AEGL-2 is the airborne concentration (expressed as ppm or mg/m3) of a substance above which it is predicted that the general population, including susceptible individuals, could experience irreversible or other serious, long-lasting adverse health effects or an impaired ability to escape.

AEGL-3 is the airborne concentration (expressed as ppm or mg/m3) of a substance above which it is predicted that the general population, including susceptible individuals, could experience life-threatening health effects or death.

Airborne concentrations below the AEGL-1 represent exposure concentrations that could produce mild and progressively increasing but transient and nondisabling odor, taste, and sensory irritation or certain asymptomatic, nonsensory effects. With increasing airborne concentrations above each AEGL, there is a progressive increase in the likelihood of occurrence and the severity of effects described for each corresponding AEGL. Although the AEGL values represent threshold concentrations for the general public, including susceptible subpopulations, such as infants, children, the elderly, persons with asthma, and those with other illnesses, it is recognized that individuals, subject to idiosyncratic responses, could experience the effects described at concentrations below the corresponding AEGL.

SUMMARY

Trimethylbenzene (TMB) isomers, including 1,3,5-, 1,2,4-, and 1,2,3-TMB, are common components of fuels and mixed hydrocarbon solvents (Delic et al. 1992). Together with other compounds of the same empirical formula, these flammable and explosive hydrocarbons are referred to as the C9 aromatics. TMB isomers are clear, colorless liquids that are insoluble in water (O’Neil et al. 2001). Little difference in toxicity has been observed between the TMB isomers. Because occupational exposures are likely to involve more than one isomer, regulatory standards are for the individual isomers and any mixture thereof.

For derivation of AEGL values, all available data on the individual TMB isomers were considered. The most appropriate end point was used as the point of departure for deriving values for each AEGL tier. Therefore, even though the point of departure might be based on data from an individual isomer, the resulting AEGL values are considered applicable to all three TMB isomers.

Human data were not available for derivation of AGEL values. No symptoms were reported at the concentrations tested in pharmacokinetic studies, and no case reports of human intoxication with the pure materials were found.

The most appropriate animal data for deriving AEGL-1 values were from neurotoxicity studies in rats exposed to 1,2,4-, 1,3,5-, or 1,2,3-TMB for 4 h (Korsak et al. 1995; Korsak and Rydzyński 1996). The effective concentration (EC50) values calculated on the basis of decrements in rotarod performance were 954, 963, and 768 ppm, respectively, indicating little difference in the effect level between the isomers. The average EC50 of 900 ppm for mild neurologic effects for the three isomers was chosen as the point of departure. A total uncer-



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