Carbon black has been massively used as a filler in the rubber tire industry since the early 19th century. Although Henderson et al. have claimed that the current TLV is too high for the workplace, the current TLV of 3.5 mg/m3
based on toxicity data reported by Nau et al. [12
] in 1962 has not yet been changed.
Carbon black has been suspected as a cause of respiratory disease, cardiovascular disorders and cancer. However, there is little consistency in the results from epidemiological studies. Robertson and Ingalls [13
] reported that carbon black did not increase the incidence of cancer, respiratory disease or cardiovascular disorders in workers at a carbon black manufacturing facility. Crosbie [14
] and Küpper et al. [15
] reported that carbon black did not cause serious respiratory effects. Sorahan et al. [16
] also could not confirm a relationship between carbon black and increased standard mortality ratio in a cohort study from 1951 and 1996, whereas Gardiner et al. [17
] reported that carbon black causes a decrease in respiratory function and induces chronic respiratory inflammation.
Recently, interest in the toxicity of carbon black, especially for nano-sized carbon black, has increased because many studies have reported that nano-sized particles are more toxic than large particles such as fine particles [1
]. Furthermore, unknown deaths in Korea have been reported in a tire manufacturing facility. Some experts insisted that nano-sized carbon black aggravated cardiovascular symptoms and it might be the cause of unknown deaths of tire manufacturing facility. Therefore, both for hazard identification of nano-sized carbon black and to verify the possibility that nano-sized carbon black may have contributed to the deaths at the tire manufacturing facility, a re-evaluation of the toxicity of nano-sized carbon black is needed.
Now, it is generally accepted that increased surface area is one of the main causes of the increased toxicity of nano materials. For carbon black, many studies in cell culture systems [20
], in instillation systems [23
] and in inhalation systems [30
] have verified that if the particle size decreases, the toxicity increases because the surface area has increased. However, the effect of aggregation on the toxicity of carbon black has not been reported. In the risk assessment of chemicals in the workplace, the effect of agglomeration on toxicity is also important because nano materials, especially well-agglomerated nano materials, might agglomerate before workers are exposed in the workplace. Therefore, this study was performed to minimize the knowledge gap regarding nano-sized carbon black, toxicity and agglomeration.
To make the different states of agglomerates, Printex 90 aerosols were generated in two steps. Namely, Printex 90 was dispersed in distilled water as the first step and the dispersed Printex 90 was aerosolized through an orifice in the nose-only inhalation chamber as the second step. For the first step, three different methods were adapted to make different agglomerates in the distilled water: 1) vortex only for group 1; 2) vortex+sonication for group 2 and 3) vortex+sonication+dispersion in a stabilizer for group 3. The polydispersity indices of group 1, group 2 and group 3 were 0.85, 0.33 and 0.70, respectively; this means that only the particles in group 2 were well-dispersed. Although the average diameters of carbon black in group 1, group 2 and group 3 were 7.14 µm, 0.22 µm and 1.88 µm, respectively, in water suspension, the diameters of the aerosols in the inhalation chamber were 1.52 µm, 1.30 µm and 0.97 µm for group 1, group 2 and group 3, respectively. It seems that the agglomeration states in water solution did not affect on the agglomeration state of aerosols in the inhalation chamber; this was because large agglomerates in the water solution were eliminated or smashed while passing through the orifice and the small carbon black aggregates re-agglomerated in the inhalation chamber. TEM analysis supported the results on aerodynamic particle sizes measured by the cascade impactor.
The average concentrations were 8.84 mg/m3
, 8.60 mg/m3
and 8.97 mg/m3
for group 1, group 2 and group 3, respectively over the 13-week exposure period. These concentrations were slightly higher than the level of 7 mg/m3
that Driscoll et al. [5
] and Elder et al. [31
] found to be associated with a mild to moderate respiratory effect. In this study, there were no changes in body weight, cytokine levels in the BAL fluid or blood biochemical and hematological parameters. However, mild changes were found in the total and differential cell counts in the BAL fluid; these results correspond to previous results preformed [5
There was little difference in the toxic effect between the three different aggregated carbon black exposed groups, even though the deposition of carbon black in the lungs of rats in group 2 and group 3 was higher than in group 1 in this study. Sometimes the relationship between agglomeration and the surface area of particles can be explained by the fractal dimension (Df
). Namely, Df
can often be used to characterize agglomerate morphology through the following equation; NP
, where Np
is the number of primary particles in the aggregate, A is a dimensionless prefactor, Ro
is the primary particle radius and Rg
is the characteristic radius of the aggregate. Katrinak et al. [33
] classified particles as fractal-like aggregates with 1.35 < Df
< 1.89 and possibly non-fractal particles with Df
> 2 (the surface area decreases if the fractal dimension is > 2). In our study, the surface area was not changed by agglomeration or sonication as determined using the Brunauer, Emmett, Teller method (data not shown), whereas carbon black aerosols seemed to be a little better dispersed in group 2 and group 3 (more fractal-like particles) than in group 1 (non-fractal-like particles) based on TEM morphology (). This is similar to the results of Kobayashi et al. [34
], who reported that agglomeration causes minimal effects of the toxicity of titanium dioxide because agglomeration does not have a considerable affect on the surface area of titanium dioxide.
In conclusion, mild respiratory toxicity occurred in rats exposed to nano-sized carbon black for 13 weeks at a concentration of approximately 9 mg/m3 through nose-only inhalation; there were few differences in toxicity between the different agglomeration states.