A review of available sources did not identify exposure limits or acute toxicity data for 1,2,3,8-tetrachlorodibenzo-p-dioxin (1,2,3,8-TCDD). Instead, analysts evaluated acute toxicity data for a similar chemical, 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD, CASRN: 1746-01-6). Data sources were selected according to the hierarchy listed in Table 3.1. Toxicity data were selected based on the type of exposure, species, and route according to the hierarchies stated in Sections 3.3.1, 3.3.2, and evaluated using factors in Tables 3.2, 3.3, and 3.4. A review of these sources identified numerous animal toxicity sources for 2,3,7,8-TCDD, from which the oral LD50 of the most preferred species (dog, monkey, and rat,) were evaluated. Many lowest reported toxicities (human and animal) were also identified; however, none reported suitable toxicity end points for TEEL development. Of the sources selected for additional evaluation, three are reported from the same source (PMID:6813132); when investigated further, it was determined that a different source reported a rat oral LD50 (43 µg/kg) that was more reliable and recent. This rat oral LD50 was selected as the TEEL-3 basis for 1,2,3,8-TCDD. TEEL-2 and TEEL-1 values were derived according to Table 3.5
• TEEL-1 = TEEL-2/11
• TEEL-2 = TEEL-3/6
• TEEL-3 = Rat oral LD50 (2,3,7,8-TCDD, CASRN: 1746-01-6; 43 µg/kg)
This review generated TEEL values inconsistent with Rev. 29A. The Rev. 29A TEEL-3 basis was rat oral LD50 toxicity data that was not found during the current review. Instead, analysts used an available rat LD50 for a similar chemical, 2,3,7,8-TCDD (CASRN: 1746-01-6, Section 3.4.6) as the TEEL-3 basis. The Rev. 29A and current TEEL-1 and TEEL-2 methodology remain consistent, with the differences in values due to the change of the TEEL-3 basis.