Deposition, Clearance, and Shortening of Kevlar Para-aramid Fibrils in Acute, Subchronic, and Chronic Inhalation Studies in Rats

The deposition and clearance of lung-deposited Kevlar paraaramidfibrils (subfibers) have been investigated as part of a subchronicand chronic inhalation toxicity testing program. Fibrils recoveredfrom lung tissue in para-aramid-exposed Sprague-Dawley ratswere microscopically counted and measured after exposures toairborne fibrils which were about 12 µm median length(ML) and <0.3 µm median diameter. In each of threestudies lung-recovered fibrils were progressively shorter withincreasing residence time in the lungs. Twenty-eight days aftera single 6-hr exposure at 400 respirable fibrils per cubic centimeter(f/cm³) the ML of recovered fibrils decreased to about 5 µm.Twenty-four months after a 3-week exposure to 25 or 400 (f/cm³)fibrils reached about 2 µm ML. After 2 years of continuousexposure at 2.5, 25, or 100 f/cm³ or 1 year exposure plus 1year recovery at 400 f/cm fibril ML approached 4 µm. Inthe 2-year study, the lung-fiber accumulation rate/exposureconcentration was similar for the three highest concentrationsand was about 3x greater than that seen at 2.5 f/cm³ indicatingthat concentrations of about 25 f/cm or more may overwhelm clearancemechanisms. Time required for fibrils to be reduced to <5µm in the lung was markedly less at lower exposure concentrationand shorter exposure time. The primary shortening mechanismis proposed to be long fibril cutting by enzymatic attack atfibril defects. However, length-selective fibril depositionand clearance may contribute to shortening in the first fewdays after exposure. The enzymatic cutting hypothesis is supportedby measured increases in numbers of short fibers following cessationof exposures, continued shortening of the fibril length distributionup to 2 years following exposure, and in vitro fibril shorteningafter 3 months in a proteolytic enzyme preparation. The conclusionis that para-aramid fibrils are less durable in the lungs ofrats than expected from the known chemical resistance of commercialyarn. These data suggest that at the low para-aramid fibrilexposures found in the workplace, this fibril-shortening mechanismmay limit the residence time of long fibers in the lungs ofexposed workers. In addition, associated cascade impactor aerodynamicmeasurements indicate that due to their ribbon shape and curlynature, para-aramid fibrils behave aerodynamically larger thanstraight fibers.