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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.
Carcinogenicity/Epidemiology
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.
Toxicity
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.
Exposure Assessment
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.
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