A review of available sources did not identify acute toxicity data or exposure limits for hafnium oxide. Instead, exposure limits for hafnium (TWAs, IDLH; CASRN: 7440-58-6) were evaluated for TEEL development. Data sources were selected according to the hierarchy listed in Table 3.1. Analysts selected lowest reported hafnium OEL-STEL (Romania, 15-minutes) as the TEEL-1 and a compound adjustment factor was applied (Section 3.2.2). The TEEL-2 was derived according to Table 3.5. The 1990 and 1994 hafnium IDLH were evaluated for the TEEL-3; the 1990 IDLH was an “effective” limit, while the 1994 IDLH was developed a limited data set (2 studies) of dissimilar chemicals. Considering this, analysts did not select the IDLH as a TEEL-3. Instead, the TEEL-3 was derived according to Table 3.5.
• PAC-1 = TEEL-1 = MW-adjusted OEL-STEL (Romania; hafnium, CASRN: 7440-58-6; 0.5 mg/m3)
• PAC-2 = TEEL-2 = TEEL-1 x 11
• PAC-3 = TEEL-3 = TEEL-2 x 6
This review generated PAC values inconsistent with Rev. 29A. The Rev. 29A TEEL-1 basis was a TLV-TWA for hafnium, while the current TEEL-1 basis is an OEL-STEL (Table 3.5, see note). The Rev. 29A and current TEEL-2 and TEEL-3 use the same methodology, with differences due to the change in TEEL-1.