This brief distills the policy recommendations presented in our comprehensive research article, “Human and Environmental Safety of Carbon Nanotubes Across their Life Cycle,” published in Nature Reviews Materials.[1]
Introduction
Carbon nanotubes (CNTs) are an exciting class of carbon-based nanomaterials with broad and expanding uses, but they have received substantial scrutiny regarding their human and environmental impacts. Microscopic cylindrical tubes of carbon that exist either as a single concentric layer, or as multiple nested tubes, the physical form of a carbon nanotube determines its physical and chemical properties; and that form can be broadly tuned for widespread applications.
Due to their superior physical and chemical properties — including high tensile strength, thermal conductivity, and tunable semiconductivity — their use in industry is wide-ranging from batteries to high-strength concrete to photovoltaic cells. The industrial use of carbon nanotubes is rapidly expanding, with annual global production already surpassing 5,000 metric tons.
Negative Perceptions and Progress
In certain forms, carbon nanotubes have been linked to negative biological effects, sparking concern among researchers that their widespread adoption may result in adverse unintended biological and environmental impacts.
However, researchers have identified many synthetic techniques and processing methods to produce carbon nanotube forms that do not exhibit these toxicities or environmental concerns.
Despite the ever-increasing variety of carbon nanotube-based materials with different properties and applications, negative perceptions persist due to earlier studies on certain types of nanotube-based materials. Carbon nanotubes have been highlighted by a nongovernmental organization in the European Union as a nanomaterial that should be substituted if possible, and potentially banned. The scientific evidence paints a nuanced picture of the health and environmental risks of carbon nanotubes, so policymakers must take a context-dependent regulatory approach to minimize risks to industry workers and the public, while also minimizing undue burdens on scientific research and industrial applications. With the increasing utilization of CNTs, human safety, environmental, economic, and all other potential impacts need to be holistically assessed throughout the life cycle of carbon nanotubes and nanotube composite materials (Figure 1).
In this policy brief, we discuss 1) what carbon nanotubes are and the variety of forms and composites, 2) current, realized, and potential future benefits of using carbon nanotubes in advanced materials, and 3) known and potential risks to humans and the environment. Finally, we offer policy recommendations to minimize risks to workers and the public, while facilitating the materials transition to a sustainable future.
Figure 1 — Stages of the Carbon Nanotube Life Cycle
The Risk-Benefit Profile of Carbon Nanotubes
From a physical and chemical perspective carbon nanotubes are an interesting, if not unlikely, material stemming from the nature of their carbon-carbon linkages. They:
- Have exceptional mechanical properties through a combination of tensile strength and stiffness.
- Can exhibit extraordinarily high electrical conductivity and can have tunable semiconducting properties.
As a result of these unique electronic and mechanical properties and physicochemical diversity, CNTs have been researched for a wide array of applications in diverse fields such as optoelectronics, light sources, environmental remediation, biological research, diagnostics, therapeutics, functional textiles, wearable devices, and construction. Industrial applications are rapidly increasing, with current markets for carbon nanotubes including lightweight, high-strength composite materials (e.g., Nanocore), high-performance electronics, and energy storage.[2]
Offsetting Highly Toxic Materials
Key industries that are leveraging the advantageous properties of carbon nanotubes include industrial manufacturing, alternative energy, and battery and energy storage. These industries are turning to carbon nanotubes to offset usage of highly toxic and environmentally damaging materials and to create more efficient materials. For example, CNTs can obviate the use of heavy metals and critical minerals in batteries. They can also make concrete exceptionally strong, thereby reducing the carbon dioxide (CO2) emissions produced by the standard way of creating concrete. In consumer products, the risk of exposure to carbon nanotubes would most likely be to industrial workers during production, to consumers in the form of the finished product, and in the environment at end-of-life or end-of-use (re-X such as reuse, repair, remanufacture, repurpose, refurbish, or recycle) (Figure 2). By minimizing risks to industry workers and taking appropriate end-of-life considerations, the potential negative environmental and toxicological effects can be mitigated.
Toxicological Impact
Scientific research indicates that toxicology of CNTs is very dependent on their specific physical forms and chemical properties.
- For certain forms of multi-walled carbon nanotubes, there is a body of work suggesting that they can cause persistent cellular inflammation and DNA damage as a result of their particular size and shape.
- In contrast, single- and few-walled nanotubes can be processed to remove metal impurities and have been reproducibly shown to impart no adverse biological effects on cells or animals.
The International Agency for Research on Cancer (IARC) — part of the World Health Organization — has thus classified a specific type of multi-walled carbon nanotube (MWCNT-7) as possibly carcinogenic to humans, while finding that for single-walled carbon nanotubes there is insufficient evidence to indicate that they have carcinogenic effects.[3]
Both single- and multi-walled carbon nanotubes have generated concern among environmental advocates due to their potential environmental persistence and accumulation up the food chain.[4] These issues merit substantial research to evaluate the potential risks and identify mechanisms to reduce any negative effects. Evidence-based practices involving appropriate end-of-life processing for CNT materials will be important for sustainable use.
