Integrating studies on carcinogenic risk of carbon black: epidemiology, animal exposures, and mechanism of action.

Objective: We sought to address the toxicology literature on carbon black (CB) since 1996, when IARC reclassified CB from group 3 to group 2B.

Methods: We reviewed epidemiology and laboratory studies from 1996 to 2006, focusing on new analyses of worker populations, on species differences in tumorigenicity of poorly soluble particles, and on the role of particle-bound organics in tumorigenicity.

Results: Some epidemiology studies have reported positive associations between cancer risk and worker's possible exposure to CB, but larger studies, in more highly exposed populations, have not shown consistent patterns of either elevated risk or dose-response. High levels of inhaled CB were linked with rat lung tumors in 1996, but today scientists increasingly recognize that rats exhibit a unique lung tumor response to all inert inhaled particles that is unlikely to be relevant to humans. On mechanism of action, new reports have continued to show that CB has a high surface area of elemental carbon, and a low quantity of organic material, which is poorly bioavailable.

Conclusion: Overall, the new epidemiological evidence decreases concerns for cancer risk compared with pre-1996 evidence. Laboratory studies support a conclusion that the mechanism of tumorigenicity of CB in rats is no different from that of any poorly soluble particle, ie, toxicity results from the particle overload per se, and not from the particles' chemistry. Thus, research published after 1996 has not identified an increase in support for CB cancer risk, but rather, points to limited and inadequate evidence for carcinogenicity.