A review of available sources identified the following: TWA exposure limits and mouse and rat acute toxicity data (LD50, LC50). Data sources were selected according to the hierarchy listed in Table 3.1. Toxicity data were selected according to 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. Inhalation exposures were scaled to 60-minute equivalents using the ten Berge equation (Section 3.3.4). Although the mouse LC50 was considered as a TEEL-3 basis, it resulted in a TEEL-2 value less than 3 x TWA (Section 3.4.2), so it was determined to be overly conservative. Instead, the most preferred species and route, a rat LC50, was selected as the TEEL-3 basis. No duration was listed, so 4 hours was assumed (Section 3.2.2). The TEEL-2 value was derived according to Table 3.5. The TLV-TWA was selected as the TEEL-1 basis.
• TEEL-1 = TLV-TWA (0.1 mg/m3) x 3
• TEEL-2 = TEEL-3/6
• TEEL-3 = Rat LC50 (165 mg/m3/4H, assumed)
This review generated TEEL-3 and TEEL-1 values that are inconsistent with Rev. 29A. The Rev. 29A and current TEEL-1 value and basis, a TLV-TWA, are consistent. The Rev. 29A TEEL-2 and TEEL-3 were derived from this TEEL-1. The current TEEL-3 basis is a rat LC50 with an assumed duration of 4-hours, from which the TEEL-2 was derived.