification of MWCNTs.”
Wider use of multi-walled carbon nanotubes in commercial products makes this study a timely alert. Commercial products refer to items made for sale in the marketplace. The message is clear: as new technologies emerge, so must careful evaluation of their health risks. This research encourages a shift in how materials are judged—not just by what they’re made of, but also by how they behave when used in real-world conditions.
Importantly, industry, researchers, and government need to work together. Identifying toxic materials is the first step in ensuring safe manufacture and use of that material. Importantly, the fact that a material is toxic does not mean that it should be banned, but rather that measures should be put into place to ensure the safe manufacture and use of that material. To make another analogy with asbestos, it was when human death was linked to asbestos that asbestos producing companies in many countries went out-of-business. In contrast, the manufacture and use of numerous other toxic compounds is routine. For example, formaldehyde is a well-known toxin that is also carcinogenic in humans. However, government safety regulations allow its widespread use in industry, consumer products, research, and medicine. Similarly, we believe that safety regulations regarding the manufacture and use of carbon nanotubes can also permit the safe use of these extremely valuable materials. However, the first step in implementing these safety regulations is identification of toxic/carcinogenic materials. The researchers conclude that the data they present indicate that the carcinogenicity of carbon nanotubes should be reassessed, and they propose that once carcinogenic carbon nanotubes are identified, that safety regulations be put into place with the goal of allowing the safe manufacture and use of these extremely valuable materials.
Journal Reference
Ahmed O.H.M., Naiki-Ito A., Takahashi S., Alexander W.T., Alexander D.B., Tsuda H. “A Review of the Carcinogenic Potential of Thick Rigid and Thin Flexible Multi-Walled Carbon Nanotubes in the Lung.” Nanomaterials, 2025; 15(168). DOI: https://doi.org/10.3390/nano15030168
About the Authors
Omnia Hosny Mohamed Ahmed, born in 1989 in Aswan, Egypt, is a clinical toxicologist and researcher. She earned her B.Sc. in Medicine from Sohag University in 2012 and completed her M.Sc. in Clinical Toxicology at Aswan University in 2019. Currently, she serves as a lecturer in the Department of Forensic Medicine and Clinical Toxicology at Aswan University. Ahmed is also affiliated with the Nanotoxicology Project Laboratory at Nagoya City University in Japan, where she contributes to studies on the pulmonary toxicity and carcinogenicity of nanomaterials, including carbon nanohorns and carbon nanobrushes. Her research has been presented at international conferences, such as the Japanese Society of Toxicology annual meeting. As a student member of the Japanese Society of Toxicology, Ahmed is actively engaged in advancing the field of toxicology through both academic and collaborative research efforts.
Dr. Aya Naiki-Ito is a medical doctor and researcher specializing in experimental pathology and tumor biology. She holds both M.D. and Ph.D. degrees and is affiliated with Nagoya City University in Japan. Her research focuses on the mechanisms of carcinogenesis, particularly in relation to nanomaterials and environmental toxins. Naiki-Ito has contributed to studies examining the toxicological effects of various substances, including carbon nanotubes, on organ systems. Her work often involves developing novel animal models to study disease progression and potential therapeutic interventions. Through her research, she aims to enhance the understanding of cancer development and contribute to the development of effective prevention and treatment strategies.
Professor Hiroyuki Tsuda is a leading expert in nanotoxicology and carcinogenesis, currently heading the Nanotoxicology Project Laboratory at Nagoya City University in Japan. With a Ph.D. from the Tokyo Institute of Technology, Tsuda has dedicated his career to studying the health impacts of nanomaterials, particularly carbon nanotubes. His research has significantly contributed to understanding the pulmonary toxicity and carcinogenic potential of these materials. Tsuda’s work includes long-term in vivo studies assessing the effects of nanomaterials on lung and pleural tissues, providing critical insights into their safety profiles. He has also been involved in developing international guidelines for training toxicologic pathologists, emphasizing the importance of rigorous scientific standards in nonclinical toxicity studies.