Net Environmental Impact
To accurately assess the net environmental impact of carbon nanotubes, policymakers must consider the positive effects achieved from their use. Although there is the potential for negative environmental impacts of carbon nanotubes, such impacts can be mitigated. There are also potential beneficial impacts of carbon nanotubes, including for minimally CO2 intensive production as compared to hard to decarbonize construction materials.
Due to their unique material properties, CNTs can be used for environmental remediation purposes including highly efficient absorption of chemicals for wastewater treatment, use in solar panels and other renewable energy applications, and as components of more efficient batteries.
Additionally, carbon nanotubes can reduce environmental impacts by curtailing demand for the extraction of energy and resource-intensive materials such as steel, concrete, or rubber, and the use of lightweight carbon nanotube composites in the aerospace industry can improve fuel efficiency and reduce carbon emissions. Although CNT composites are not yet cost-effective — research does, however, show promise in reducing costs — these potential benefits must be considered when looking holistically at their environmental impact.
Figure 2 — Potential Routes of Exposure to Carbon Nanotubes Across Their Life Cycle
Concerns About Commercial Usage and Regulation
As carbon nanotubes enter commercial use, there are legitimate concerns about toxicological and environmental impacts. How to effectively regulate nanomaterials — to ensure safety to all stakeholders throughout global supply chains while not unduly hindering innovation or their safe usage for low-risk applications — remains an open question.
The EPA has authority via Section 4 of the Toxic Substances Control Act of 1976 (TSCA) to require manufacturers, importers, or processors to test chemical substances and mixtures, including nanomaterials such as CNTs, and to collect environmental and health data about when EPA risk assessors have insufficient data. The EPA also allows the manufacturing of new nanoscale materials under the terms of certain regulatory exemptions, but only in circumstances where exposures are tightly controlled to protect against unreasonable risks. Manufacturers and researchers thus should continue to demonstrate a safety profile of carbon nanotubes that shows that the benefits outweigh the costs for that specific application.
Policy Recommendations
Taking a science-based and data-driven life cycle approach for carbon nanotubes, we propose the following policy recommendations:
- More precise and accurate classification and identification of carbon nanotubes should be adopted to allow for improved communication among researchers, industry and policymakers.
- Policymakers and industry leaders should take a context-dependent approach to regulations and risk mitigation involving carbon nanotubes and carbon nanotube composite materials and consider both risks and benefits arising from the replacement of toxic and environmentally damaging materials.
- Safety and environmental assessments of carbon nanotubes should expand beyond primary risks to occupational workers to all stakeholders across their lifecycle.
- Carbon nanotube policies should promote principles of a circular economy.
We detail each of these proposals below.
1. A Precise Classification and Identification System
A significant obstacle to the understanding of carbon nanotubes and their composite materials is their chemical diversity, and more importantly, the absence of systems to classify and identify materials based on their distinct toxicological and environmental considerations. Despite the chemical and physical diversity of CNTs, their structural and functional diversity is often overlooked.
More, and More Specific, CAS Numbers Are Needed
The Chemical Abstracts Service (CAS) database has several CAS numbers for carbon nanotubes, and new CAS numbers can be initiated as necessary, however all commercial carbon nanotube materials currently use a single number (308068-56-6) to refer to any type of carbon nanotube, irrespective of their physical structure, method of synthesis, or chemical functionalization. Unfortunately, while there are other CAS numbers for more specific subtypes of CNTs and manufactured raw chemical products, they are not widely used or recognized by industry, academia, or policymakers.
Consequently, the properties of carbon nanotube materials cannot be distinguished by nomenclature. This lack of specificity leads to generalized statements about toxicological and environmental concerns of carbon nanotubes that may only apply to a limited subset of these materials.
To better enable material comparisons and avoid confusion among policymakers, it would be beneficial for manufacturers of CNT products and academics to use more specific CAS numbers in product listings and scholarly publications — instead of the general carbon nanotube CAS number — whenever feasible. Advantages of this approach include:
- The identification and cataloging of existing and new carbon nanotube-based products could enhance the evaluation and distinction of the human and environmental health and safety attributes of carbon nanotube materials produced at scale.
- Academic researchers citing more specific registry numbers would bring clarity when making research assertions, thereby avoiding incorrect conflation and overgeneralization.
- By modifying the nomenclature to accommodate the various types of carbon nanotubes, research reproducibility could be improved through more accurate methodological reporting. This would assist scientists in evaluating the toxicological and environmental impacts of the numerous forms of CNTs and offer policymakers improved guidelines for use based on their distinct toxicological and environmental considerations.
