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Introduction to Multi-Walled Carbon Nanotubes and Their Risks
Multi-walled carbon nanotubes (MWCNTs) are cylindrical nanostructures consisting of multiple layers of graphene sheets. Due to their unique physical, chemical, and mechanical properties, MWCNTs have gained significant attention in various fields, including electronics, materials science, and medicine. However, the increasing production and use of MWCNTs raise concerns regarding their potential health risks, particularly their carcinogenicity when inhaled.
The International Agency for Research on Cancer (IARC) has classified MWCNT-7 as a Group 2B material, which indicates it is possibly carcinogenic to humans. Other types of MWCNTs remain classified as Group 3 materials, suggesting inadequate evidence of carcinogenicity in experimental animals (Nanomaterials, 2025). This review explores the carcinogenic potential of MWCNTs in the lungs by discussing their mechanisms of action, comparative toxicity in animal studies, long-term health effects, and regulatory perspectives.
Mechanisms of Carcinogenicity in Multi-Walled Carbon Nanotubes
The carcinogenic potential of MWCNTs is believed to arise from several mechanisms, including direct cytotoxic effects, inflammation, and oxidative stress. MWCNTs can induce cellular damage through reactive oxygen species (ROS) generation, leading to DNA damage and promoting tumorigenesis. The persistence of MWCNTs in the lungs can trigger chronic inflammatory responses, a key contributor to cancer development.
The Role of Fiber Dimensions
Stanton’s hypothesis suggests that the carcinogenicity of fibers, including asbestos and MWCNTs, is related to their length and rigidity. Longer and straight fibers are more likely to cause carcinogenic effects due to their ability to evade macrophage clearance and induce chronic inflammation. Studies have shown that rigid MWCNTs exhibit more significant lung toxicity and carcinogenic potential compared to their flexible counterparts (Nanomaterials, 2025).
Inflammatory Responses and Cytokine Release
When MWCNTs are inhaled, they interact with lung epithelial cells and macrophages, leading to the release of pro-inflammatory cytokines such as IL-1β, TNF-α, and IL-6. These cytokines facilitate a cascade of inflammatory responses that can promote tumor development in the lung tissue. For instance, studies have indicated that MWCNTs can stimulate the upregulation of matrix metalloproteinases (MMPs), which play a crucial role in tissue remodeling and cancer progression (Nanomaterials, 2025).
Comparative Analysis of Lung Toxicity in Animal Studies
Numerous animal studies have been conducted to assess the lung toxicity of MWCNTs. The intratracheal instillation and inhalation of MWCNTs have revealed varying degrees of pulmonary inflammation and fibrosis, particularly with rigid MWCNTs.
| Study | Route of Administration | Animal Model | Key Findings |
|---|---|---|---|
| Takagi et al. (2008) | Intraperitoneal Injection | p53+/− Mice | Developed mesothelioma in 14 of 16 mice injected with MWCNT-7. |
| Sakamoto et al. (2009) | Intrascrotal Injection | Fischer 344 Rats | Developed mesothelioma in 6 of 7 rats; MWCNT-7 induced inflammation. |
| Murphy et al. (2013) | Pharyngeal Aspiration | C57BL/6 Mice | Long rigid MWCNTs induced pulmonary inflammation and fibrosis. |
| Xu et al. (2014) | Intratracheal Instillation | F344 Rats | Indicated that MWCNTs can translocate to the pleural cavity and induce mesothelial lesions. |
These studies collectively emphasize the potential hazards of MWCNT inhalation, particularly for rigid types, which induce more pronounced inflammatory responses and long-term health risks.
Long-Term Effects of Multi-Walled Carbon Nanotubes on Health
The long-term effects of MWCNT exposure in humans remain inadequately understood due to limited epidemiological studies. However, findings from animal models suggest that chronic exposure to MWCNTs can lead to permanent lung damage, including fibrosis and cancer. For instance, studies have shown that MWCNT-7 can lead to lung tumors and mesothelioma in rodent models after prolonged exposure (Nanomaterials, 2025).
Implications for Occupational Health
Workers in industries that manufacture or utilize MWCNTs could be at increased risk of developing respiratory diseases, including lung cancer and mesothelioma. Regulatory frameworks need to be established to monitor and mitigate exposure in occupational settings. The potential for airborne MWCNTs to contribute to lung toxicity necessitates stringent safety measures and guidelines to protect workers’ health.
Regulatory Perspectives on Multi-Walled Carbon Nanotubes Safety
Given the potential risks associated with MWCNTs, regulatory bodies, including the IARC, have called for more research to assess their safety. Currently, regulations surrounding MWCNTs vary by country, with some jurisdictions requiring safety data and risk assessments for nanomaterials used in consumer products.
Recommendations for Future Research
Further studies are essential to elucidate the long-term health impacts of MWCNT exposure, particularly regarding their carcinogenicity and mechanisms of action. It is crucial to conduct comprehensive risk assessments that consider the physicochemical properties of MWCNTs, exposure routes, and the biological responses elicited in various tissues.
Conclusion
The carcinogenic risks posed by multi-walled carbon nanotubes in the lungs are significant and warrant attention from researchers, regulators, and industry stakeholders. Given their unique properties and increasing use in various applications, understanding the mechanisms of toxicity and implementing effective regulatory measures is essential for safeguarding public health.
FAQ
What are multi-walled carbon nanotubes? Multi-walled carbon nanotubes (MWCNTs) are cylindrical nanostructures made of multiple layers of graphene sheets. They have unique mechanical, electrical, and thermal properties.
Why are MWCNTs considered carcinogenic? MWCNTs are considered potentially carcinogenic due to their ability to induce chronic inflammation, oxidative stress, and cytotoxicity in lung tissues, leading to DNA damage and tumor formation.
What types of studies have been conducted on MWCNTs? Numerous animal studies have assessed the lung toxicity of MWCNTs through various routes of administration, including inhalation, intratracheal instillation, and intraperitoneal injection.
How can exposure to MWCNTs be regulated? Regulatory measures should include risk assessments, monitoring of workplace exposure, and the establishment of safety guidelines to protect workers and the general public from potential health risks associated with MWCNTs.
References
- Nanomaterials (2025). A Review of the Carcinogenic Potential of Thick Rigid and Thin Flexible Multi-Walled Carbon Nanotubes in the Lung. Retrieved from https://doi.org/10.3390/nano15030168
- IARC (2017). IARC Working Group on the Evaluation of Carcinogenic Risks to Humans: Some Nanomaterials and Some Fibres