Pulmonary evaluation of permissible exposure limit of syntroleum S-8 synthetic jet fuel in mice

Abstract

No current studies have systematically examined pulmonary health effects associated with Syntroleum S-8 synthetic jet fuel (S-8). In order to gain an understanding about the threshold concentration in which lung injury is observed, C57BL/6 male mice were nose-only exposed to S-8 for 1 h/day for 7 days at average concentrations of 0 (control), 93, 352, and 616 mg/m(3). Evaluation of pulmonary function, airway epithelial barrier integrity, and pathohistology was performed 24 h after the final exposures. Significant decreases were detected in expiratory lung resistance and total lung compliance of the 352 mg/m(3) group, for which no clear concentration-dependent alterations could be determined. No significant changes in respiratory permeability were exhibited, indicating that there was no loss of epithelial barrier integrity following S-8 exposure. However, morphological examination and morphometric analysis of distal lung tissue, by using transmission electron microscopy, revealed cellular damage in alveolar type II epithelial cells, with significant increases in volume density of lamellar bodies/vacuoles at 352 and 616 S-8 mg/m(3). Moreover, terminal bronchiolar Clara injury, as evidenced by apical membrane blebs, was observed at relatively low concentrations, suggesting if this synthetic jet fuel is utilized, the current permissible exposure limit of 350 mg/m(3) for hydrocarbon fuels should cautiously be applied.

FIG. 1.

The schematic of S-8 vapor/aerosol generator, mouth-only mouse exposure chamber, and instrumentations.

FIG. 2.

Total hydrocarbon/aerosol concentrations and aerosol size distribution of Syntroleum S-8 jet fuel at the exposure chamber level. The fuel vapor and aerosol concentrations were monitored using an “in-line real-time” THC analysis system and the aerosol spectrometer (A). The aerosol sizes were measured by using a seven-stage IN-TOX cascade impactor (B).

FIG. 3.

Respiratory clearance of ^99mTC-DTPA in mice following repeated inhalation exposures to aerosolized S-8 jet fuel 1 h/day for 7 days.

FIG. 4.

Representative transmission electron micrographs of terminal bronchiole ciliated (Ci) and nonciliated (NC, Clara) epithelial cells from mice following S-8 jet fuel exposure 1 h/day for 7 days. M: mitochondria; N: nucleus; S: secretory granules. Uranyl acetate and lead citrate. Magnification ×1800. (A) Clara cells in controls have numorous mitochondria in columnar profile distributed throughout the Clara cell cytoplasm; (B) at 93 mg/m³, Clara cells had round mitochondria (M) with few cristae that were distributed most apically in the cell cytoplasm with membrane blebbing (arrow) on apical surface; (C) at 352 mg/m³, more Clara cells had apical membrane blebbings (arrows) and heavy consolidation of cytoplasmic spaces with swollen mitochondria (M) in the apex of the cells; (D) at 616 mg/m³, Clara cells had widespread loss of cytoplasmic density (*) and necrotic cells or debris (arrows) were frequently found in the airway lumen.

FIG. 5.

Representative transmission electron micrographs of alveolar type II epithelial cells from mice following S-8 jet fuel exposure 1 h/day for 7 days. N: Nucleus. Uranyl acetate and lead citrate. Magnification: ×8000. Note the apparent number increases in surfactant-producing lamellar bodies (arrows) of alveolar type II epithelial cells with swollen mitochondria (arrow heads) in S-8 groups (B–D). At 616 mg/m³ (D), cells had severely swollen mitochondria (arrow heads) and mess lamellar bodies organization (arrows) with decreased granulation (*) in the matrix occurred throughout the cytoplasm, a sign of cell necrosis.

FIG. 6.

Cellular organ volume density in epithelial cells of mice distal lung following S-8 jet fuel exposure 1 h/day for 7 days. By using a morphometric analysis of the electron micrographs (magnification ×8000). (A) Volume density of surfactant-producing lamellar bodies in alveolar type II epithelial cells; (B) volume density of cytoplasmic vacuoles in the Clara cells. *p < 0.05 when compared with controls.