tainty factor of 10 was used. A factor of 3 for interspecies differences was used because the mechanism of action for hydrocarbon narcosis is not expected to differ between rats and humans, and a factor of 3 was applied for intraspecies variability because the threshold for narcosis differs by no more than 2- to 3-fold among the general population (NRC 2001). Because the point of departure is based on a systemic effect, values were scaled using the equation Cn × t = k, where n ranges from 0.8 to 3.5 (ten Berge et al. 1986). In the absence of an empirically derived, chemical-specific exponent, scaling was performed using n = 3 for extrapolating to the 30-min and 1-h durations and n = 1 for the 8-h duration. According to Section 2.7 of the Standing Operating Procedures for Developing Acute Exposure Guideline Levels for Hazardous Chemicals (NRC 2001), 10-min values should not be scaled from experimental exposure durations of 4 h or longer. Therefore, the 30-min AEGL-1 value was adopted as the 10-min value.
Few data were available for deriving AEGL-2 values. Rats repeatedly exposed to 1,2,4-TMB at 2,000 ppm for 6 h exhibited irritation, respiratory difficulty, lethargy, and tremors (Gage 1970); therefore, 2,000 ppm was chosen as the basis for deriving the AEGL-2 values. That point of departure also is supported by the weight of evidence on neurologic deficits measured at this concentration (Korsak et al. 1995; Korsak and Rydzyński 1996). The point of departure might not be a no-effect-level for AEGL-2 values, because the effects could lead to an impaired ability to escape. However, because the study involved repeated exposures, 2,000 ppm was considered a conservative estimate of effects for a single exposure. A total uncertainty factor of 10 was applied, which included a factor 3 for interspecies differences and 3 for intraspecies variability. Use of larger uncertainty factors was unnecessary because the mechanisms for irritation and narcosis are not expected to differ between humans and animals. Values were scaled using the same method used to derive AEGL-1 values, and the 30-min AEGL-2 value was adopted as the 10-min value.
Data were insufficient to derive AEGL-3 values for TMB. AEGL values for TMB are presented in Table 8-1.
Trimethylbenzene (TMB) isomers include 1,3,5-, 1,2,4-, and 1,2,3-TMB, which are common components of motor vehicle and aviation fuels and mixed hydrocarbon solvents (Delic et al. 1992). Together with other compounds of the same empirical formula, these substances are referred to as the C9 aromatics. The primary hazards associated with these compounds are fire and explosion. TMB isomers are clear, colorless liquids that are insoluble in water (O’Neil et al. 2001). 1,2,4-TMB is purified by superfractionation and is used as a component of liquid scintillation cocktails (Earhart and Komin 2000). The 1,3,5- and 1,2,3-TMB isomers are produced synthetically and the derivatives are used in specialty solvents (Delic et al. 1992; Earhart and Komin 2000).