PAC Database - Detailed Chemical Data

Chemical Identity
Chemical CAS# UN# Health Code Numbers
Silica amorphous hydrated 7631-86-9

Formula Synonyms
Physical Properties
Mol Wt State MP (°C) BP (°C) VP (Hg) VP (°C) SG LEL (ppm)
60.08 S 2.2
PAC Values
PAC Values (Original Units: ppm)
7.3 200 1200
Converted PAC Values (mg/m3)
18 490 2900
Technical Basis
Multiple exposure levels and toxicity starting points were explored for this chemical. Analysts recommend using REL TWA to determine TEEL-1 values. Analysts also reviewed several available rat oral dosages. Out of those reviewed, a rat oral dosage of 16000 mg/kg (20% mortality rate) resulted in a TEEL-3 of 3200mg/m3. For TEEL-3, an IDLH value of 3000 mg/m3 was found (1994). This value is comparable to the IDLH concentration.

TEEL-1 = Multiply NIOSH REL-TWA exposure limit by 3
TEEL-2 = TEEL-3/6; use the gap adjustment factors from Table 3.5
TEEL-3: IDLH value (1994) = 3000 mg/m3

Both analysts used same method, resulting in the same TEEL values. These results are slightly more conservative than the values listed in Rev29A, where a rat oral LD50 starting point was used. IDLH values are preferred over LD50 data according to the TEEL handbook. The number of waters for this agent is not specified, thus the CAF could not be used to adjust from amorphous silica to hydrated amorphous silica. Conversion to ppm were done using the MW for amorphous silica. TEELs were rounded to two significant figures and converted from referenced units to either ppm or mg/m3, providing both values. The review results in a recommendation to revise previous thresholds.
Temporary Emergency Exposure Limits for Chemicals: Methods and Practice (DOE-HDBK-1046-2016)
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