Carbon black has been the subject of extensive scientific health studies during the past several decades. Although carbon black is classified by the International Agency for Research on Cancer (IARC) as a Group 2B carcinogen (possibly carcinogenic to humans) based on "sufficient evidence" in animals and "inadequate evidence" in humans, recent evidence indicates that the phenomenon of carcinogenicity in the rat lung is species-specific, resulting from persistent overloading of the rat lung with poorly soluble particles <1.0 micrometer in diameter. Mortality studies of carbon black manufacturing workers do not show an association between carbon black exposure and elevated lung cancer rates. (See Human Studies and Carcinogenicity sections.) Studies have demonstrated, however, that regular exposure to carbon black and other poorly soluble particles may play a role in declining lung capacity as measured by forced expiratory volume in one second (FEV1). Good occupational hygiene practices should be followed to maintain worker exposures below the occupational exposure limit.
The International Agency for Research on Cancer (IARC) evaluation (Monograph 65, 1996 publication) concluded, "There is sufficient evidence in experimental animals for the carcinogenicity of carbon black." This categorization was based upon IARC's guidelines, which require such a classification if one species exhibits carcinogenicity in two or more studies. Based on this evaluation, along with its finding of inadequate evidence of carcinogenicity in humans, IARC designated carbon black as a Group 2B carcinogen, that is "possibly carcinogenic to humans." Some other research and regulatory organizations that have classified carbon black as to its carcinogenicity include: The American Conference of Governmental Industrial Hygienists (ACGIH) classifies carbon black as A4, Not Classifiable as a Human Carcinogen; The German MAK Commission classifies carbon black as a suspect carcinogen category 3B; The National Toxicology Program (NTP) has not listed carbon black as a carcinogen; The Occupational Safety and Health Administration (OSHA) has not listed carbon black as a carcinogen; The National Institute for Occupational Safety and Health (NIOSH) criteria document (1978) on carbon black recommends only carbon blacks with polycyclic aromatic hydrocarbon contamination levels greater than 0.1% (1,000 parts per million) be considered suspect carcinogens; and The Office of Environmental Health Hazard Assessment (OEHHA) of the California Environmental Protection Agency added "carbon black (airborne, unbound particles of respirable size)" (CAS No. 1333-86-4) to the Proposition 65 substances list on February 21, 2003. This listing, triggered by the "authoritative body" mechanism in the California Code of Regulations, was based solely on IARC's 1996 reclassification of carbon black as a Group 2B carcinogen.
Concern has been expressed about the polycyclic aromatic hydrocarbon (PAH or sometimes referred to as polynuclear aromatics (PNA)) content of manufactured carbon blacks. In non-adsorbed forms, some PAHs have been found to be carcinogens in animal studies. In-vitro studies indicate that the PAHs contained in carbon black are not bioavailable. Modern production and quality control procedures are generally able to maintain extractable PAH levels to less than 0.1% (<1000 ppm) in carbon black with PAHs regulated as carcinogens representing a smaller fraction of the extractables. Extractable PAH content depends on numerous factors including, but not limited to, the manufacturing process and the ability of the analytical procedure to identify and measure extractable PAHs. Specific questions concerning PAH content should be addressed to your carbon black supplier.
Long-term inhalation studies, up to two years, have resulted in chronic inflammation, lung fibrosis, and lung tumors in some rats experimentally exposed to excessive concentrations of carbon black. Tumors have not been observed in other animal species (i.e., mice and hamsters) under similar study conditions. These same effects are observed when rats have been exposed to several other poorly soluble dust particles. Many researchers conducting rat inhalation studies believe the observed effects result from the massive accumulation of small dust particles in the rat lung after exposure to excessive concentrations. These accumulations overwhelm the natural lung clearance mechanisms of the rat and produce a phenomenon that is described as "lung overload." The effects are not thought to be the result of a specific toxic effect of the dust particle in the lung. Many inhalation toxicologists believe the tumor response observed in the above referenced rat studies is species-specific and does not correlate to human exposure.
Industrial or occupational hygiene management of the work environment includes ongoing efforts to anticipate and identify potential exposure conditions, measurement of exposures, and implementation of appropriate controls to reduce exposures to the lowest feasible levels. Industrial hygiene experience within the manufacturing environment suggests the activities with the greatest potential for occupational exposure to carbon black are those related to manual handling (e.g., bag slitting, bulk weighing, sample preparation, and dry batch preparation). Maintenance operations should also be carefully observed and evaluated for potential exposures. Each employer must conduct a hazard assessment based on knowledge of their own work environment activities and conditions.