The question of whether carbon nanotubes can cause mesothelioma is not new and there have been a number of pre-clinical  (i.e. animal) studies that have looked at this. Some indicated the potential for disease, while others did not, and this is mostly due to the experimental design used for each: short time frame, variable anatomical locations and carbon nanotube samples being of mixed sizes etc., thus leading to inconclusive results.
 
This paper is the first that I’m aware of which has thoroughly investigated the carcinogenic potential of carbon nanotubes based on their size. It is important to note that the ability of carbon nanotubes to induce mesothelioma was restricted to those with similar properties to the known carcinogen of mesothelioma: asbestos.
 
In this study they used amosite (brown asbestos), as opposed to crocidolite (blue asbestos) which is what most Australians would be familiar with. Nonetheless, both amosite and crocidolite come from the amphibole asbestos group and have the long, thin, needle-like physical appearance, which is believed to enable them to persist for a long period of time in the body, once inhaled. Short asbestos and carbon nanotube fibres are easily removed in the body by immune cells, and therefore have little propensity for disease.
 
The strengths of this study include that it was well designed, they used a well-known mouse model and well defined samples, and they used the correct anatomical location for human disease (i.e. the pleural cavity). A potential limitation is that it was not an inhalation study - i.e. carbon nanotubes were not breathed into the lungs. Instead the samples were injected directly into the space between the lungs and inner chest wall - the pleural cavity.
 
The dosages used in the study are within the range of the accepted relative exposure levels for carbon nanotubes (see https://www.cdc.gov/niosh/docs/2013-145/default.html), so it is not unreasonable to presume that high-dose or long term low-dose exposure to particular carbon nanotubes could have an adverse health risk.
 
However, the likelihood that the average person would be at risk of exposure from everyday items (cosmetics, sunscreen, building materials etc.) would be low. This is because the carbon nanotubes are not in a respirable format. They are ‘locked up’ in the material and only if this material is damaged in a way that enables release of airborne, respirable fragments could an increased risk exist.
 
It would be reasonable to presume a risk to people involved in manufacturing processes where raw carbon nanotubes are produced, particularly those that fall into the ‘long’ category. Not only does this work have implications for further understanding the early stages of diseases like mesothelioma, but it may also inform how best to protect people from exposure during manufacturing.

These research results add to a growing body of evidence that a variety of materials can cause cancer (mesothelioma) to develop in the lungs. In the present case, the exposure to these particles is most likely in the workplace where they are being used to manufacture composite materials, rather than in the use of consumer products in which nanoparticles might be incorporated.
 
The details of how nanoparticles can cause mesothelioma to develop in the lungs are still being worked out, but evidence is accumulating about just which materials can cause trouble. Asbestos fibres are the best-known cause, but it's only some types of asbestos and some sized fibres. There has been scientific interest in whether other small particles can produce these effects, too. Attention has focussed on manufactured nanoparticles - that is, particles about a billionth of a metre in size. Various materials at this size have been tested and it is becoming clear that the surface properties of the nanoparticles, as well as their size, are what determines whether the particles are dangerous.
 
In the present case, the particles consist of carbon in the form of manufactured carbon nanotubes. The sizes of the carbon nanotubes used in these experiments was not revealed, but they came in two sizes - 'short' and 'long', with only the longer ones producing change in lung tissue.
 
Mice were used as the experimental animals. The nanotubes were injected into the fluid that surrounds the lung, in the pleural cavity. Changes in DNA and tissue were observed. The pleural cavity is where irritant materials end up, following ingestion with air into the lungs. The researchers also did the same experiment with asbestos fibres that are known to be carcinogenic, and got similar results.
 
A critical point to bring out is that exposure of the general public to these nanotubes is unlikely. The researchers mention the use of carbon nanotubes incorporated into sports equipment, computers and
(unspecified) building materials. The key word is 'incorporated' because there are no free nanotubes floating around to impact on users of these composite products, so we won't be exposed to them.
 
Who is exposed, then? The mention in paper that mice were exposed to 'occupationally relevant doses' tells us that the real target of the 'danger' message is workers who might be exposed to the nanotubes when they are being used to manufacture composite materials. Rather than indulging in the common 'nanoscare', the researchers see their work as helping manufacturers to choose safer alternatives when selecting nanomaterials for use in their products.
 
Overall, the research is sound, the conclusions reasonable, and the views expressed are helpful rather than merely scary.

This study is an excellent piece of investigative research exploring risks posed from the development of new advanced materials.

While we are profoundly more knowledgeable about health risks due to exposure to materials than we were thirty years ago, there’s still a lot we don’t know, especially as the rate of novel materials being designed accelerates. It is imperative that we understand the risks posed to the people handling new materials as well as to the broader community and environment.

This research identifies a possible health risk consistent with aspects of asbestos-induced disease due to a specific type of carbon nanotube. Carbon nanotubes have great potential in many applications due to electrical conduction, optical and mechanical properties. Currently, for consumers, they are not too common in day-to-day products but are used in some more specialised products where they provide superior strength. Their use is very likely to accelerate, however, and to occur more frequently in day-to-day products.

It is important to note that the observations made in this research are due to the physical dimensions specific to a particular nanomaterial, and do not translate to other nanomaterials that generally pose little risk. The greatest risk occurs for workers involved in the manufacture and handling of these materials. This research provides important awareness for guidance on creating safe workplaces.

For the general public, a greatly reduced risk exists since these materials are typically embedded within other materials such as resins within products. There is very little chance of the nanotubes, firstly, maintaining their original properties and causing the health risk, and, secondly, becoming isolated from the product and able to be inhaled by an individual.

While research and development leads to new materials that offer exciting innovations in our lives, there is a duty of care to fund this type of quality research to appropriately determine the risks posed as we embrace new technologies.