2. A Context-Dependent Approach to Regulating CNTs
Carbon nanotubes comprise an extremely varied group of materials with a wide range of physical and chemical properties. Depending on the method of synthesis, certain inherent features like the number of walls, and some adjustable features such as metallic impurities and chemical modifications or derivatization, the toxicological and environmental impact of CNTs and their composite materials can be finely adjusted. Broad policies that try to regulate carbon nanotubes and their nanocomposite materials as a single entity, with a single risk-benefit profile, could stifle innovation and unnecessarily burden safe applications of these materials if they are overly strict, while potentially endangering occupational workers and end consumers if they are excessively lenient.
When assessing risk-benefit profiles, policymakers should consider the specific contexts in which the carbon nanotubes are used. For instance,
- Alternative energy — Certain types of multi-walled carbon nanotubes, despite having a more adverse toxicological profile compared to single-wall carbon nanotubes, play a crucial role in alternative energy sectors, e.g., in photovoltaic cells and as capacitors in high-efficiency batteries. These applications could potentially replace heavy metals, thereby contributing significantly to the ongoing materials transition.
- Concrete — Carbon nanotubes can be used as a high-strength nanocomposite additive in concrete. Currently the production of concrete is highly energy-intensive, and the integration of CNTs could considerably lower emissions, especially if produced via methane pyrolysis, where no CO2 is produced.
Such specialized uses may not pose a substantial risk to human health or the environment, especially when compared to the heavy metals they replace, and any potential impacts can be mitigated through the careful development of well-defined policies governing their disposal at the end of their life cycle.
Policymakers should strike a balance between fostering innovation and reducing risk: A policy approach that considers the wide variety of forms and functions of carbon nanotubes is essential.
3. Safety and Environmental Assessments Across the CNT Lifecycle
Policies on carbon nanotubes and their composites should require toxicological impact studies throughout the material’s life cycle, from inception to disposal.
- Supply chain — The scope of these studies should extend beyond the respirable risks for occupational workers, to evaluate the risk across all exposure pathways throughout the entire supply chain.
- Toxicological effects — Investigations into the toxicological effects linked with exposure to carbon nanotubes in their intended form (like nanocomposites or fibers) will aid policymakers in devising effective regulations for consumer protection.
- Long term — Research on the long-term fate of carbon nanotubes in different formulations and their potential environmental release through various end-of-life or end-of-use routes will ensure a comprehensive risk assessment of carbon nanotubes, mitigation of any potential impacts, and proper accounting for all adverse externalities.
Toxicological considerations at the end of life are crucial in determining whether carbon nanotubes and materials incorporating them can be safely reused, repurposed, or recycled, or, if there are no available or feasible life cycle extension pathways, how to ensure safe disposal.
Currently, CNTs are predominantly used in industrial processes as battery components or as part of nanocomposite materials. When these materials reach their end of life, they are typically landfilled, incinerated, or shipped to developing economies where there is a lack of responsible end-of-life management or proof of legitimate disposal. There is also a significant research gap regarding the toxicological or ecological effects of carbon nanotubes that become aerosolized and dispersed through the incineration of nanocomposites containing CNTs, or through the escape of carbon nanotubes from landfills via rainwater drainage.
4. CNT Policies Should Promote Circular Economy Principles
Carbon nanotubes, with their distinctive electronic and mechanical characteristics, are part of the ongoing energy and materials revolution that is reshaping our world. These materials show considerable potential for promoting sustainable development principles and may have less environmental and social impact than the materials that they replace. This highlights the urgency for policymakers to devise policies that not only foster innovation but also integrate the fundamental tenets of a circular, sustainable economy.
For a smooth shift toward sustainable, climate-neutral expansion, it is crucial to incorporate carbon management strategies throughout the entire carbon life cycle. The concept of a circular carbon economy provides a structure for effectively handling carbon at each stage of its life cycle. For example, the carbon feedstock to produce carbon nanotubes could originate:
- From bio-based chemistry that captures atmospheric carbon via photosynthesis using renewable biological resources such as biomass.
- Through chemical valorization of CO2 and other carbon-based emission sources generated from industrial sites, landfills, agriculture, forestry, and land use and captured before they enter the atmosphere (e.g., from methane pyrolysis that directly converts methane in natural gas to hydrogen and solid carbon materials without the creation of CO2).
- From chemical recycling of CNT waste and CNT products at end of use that otherwise would be incinerated or sent to landfill (Figure 3).
With the swiftly growing demand for CNTs in diverse fields, large-scale carbon nanotube materials present a significant potential to fulfill global material requirements. They are poised to become more sustainable constituents of the economy while also contributing to the reduction of CO2 emissions.
