Carbon nanotube hazards, just like the hazards of other nanomaterials, require to be identified. While identification tests are part of the work in many laboratories in the world dealing with the biological effects of carbon nanotubes, there are also institutions that are engaged in assessing situations where individuals or the environment can be exposed to CNT (exposure). Data from these two approaches can be combined to assess the risks of CNT. Since carbon nanotubes are not yet applied to products from which they can be released in an uncontrolled way, such risks do practically not exist at present. This may change rapidly in case the quantity of carbon nanotubes applied should increase from a few to many thousand tons. If so, the workplaces (manufacturers of CNT and CNT-containing products), the buyers (users), and the environment (use and disposal) have to be observed carefully for the behavior of the substances applied. Numerous results have been obtained so far within the framework of the projects that have been conducted on the subject:

• TRACER, supported by the German Federal Ministry of Education and Research (BMBF), Germany, 2006-2009,
• NanoRisk, supported by the Swiss Federal Office for Health (BAG), Switzerland, 2005-2007,
• NanoMMUNE, supported by the EU, international consortium, 2008-2011,
• CarboSafe, supported by the German Federal Ministry of Education and Research (BMBF), Germany, since 2008,
• Nanosafe2, supported by the EU, international consortium: http://www.nanosafe.org/,
• Particle Risk, supported by the EU, international consortium.

The data obtained within the framework of the many studies carried out are used to assess possible risks (risk assessment) and to react, if necessary, to existing ones (risk management). In the final analysis, new materials such as CNT should not bear increased risks when used in everyday life in a large variety of products.

Literature:

1. Aitken, RJ et al 2006, Occup. Med. (Lond) 56, 300-306
2. Donaldson, K et al 2006, Toxicol. Sci. 92, 5-22
3. Helland, A et al 2007, Environ. Health Perspect. 115, 1125-1131
4. Jain, AK et al 2007,Nanotoxicology 1, 167-197
5. Kaiser, JP et al 2009, Nanomedicine 4, 57-63
6. Lam, CW et al 2006, Crit Rev. Toxicol. 36, 189-217
7. Smart, SK et al 2006, Carbon 44, 1034-1047
8. Tsuji, JS et al 2006, Toxicol. Sci. 89, 42-50
9. Warheit, DB 2006, Carbon 44, 1064-1069
   

Your car has broken down, so you want to cross the freeway by foot to seek for help. Almost instantly, you become aware of the fact that crossing will bear a considerable risk. Trying to assess the success of your unintentional adventure, you see that it all depends on the number of cars driving past. Surely you will think twice walking to the bridge nearby to cross the autobahn safely.
This example clearly shows that even if the situation does not change (i.e. even if the number of cars on the freeway remains unchanged), you can find a way to cross without risk.
We have to proceed in the same way in the case of nanomaterials, or in the case of carbon nanotubes (CNT) that are discussed hereunder.

Risk assessment must be preceded by identification of the potential hazards without which, indeed, risks would not exist at all (compare the example given above: cars may be dangerous). In addition, it is necessary to determine whether the environment or humans are exposed to CNT (compare our example: cars actually drive on the autobahn; if there are none, we don’t care how dangerous they are). Humans currently are not concerned. Since they may well become affected as the quantity of CNT produced increases, precautionary measures must be taken that anticipate or even presuppose exposure and promote the necessary studies in advance.
Eventually, all risk assessment must be based on information on the production quantities, the products, uses and disposal of carbon nanotubes as well as on the knowledge of their biological effects and environmental exposure . The effect depends on the dose occurring and remaining in the respective environment or organism.
Some hundred tons of carbon nanotubes are presently produced each year. Since quantities are expected to soon amount to many thousand tons, it makes sense to take a closer look at further aspects. Most of the CNT-containing products are composite materials, i.e compounds with plastics integrated in a matrix. This is important as regards the biological effects: Since, probably, there is no direct contact, the CNT will not be taken up into the body. Disposal issues, therefore, seem to be more relevant. Since CNT are modified carbons whose combustion in the presence of air leaves no other residues than water and CO₂, no problems are expected to occur. It is essential instead to identify the products that contain “free CNT” or that are applied directly to the body or in the body. The effects of increased uptake that can be expected must be tested in in vitro and in vivo tests or within toxicological studies. While much is known today on the biology of CNT, the effects of an increased uptake are analyzed world-wide at present. Reliable results will be available within some years.
A first appraisal based on existing knowledge is made regarding the two major findings of tests on animals and humans:

1. The particularly long carbon nanotubes (>> 30 mm), other than the short ones, cause disadvantageous biological effects in animals ^{[1, 3, 6]}.
2. In addition, a strong biological effect is observed to be caused by CNT containing metal residues left over from production ^{[2, 4, 5 ]}.

Since only short carbon nanotubes are produced today, the risk assessed is low. An increased risk is expected from future longer variants made up of stiffer bundles.

Literature:

1. Belyanskaya, L et al 2009, Neurotoxicology 30, 702-711
2. Kagan, VE et al 2006, Toxicol. Lett. 165, 88-100.
3. Poland, CA et al 2008, Nature Nanotech. 3, 423-428.
4. Pulskamp, K et al 2007, Toxicol. Lett. 168, 58-74.
5. Shvedova, AA et al 2009, Pharmacology & Therapeutics Vol121 (2),192-204
6. Wick, P et al 2007, Toxicol. Lett. 168, 121-131.

We are all used to getting around by car, thus incurring the risk of accidents. After all, approximately 4000 road users are killed in accidents each year in Germany alone^[1]. We accept this, however, for the benefit of increased mobility and personal freedom, and try to avoid “risk factors” by exercising particular caution in traffic, by avoiding alcoholic drinks, and by not driving when exhausted.
A comparative risk management will be applied to nanomaterials or carbon nanotubes. We know that biological effects are in particular produced when inhaling the large and long CNT. This can possibly be avoided by manufacturing only shorter types of CNT and by preventing inhalation, e.g. in workplaces, by means of suitable protective measures. In addition, CNT must not be used in products from which they can be released until proof has been established of the fact that they do not cause any harmful effects.

Literature and links:

1. Statistisches Bundesamt, Statistisches Jahrbuch 2008, S. 439
2. ISO: http://www.iso.org/iso/catalogue_detail?csnumber=43170
3. Wikipedia: http://en.wikipedia.org/wiki/ISO_31000