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length was between 3-210 μm. The mean fiber diameter and lengths were 2.5 and 52 μm, respectively.
In a separate test series, USCG burned 48x48 cm2 sections of graphite/epoxy composite in heptane pool fires [28].
The fiber sampling system consisted of a cascade impactor placed inside the exhaust duct. Analysis of the fiber size
distribution indicated that the diameters ranged between 0.5-5.0 μm with a mean diameter of 2.4 μm. The scatter in
the fiber lengths was much greater in pool fire data, and ranged between 5-900 μm with a mean fiber length of 77
μm. Results from both the Cone calorimeter and large scale tests indicate that fibers are significantly oxidized
during the burning process, with fiber diameters reduced from 7 μm originally to an average value of less than 3 μm.
The US Coast Guard studies did not include toxicological evaluation of combustion products.
Additional data on the characteristics of the airborne carbon fibers have come from recent aircraft postcrash investigations.
Mahar [29] measured fiber concentration (fibers/cm ) and aerodynamic diameter of the fibers collected at
a military jet crash site. Fiber samples were collected from the personal respirator filters worn by the investigators
fitted with 25-mm cassettes containing 0.8-μm mixed cellulose ester filters. Mahar reported that there is a significant
increase in the particulate levels during cleanup operations with the disturbance of the aircraft wreckage. Less than
20 percent of the collected fibers were found respirable with aerodynamic diameter smaller than 10 μm. Microscopic
analysis revealed that respirable fibers were approximately 2 μm in diameter and 7-8 μm long. The total concentrations
of the fibers collected from breathing air zones ranged from 0.02-0.06 fibers/cm. The Navy guidelines limit
exposure to a time-weighted average of 3.5 fibers/cm of air and a maximum of 10 fibers/cm over a 40-hour workweek
[29].
Recent Studies
The Civil Aviation Authority [30] in the United Kingdom investigated the toxic nature of combustion products of
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composite materials used in structural components of a public transport aircraft and a helicopter. Samples were
subjected to a flaming heat source at a temperature of 1150 C+/- 50 C in a combustion chamber. The chemical
analysis of the combustion products via gas chromatography and mass spectroscopy revealed several organic
chemicals, the exact composition of which was not reported. Although no fibers were found in the soot filters, visual
inspection of the burnt samples indicated evidence of surface pitting and fiber fibrillation. In further investigations of
various air crashes in England in past 10 years, the Defense Evaluation and Research Agency (DERA) [30-31]
found that typically 35-50 % of the free fibers still attached to the wreckage pieces and which could be released
during handling were smaller than 1 mm in length. These fibers could cause significant skin and eye irritation, and
irritation of upper respiratory tract, they are unlikely to be inhaled deep into the lungs. There were however a small
fraction of airborne carbon fibers and some still attached to composite parts with signs of intense fire, which showed
tapering and surface pitting. These results areconsistent with the previous NASA test results showing reduction in
fiber size to smaller diameters and could pose a risk of deep lung penetration depending on their lengths.
The US Air Force Toxicology Division conducted a series of tests [32] for evaluating combustion toxicity of advanced
composite materials used in military aircraft. Preliminary studies focused on the morphology and chemical composition
of organic compounds associated with particulates carried in the smoke from burning composites. The test
materials consisted of carbon fiber impregnated in a modified bismaleimide resin [32]. The SEM analysis did not
reveal the presence of any fiber-shaped particles i.e., L/D >3. Forty percent of the particles were of respirable
dimension with aerodynamic diameter ≤ 5 μm. These particles are small enough to penetrate deep below the
tracheobronchial airways. Approximately 15 percent of particles had an aerodynamic diameter ≤ 1 μm, which can
be deposited in the alveoli. The study did not report the fiber length measurements. The study did however identify
a large number of organic species that were adsorbed on the particulate matter. Lipscomb [33] confirmed 90 different
chemicals which can be broadly classified as polycyclic aromatic hydrocarbons, nitrogen-containing aromatics
such as aniline, and phenol-based organic compounds. Several of these chemicals, e.g., aniline, quinoline, and
toluidine, are known to induce carcinogenic and mutagenic effects in animals [33].
　
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