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the lungs facilitate removal of these fibers. The primary function of these cells is to remove bacteria, dead cells, and
foreign particles and fibers by ingestion [13]. Hesterberg [15] has given a detailed description of the biological
mechanism in the clearance of fibers by the macrophage cells.
Carbon Fiber Toxicity
Compared to asbestos and other mineral fibers, very little work has been done on the inhalation toxicology of carbon
fibers. Few studies have been published on the health effects related to chronic exposure from carbon fibers and
dusts in the manufacturing environment. Two conferences [16-17] brought together experts from the aerospace and
composites industries and government agencies to address the health implications of exposure to carbon fiberreinforced
composites. These proceedings primarily focussed on the hazards related to exposure from carbon
fibers and dusts during the machining and handling of fiber composites. In a review of the to date toxicology research
on carbon fibers, Thomson [18] concluded that there are no long-term health risks associated with exposure
to PAN-based carbon fibers alone under occupational conditions. The health effects are limited to temporary irrita-
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tions of the skin and upper respiratory tract since given that exposure under occupational conditions is limited to
relatively large-diameter, nonrespirable fibers (nominal diameter ~ 6-8 μm). In a separate review of studies that
included some consideration of the health effects of exposure to composites duringvarious stages of manufacturing,
Luchtel [19] concluded that, “carbon fibers and composite dusts should be regarded as more hazardous than the socalled
nuisance dusts, and present a low health risk.” The author noted that the pulmonary effects of the composite
materials are not in the same class as asbestos in terms of toxicity, however adequate respirable protection should
be used to minimize the occupational exposure. One consistent shortcoming of the previous studies is the lack of
complete information about the characteristics of the material being studied, i.e., the size distribution of fibers, their
Dea, multiple dosages, if used, and the type of resin matrix. In addition, these studies used unequal time periods for
the animal exposure and post-exposure recovery. Today, standard design requirements for the toxicological evaluation
in a lifetime inhalation study with rats call for an exposure time of at least 2 years and a post-exposure evaluation
period of 2-3 years [20]. A post-exposure recovery period allows evaluation of reversibility of effects. In the
studies reviewed by Luchtel, the exposure times used by all the researchers were very short compared to the
required 2 years. Similarly, post-exposure times for recovery were too short compared to the current standard
guidelines.
Further studies are needed to assess the carbon fiber toxicity in the presence of surface contaminants from the
combustion environment. Fibers and other small particles, with high surface-to-volume ratio, generated in fires can
carry with them a diverse package of chemical species with potentially harmful effects. It has been suggested that
adsorbed chemicals may enhance the pathology of inhaled particles similar to the example of diesel soot [6, 21]. In
the case of inert fibers such as carbon, the presence of fire-caused surface contaminants might affect fiber retention
in the lung.
COMBUSTION PRODUCTS FROM COMPOSITE FIRES
Fiber Size Characterization
Characterization of fiber dimensions is an important requirement in any toxicology study in order to determine their
respirability. Bell [22] described an extensive series of tests conducted by the National Aeronautics and Space
Administration (NASA) to ascertain the extent of carbon fiber release during an aircraft crash. In one series of tests
conducted at the Naval Weapon Center (NWC), China Lake, CA, full composite sections of a Boeing 737 spoiler and
an F-16 fuselage were subjected to flames for 4-6 minutes in 15.2 m pool of JP-5 jet fuel to simulate aircraft fires.
Fibers released during the fire were collected through adhesive coated papers 20x25 cm located on an elevated
platform 0.3 m above the ground. However, this sampling procedure yielded very low amounts of single fibers. The
massive smoke plume that reached a height of ~1000 m carried the majority of the single fibers away from the test
location to distances beyond the instrumentation limit of 2000 m. Sampling size was also reduced due to difficulties
in separating the fibers from the paper, thus limiting the number of fibers analyzed for size distribution [23]. A second
series of large-scale fire tests were conducted at the U.S. Army’s Dugway Proving Ground, UT to measure the
　
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