ity, because no data were available to estimate human variability and it was not clear that trimethoxysilane acts as a simple chemical irritant in the lungs (NRC 2001). Time scaling was performed using the concentration-time relationship equation Cn × t = k, where C = concentration, t = time, k is a constant, and n generally ranges from 0.8 to 3.5 (ten Berge et al. 1986). An empirical value for n of 1.45 was calculated for trimethoxysilane. AEGL values for trimethoxysilane are presented in Table 7-1.

AEGL-1 values were not recommended for tetramethoxysilane because of inadequate data. AEGL-2 values for tetramethoxysilane were derived from an inhalation study in which rats were exposed to tetramethoxysilane at concentrations up to 45 ppm for 6 h/day, 5 days/week for 28 days (Kolesar et al. 1989). No deaths or effects on the respiratory or ocular epithelium were observed at 0, 5, and 10 ppm. At 15 ppm, nasal changes indicative of minimal acute inflammation were found in two of 20 rats and acute keratitis of cornea was observed in four of 20 rats, indicating a no-effect level for irreversible effects. At 30 and 45 ppm, lesions more severe than defined by AEGL-2 and deaths were observed, respectively. Therefore, 15 ppm was used as the point of departure for calculating AEGL-2 values. Extrapolation to different exposure durations was performed using the equation Cn × t = k (ten Berge et al. 1986), where n = 3 for extrapolation to 30 min, 1 h, and 4 h, and n = 1 for extrapolation to 8 h. The 30-min value was adopted as the 10-min value because extrapolating from durations of more than 4 h to 10 min is not recommended (NRC 2001). A total uncertainty factor of 30 was used. A factor of 3 was applied for interspecies differences because in a 5-day inhalation study with trimethoxysilane, a structural analog, effects were similar in rats, mice, and hamsters (Dow Corning Corp. 1981). A default value of 10 was used for the intraspecies variability because there were no data to estimate human variability and it was not clear that tetramethoxysilane acts as a simple chemical irritant in the lungs (NRC 2001).

AEGL-3 values for tetramethoxysilane were derived from a 4-h LC50 inhalation study in rats (Dow Corning Corp. 1992). The data were analyzed with EPA’s Benchmark Dose Calculation Software, version 1.3.2 (EPA 2005), and values were calculated using log-probit analysis. A BMCL05 (benchmark concentration, 95% lower confidence limit with 5% response) of 26 ppm was used as the basis for determining AEGL-3 values. For completeness, a BMC01 (benchmark concentration with 1% response) of 30 ppm was also derived, but the lower BMCL05 value was used. Extrapolation to different exposure durations was performed using the equation Cn × t = k (ten Berge et al. 1986), where n = 3 for extrapolation to 30 min and 1 h and n = 1 for extrapolation to 8 h. The 30-min value was adopted as the 10-min value because extrapolating from 4 h to 10 min is not recommended (NRC 2001). A total uncertainty factor of 30 was used. An uncertainty factor of 3 was used for interspecies differences because in a 5-day inhalation study with trimethoxysilane, a structural analog of tetramethoxysilane, effects were similar in rats, mice, and hamsters (Dow Corning Corp. 1981). A default value of 10 was used for intraspecies variability because there were no data to estimate human variability and it was not clear that tetramethox-



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