From exposure under real-life conditions of NC7000™ use, no indications of a harmful effect have been demonstrated:
- Based on hazard assessment on NC7000™, it has been possible to conclude that hazard of NC7000™ is similar to any industrial insoluble particles inducing transient pulmonary inflammation at high doses
- Exposure is likely to be micro-sized aggregates/agglomerates of NC7000™ and not free isolated NC7000™
Hazard of NC7000™ powder
For the time being a case-by-case approach is still appropriate and the hazard assessment described in this section is based solely on the information collected with NC7000™.
Although a large number of studies on the toxicity of MWCNTs have been published in scientific peer-reviewed literature, a clear characterization of the test materials (including sample preparation essential to ensure reproducibility and reliability in the toxicity test using suspension in vivo and in vitro methods), is often missing. Importantly, modifications of some specific characteristics of the MWCNT tested such as variations in size and shape, surface chemistry or purity levels might give rise to differences in the (eco) toxicological profile.
That means, on the other hand, that the values used for any risk assessment should not be associated to all MWCNTs or for CNTs in general. The results obtained with one particular type of MWCNT may not necessarily be relevant for other CNT with other dimensions and properties.
NC7000™ case: In compliance with generally accepted OECD guidelines, tests were conducted on intake and absorption via the mouth, skin and respiratory passages. The studies found no indications of a harmful effect from potential exposure under realistic industrial conditions. A long-term inhalation study similarly showed no indications of a harmful effect on health outside the respiratory passages. The effects observed in the lungs correspond to those typically described for insoluble particles meaning a local transient pulmonary inflammation at sites of predominant deposition for high dose exposure (Ma-Hock et al. 2009, Treumann et al. 2013]). From this study, an internal limit value of 2.5 µg/m3 was derived for workers exposure. Above and beyond the workplace limit value, comprehensive safety requirements are imposed on the manufacturing process.
Figure 6 summarizes the toxicological information generated on NC7000™ after dermal, oral or inhalation exposure. In parallel, ecotoxicological data (OECD 201, 202, 204, 211, 212) indicate no specific toxic effects of NC7000™. These results were generated in several recognized research centers either via funded research programs or via Nanocyl own funds.
Figure 6. Summary of (eco)toxicological data.
Regarding carcinogenicity of MWCNTs (chronic inhalation effects), different scientific studies propose to consider two main structures of materials:
- Category 1: Short, Thin, Flexible, Entangled (STFE) MWCNT
- Category 2: Long, Thick, Rigid ones (LTR)
The first category of MWCNTs or Short, Thin, Flexible, Entangled (STFE) MWCNTs such as NC7000™, does not behave like asbestos. This has been demonstrated in a 2-years in vivo carcinogenicity study in which rats were intraperitoneally exposed to a single dose of NC7000™ (2 or 20mg/rat) by comparison with asbestos. After 24 months, NC7000™ did not induce mesothelioma in this bioassay. This is contrary to asbestos that induced a clear carcinogenic response (34.6% animals with mesothelioma vs. 3.8% in vehicle controls) (Ref Muller et al, 2009).
The second category is clearly distinct from Category 1 MWCNT. This distinction between Short, Thin, Flexible, Entangled (STFE) MWCNT and Long, Thick, Rigid ones (LTR) was highlighted during the discussions at the International Agency for Research on Cancer (IARC)8. The experts concluded that there is sufficient evidence in experimental animal for the carcinogenicity of the Mitsui-7 MWCNT (second category). This specific Long, Thick and Rigid MWCNT type is now classified in IARC Group 2B like carbon blacks (possibly carcinogen to human).
However, all the other types of CNT (including NC7000™) are classified in the Group 3 (not classifiable as to their Carcinogenicity to Humans). Experts stated that “there is inadequate evidence in experimental animal tests for the carcinogenicity of all the other types of CNT that do not show similar dimensions and shapes to the Mitsui-7 MWCNT”.
Based on those above information, current good practice for the handling of carbon nanotubes is recommended to minimize potential exposition of workers especially for the exposure by inhalation. Handling guidelines, similar to those applied for any industrial insoluble particles, are sufficient to protect the workers from high dose exposure to NC7000™.
Exposure to NC7000™ powder
The industrial highest likelihood to release NC7000™ dust is during the manufacturing and the handling of NC7000™powder at industrial plants. However, this corresponds to situation in which exposure can be best controlled.
Internal measurement campaigns conducted at Nanocyl confirm that, without taking into account the risk management measures in place, powder handling tasks (e.g. transfer, weighing, bagging, dumping, maintenance of the reactor,…) were associated with higher exposure levels in the production area. However, due to the strong tendency of NC7000™ to agglomerate together (Vander Waals forces), exposure to a free isolated NC7000™ is statistically negligible and was never observed during the plant field measurement campaign.
This is confirmed by a dustiness study conducted with the NC7000™ powder in a clean laboratory atmosphere and showing that the average spherical agglomerate of NC7000™ is 14 µm (EN 15051- Gravimetrical dustiness test-internal report).
It is also important to note that conventional protective devices such as filter media and FFP3 cartridge for respirators reduce the exposure of this kind of aerosol. Overall, due to the strong tendency of NC7000™ to agglomerate in the micrometer range, risk management measures to ensure the safety are similar to those used for typical powders such as silica or carbon black.