Figure 3 — Circular Carbon Economy
A well-defined set of guidelines, best practices, and standard procedures, developed with contributions from the academic community, industry, NGOs, and regulatory authorities, will offer regulatory and scientific assurance, significantly enhancing public comprehension and confidence in carbon nanotubes. Life cycle evaluations that are appropriately scoped and framed, using regionally relevant datasets, can provide insights to the environmental, health, and safety facets to a wider audience. Given the interconnected nature of global supply chains, the impacts are not limited to a single industry or geographic region. Gathering data from a wide spectrum will shed light on potential compromises, unforeseen outcomes, and the shifting nature of the risks.
Conclusion
Carbon nanotubes have exceptional physicochemical characteristics that hold promise for applications in electronics, structural materials, and clean energy. However, they also carry potential health and environmental risks. Recent studies suggest ways to alleviate these risks, advocating for a balanced regulatory strategy that weighs both the advantages and dangers. As the use of nanotubes spreads, it is crucial for policymakers to implement policies that are sensitive to specific contexts, prioritizing the safety of workers and the public while facilitating a shift toward a more sustainable future.
To ensure a sustainable and evidence-based materials and energy transition partially enabled by carbon nanotubes and their composites, policymakers should implement four important frameworks — these mirror the recommendations detailed above:
- Classification and identification — Classification and identification of carbon nanotube materials should be conducted carefully to facilitate consistent communication among researchers, industries, and policymakers. In this regard, a single CAS number should not be used for all carbon nanotube materials. Each type of manufactured carbon nanotubes should be assigned unique CAS numbers, which would be utilized by academia and industry to enhance reproducibility and comparisons.
- Context-dependent regulations — Regulations for carbon nanotubes and their composites should be context-dependent, taking into account the benefits derived from replacing toxic and environmentally harmful materials.
- Safety and risk assessments through lifecycle — Safety and risk assessments should encompass all stages of the carbon nanotube lifecycle, not just manufacturing and raw material handling. If addressed appropriately, these measures will equip regulators with the tools to understand carbon nanotube materials, while ensuring that any restrictions on synthesis, production, manufacturing, use, transportation, and disposal are minimally disruptive to important industries.
- Promotion of circular carbon economy — Policies related to carbon nanotubes should promote the transition to a circular carbon economy.
In addition to setting these frameworks in place, international efforts should be made to establish a coordinated system for classifying and testing carbon nanotubes and to create a central repository of open-source scientific information, risks, benefits, and uncertainties associated with carbon nanotubes. This will help to reduce regulatory barriers to international trade and commerce.
Carbon nanotubes hold great potential as a key component in future strategies for decarbonization and sustainability. From a life cycle standpoint, the use of CNTs could result in significantly lower energy and material needs, as well as fewer environmental and social impacts — by diminishing the need for primary resource extraction and the processing of energy-intensive metals, minerals, and materials, along with their complex supply chains.
As we move toward a revolution in sustainable energy and materials, it is critical that the advanced materials sector, including carbon nanomaterials, has a transparent and consistent journey from inception to end-of-life. This journey should be supported by suitable, scientifically driven standardization and classification, guided by policies based on life cycle considerations, and steered by industry best practices that are informed and backed by robust evidence of the advantages and disadvantages to people, the economy, and the environment.
Acknowledgements
The authors would like to thank Michael Dennis, Jeffrey Wilson, Mark Schmidt, and Manuel Guzman of Chemical Abstract Services (CAS), a division of the American Chemical Society, for their valuable contributions and continued support in this ongoing effort.
Notes
[1] Full-text access is available here: https://rdcu.be/drPzl (Mijin Kim et al., “Human and Environmental Safety of Carbon Nanotubes Across Their Life Cycle,” Nature Reviews Materials 9 [January 2024]: 63–81, https://doi.org/10.1038/s41578-023-00611-8).
[2] According to its website, Nanocore ApS has “developed a new type of coupling agent to incorporate the strongest molecules known, carbon nanotubes, into composite materials:” “About,” Nanocore, accessed February 22, 2024, https://www.nanocore.com/about/.
[3] Yasuo Morimoto and Norihiro Kobayashi, “Evaluations of the Carcinogenicity of Carbon Nanotubes, Fluoro-Edinite, and Silicon Carbide by the International Agency for Research on Cancer (IARC),” Nihon Eiseigaku Zasshi 71, no. 3 (2016): 252–259, https://pubmed.ncbi.nlm.nih.gov/27725428/?, original article in Japanese: https://doi.org/10.1265/jjh.71.252.
[4] There is evidence of accumulation via the food chain in research studies. However, whether there will be long term or widespread food chain usage of CNTs remains an open question.
This material may be quoted or reproduced without prior permission, provided appropriate credit is given to the author and Rice University’s Baker Institute for Public Policy. The views expressed herein are those of the individual author(s), and do not necessarily represent the views of Rice University’s Baker Institute for Public Policy.
