Studies explicitly dealing with the release of nanoscale carbon black from products during use are currently not available. In small doses, carbon black does not cause any harm.
An epidemiological study on carbon black exposure in carbon black-producing factories in England showed that some workers after increased exposure were suffering from cough, sputum production, and a decreased lung function . In Canada and the USA, another study revealed a correlation between the occurrence of chronic obstructive pulmonary diseases and increased carbon black exposure . Besides, studies of printing ink proved the carcinogenic effect of carbon black in mists of ink . A case study published in 2010 states that carbon particles from toners that have been inhaled can affect the health of the persons exposed .
The International Agency for Research on Cancer (IARC) has classified carbon black used in toners into group 2B. This means that „carbon black is possibly carcinogenic to humans”. However, IARC at the same time says that „there is inadequate evidence in humans for the carcinogenicity of carbon black“.
Another study carried out in 2001 shows that there is no correlation between lung cancer and carbon black . It seems that mixtures of the type used in toners may be more critical than pure carbon black.
Life cycle and possible release paths of nanoscale carbon black from products shown here as an example for tires. © Kuhlbusch 2010, UBA Study .
In 2004, Kuhlbusch et al. have shown that mainly carbon black-containing particles > 400 nm are released during carbon black bagging operations . Further studies by the authors reveal that during carbon black production, no release of nanoscale carbon black is expected to occur under undisturbed conditions. However, a release may occur as a result of leakages or during accidents .
Studies on Living Organisms – in vivo
In principle, Carbon Black can get into the organism via all entry portals such as the lung, the intestinal tract, and the skin (if it is injured). Most frequently, Carbon Black is taken up by the respiratory tract. It was found that high doses of inhaled Carbon Black do not cause damage to human organisms and the organisms of animals . Only very high doses can cause damage to the lung or lead to tumors of the lung in the extreme case.
In long-term studies where rats were made to inhale high doses of up to 1,1 mg/cm3 of Carbon Black for six hours/day on five days/week over 13 weeks with convalescence periods of three and eight weeks, no harmful effects were observed to occur. Only at still higher concentrations (> 2,5 mg/cm3), cell and tissue damage through to lung tumors were found to occur in the animals .
While in the case of inhalation, test animals are made to take in particles through the nose for a certain period of time, a particle suspension is instilled in the nasopharyngeal zone during instillation. Very high doses of nanoscaled Carbon Black administered by instillation cause an increase in the leukocytes which are known to be inflammation tracers . Extreme doses can cause tissue damage and, moreover, strongly impair the phagocytosis capability of the macrophages . Lungs of mice instilled with Carbon Black (0,1 - 0,5 mg/lung), however, did not show any signs of inflammation. Deposits of the particles showed as local black spots in the lung tissue .
Studies Outside the Organism - in vitro
Different studies conducted in the recent years have shown that certain doses of nanoscale carbon black (primary particle size approx. 14 nm), also referred to as ultra-fine carbon black in previous investigations, cause more oxidative stress and are more toxic to cells than coarser particles (> 100 nm) . Dose-dependent cytotoxicity, secretion of inflammatory markers, and reactive oxygen species were detected for different cell types . Pulskamp et al. have shown that in macrophages of the rat and in human epithelial lung cells, defined doses of 14 nm carbon nanoparticles can cause formation of dose-dependent reactive oxygen species (ROS) and that, in both cell lines, cell activity decreases accordingly . Other studies show that, depending on their type and origin, different cell types can react differently on the treatment with such particles .
Yet a further study observed an increase in apoptotic cells followed by apoptosis – the programmed death of cells - in cell layers treated with high concentrations of 20 µg of carbon black per cm2. During apoptosis, the damaged cells start shrinking, the DNA decomposes, and the cell destroys itself.
It has been reported by several authors that carbonic particles or fibers can cause problems in cell culture test systems . The false-positive, invalid results obtained due to interferences of the particles with pigments or dyes prevent one from making explicit statements on the toxicity of carbonic particles.
TEM image of Carbon Black agglomerates. © NanoCare Final Scientific Report
With this in view, the MTT assay was replaced by the WST test (water soluble tetrazolium) within the NanoCare project and at least one additional vitality test was carried out to obtain valid results.
Neither did any of the eleven cell lines of different origins tested in layers containing up to 10 µg of carbon black particles per cm2 exhibit stress symptoms nor did carbon black cause any cell-damaging effects. Moreover, a test for cell culture apoptosis proved negative. As shown in other studies, carbon black caused the formation of reactive oxygen species. Further in vitro experiments on human lung cells proved that the latter do not get stressed before being exposed to high doses of 25 µg particles per cm2 cell layer. The cell vitality was observed to decrease strongly at and above such particle concentrations .
Within NanoCare complex, so-called co-culture systems were used in addition to simple culture systems with only one cell line. By simulating the interaction of the cells, such systems allow a better representation of the in vivo situation in the body. The carbon black particles were found to trigger small biological effects in the respective systems .
In general, it could be shown that carbon black tends to agglomerate considerably. The agglomerates can be detected in the cells. Medium doses of carbon black cause the formation of ROS while high doses can damage cells in in vitro experiments . In addition, a genotoxic effect is described for high concentrations of carbon black contained in toners together with several other components (e.g. artificial resins, magnetizable metal oxides, dyes, and other additives) .