This 90-day repeated-dose inhalation toxicology study of brake-dust (BD) (brakes manufactured with chrysotile) in rats provides a comprehensive understanding of the biokinetics and potential toxicology in the lung and pleura. Exposure was 6 h/d, 5d/wk., 13wks followed by lifetime observation (~20 % survival). Control groups included a particle control (TiO2), chrysotile, commercial crocidolite and amosite asbestos.
Aerosol fiber distributions of the chrysotile, crocidolite and amosite were similar (fibers L > 20 μm/cm3 : chrysotile-Low/High 29/72; crocidolite 24; amosite 47 fibers/cm3 ; WHO-fibers/cm3 : chrysotile-Low/High 119/ 233; crocidolite 181; amosite 281 fibers/cm3 ). The number of particles/cm3 in the BD was similar to that in the chrysotile, crocidolite & amosite exposures (BD 470–715; chrysotile 495–614; crocidolite 415; amosite 417 particles/cm3 ).
In the BD groups, few fibers L > 20 μm were observed in the lungs at the end of exposure and no fibers L > 20 μm at 90d post exposure. In the chrysotile groups, means of 204,000 and 290,000 fibers(L > 20 μm)/ lung were measured at 89d. By 180d, means of 1 and 3.9 fibers were counted on the filter corresponding to 14,000 and 55,000 fibers(L > 20 μm)/lung.
In the crocidolite and amosite groups mean lung concentrations were 9,055,000 and 11,645,000 fibers (L > 20 μm)/lung at 89d. At 180d the means remained similar with 8,026,000 and 11,591,000 fibers (L > 20 μm)/lung representing 10–13% of the total lung fibers.
BAL determined the total number of macrophages, lymphocytes, neutrophils, eosinophils, epithelial-cells and IL-1 beta, TNF-alpha and TGF-beta. At the moderate aerosol concentrations used in this study, neutrophil counts increased ~5 fold in the amphibole asbestos exposure groups. All other groups and parameters showed no important differences at these exposure concentrations. The exposure and lung burden results provide a sound basis for assessing the potential toxicity of the brake dust in comparison to the TiO2 particle control and the chrysotile, crocidolite and amosite asbestos control groups. The BAL results provide an initial indication of the differential response. Part 2 presents the presentation and discussion of the histopathological and confocal microscopy findings in this study through 90 days post exposure.
The interim results from this 90-day multi-dose, inhalation toxicology study with life-time post-exposure observation has shown an important fundamental difference in persistence and pathological response in the lung between brake dust derived from brake-pads manufactured with chrysotile, TiO2 or chrysotile alone in comparison to the amphiboles, crocidolite and amosite asbestos.In the brake dust exposure groups no significant pathological response was observed at any time. Slight macrophage accumulation of particles was noted. Wagner-scores, were from 1 to 2 (1 = air-control group) and similar to the TiO2 group.Chrysotile being biodegradable, shows a weakening of its matrix and brakeing into short fibers & particles that can be cleared by alveolar macrophages and continued dissolution. In the chrysotile exposure groups, particle laden macrophage accumulation was noted leading to a slight interstitial inflammatory response (Wagner-score 1–3). There was no peribronchiolar inflammation and occasional very slight interstitial fibrosis.The histopathology and the confocal analyses clearly differentiate the pathological response from amphibole asbestos, crocidolite and amosite, compared to that from the brake dust and chrysotile. Both crocidolite and amosite induced persistent inflammation, microgranulomas, and fibrosis (Wagner-scores 4), which persisted through the post exposure period. The confocal microscopy of the lung and snap-frozen chestwalls quantified the extensive inflammatory response and collagen development in the lung and on the visceral and parietal surfaces.The interim results reported here, provide a clear basis for differentiating the effects from brake dust exposure from those following amphibole asbestos exposure. The subsequent results through life-time post-exposure will follow.