where AC is the acceptable concentration and ppm is parts per million.


The 24-h SMAC was set with the same starting point except that it was assumed that 50% of the TMS was eliminated in 24 h and the average breathing rate was lower (0.015 m3/min) because of sleep periods. The calculation was as follows:

The long-term SMACs were set from the oral CNS-depression NOAEL of 100 mg/kg observed in rats given doses over 31 d (Dow Corning Corp. 1972). The calculation was as follows:

Because the relative uptake of an oral and an inhalation dose was unknown, no factor was used to relate the oral and inhalation routes. Basically, it was assumed that the two routes of exposure have comparable uptakes.

Experience with samples of air obtained in the International Space Station shows that the concentration of TMS is typically about 0.1 mg/m3 (0.03 ppm) or less, with occasional excursions up to 0.3 mg/m3 (1.1 ppm) and one excursion to 0.65 mg/m3 (2.4 ppm). The most likely source is silicone lubricants, which would have in episodic entry into the atmosphere. TMS is commonly found in off-gas tests of flight hardware, presumably from lubricants in hardware joints or from off-gassing from silicone-based seals.

TABLE 18-1 Previously Set SMACs for TMS

Duration of exposure

SMAC, ppm

Target toxicity to prevent

1 h

150

CNS depression

24 h

20

CNS depression

7 d

10

CNS depression

30 d

10

CNS depression

180 d

10

CNS depression

Source: Data from Kaplan et al. 1994.



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