Selected chemical and physical properties of 1,2,3-trimethylbenzene, 1,2,4-trimethylbenzene and 1,3,5-trimethylbenzene are listed in Table 1-1. In the atmosphere the trimethylbenzene compounds are expected to exist primarily in the vapor phase. The atmospheric half-lives listed in Table 1-1 were calculated using measured HO radical reaction rate constants of 26.4x10-12 cm3/molecule-sec for 1,2,3-trimethylbenzene at 24 degrees C, 33.5x10-12 cm3/molecule-sec for 1,2,4-trimethylbenzene at 23.8 degrees C, and 47.2x10-12 cm3/molecule-sec for 1,3,5-trimethylbenzene at 24 degrees C (Atkinson, 1985) and assuming an ambient HO radical concentration of 10c molecules/cm3. In water, volatilization and biodegradation may be important fate processes for the trimethylbenzenes (Wakeham et al., 1983). With respect to volatilization, appropriate residence times for 1,3,5-trimethylbenzene in water from Narragan ? Bay, RI, were estimated to be 220 hours (Wakeham et al., 1983). Zoeteman et al. (1980) estimated the half-life of 1,3,5- trimethylbenzene in Rhine River surface water (Netherlands) to be about 1 day. Based on recommended values for Henry's Law constant of 3.19x10-3, 5.18x10-3 and 5.92x10-3 atm- m3/mol at 25 degrees C for 1,2,3-, 1,2,4, and 1,3,5-trimethylbenzene, respectively, volatilization half-lives from a body of water 1 m deep flowing 1 m/sec with a wind speed of 3 m/sec were calculated to be 3.6 hours for 1,2,3-trimethylbenzene and 3.4 hours for 1,2,4- and 1,3-5- trimethylbenzene (Lyman et al., 1982; U.S. EPA, 1986b). Estimated BCF and Koc values that suggest that bioaccumulate in aquatic organisms would be insignificant and that moderate adsorption to suspended solids and sediments may occur. The half-lives of the trimethylbenzenes in soil could not be located in the available literature. Based on aquatic data, both vaporization and biodegradation are expected to play significant roles in determining the half-lives of these compounds in soil; because of their estimated Koc values, they are expected to be moderately mobile in soil (see Table 1-1). Subchronic reference dose (RfDS). Oral (RfDSO). There are no data from which to derive an RfDSO for trimethylbenzenes. Inhalation (RfDSI). The subchronic inhalation data consists only of three abstracts from the foreign literature (Bernstein, 1972; Wiglusz et al., 1975a,b). Each abstract reported only a single level of exposure at which effects were seen. Information regarding the use of controls or other levels of exposure was not provided. These studies are not adequate for quantitative risk assessment. Reference dose (RfD). Oral (RfDO). There are no data from which to derive an RfDO or CS for trimethylbenzenes. Inhalation (RfDI). There are no adequate subchronic or chronic inhalation data that define dose- specific adverse effects; therefore, an RfDI or a CS for trimethylbenzene cannot be derived.