A review of available sources did not identify exposure limits or acute toxicity data for hexamethylcyclotrisiloxane. Although hexamethylcyclotrisiloxane is a solid, it is volatile. As such, a structurally similar, liquid phase chemical, octomethylcyclotetrasiloxane (CASRN: 556-67-2; C8H24O4Si4), was evaluated as a TEEL basis (Section 3.4.6).

Octomethylcyclotetrasiloxane has both exposure limits and acute toxicity data, from which TEELs have been developed. A review of available sources identified the following for octomethylcyclotetrasiloxane: a WEEL-TWA exposure limit, LD50 (rabbit, rat), rat LC50 and rabbit TDLo toxicity data. 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. The most preferred route and species, a rat LC50, was selected as the TEEL-3 basis and scaled to a 60-minute equivalent using the ten Berge equation (Section 3.3.4). The rabbit TDLo reported ataxia as a toxicity endpoint, appropriate for TEEL development, and was selected as the TEEL-2 basis. The WEEL-TWA was selected as the TEEL-1 basis.
• TEEL-1 = WEEL-TWA (octomethylcyclotetrasiloxane, CASRN: 556-67-2, 10 ppm) x 3
• TEEL-2 = Rabbit subcutaneous, soluble TDLo (octomethylcyclotetrasiloxane, CASRN: 556-67-2, >10,000 mg/kg; toxic endpoint includes ataxia, a loss of muscle function)
• TEEL-3 = Rat LC50 (octomethylcyclotetrasiloxane, CASRN: 556-67-2, 36000 mg/m3/4H)
This reviewed generated TEEL values inconsistent with those in Rev. 29A. The Rev. 29A TEELs were derived from an assumed HHR= 2; this review instead uses TEELs developed for a similar chemical, octomethylcyclotetrasiloxane (CASRN: 556-67-2, Section 3.4.6).