If death does not occur as a result of HF exposure, renal effects from the uranium moiety might occur (Just 1984).

In the absence of relevant chemical-specific data for deriving the lowest acute exposure guideline level (AEGL) values for UF6, modifications of the AEGL-1 values for HF were used. The use of HF as a surrogate for UF6 was deemed appropriate for the development of AEGL-1 values, which are based on irritation symptoms, because it is likely that HF, a hydrolysis product, is responsible for those low-level effects. The HF AEGL-1 values were based on the threshold for pulmonary inflammation in healthy human adults (Lund et al. 1999). Because a maximum of 4 moles (mol) of HF are produced for every mole of UF6 hydrolyzed, a stoichiometric adjustment factor of 4 was applied to the HF AEGL-1 values to approximate AEGL-1 values for UF6; the AEGL-1 values for UF6 are constant across time up to 1 hour (h) because the HF AEGL-1 values were held constant across time. AEGL-1 values for UF6 were derived for only the 10-minute (min), 30-min, and 1-h time points because derivation of 4- and 8-h values resulted in AEGL-1 values greater than the 4- and 8-h AEGL-2 values calculated for UF6. That would be inconsistent with the total database.

The AEGL-2 values were based on renal pathology in dogs exposed to UF6 at 192 milligrams per cubic meter (mg/m3) for 30 min (Morrow et al. 1982). An uncertainty factor (UF) of 3 was used to extrapolate from animals to humans, and a UF of 3 was also applied to account for sensitive individuals (total UF=10). This total UF of 10 is considered sufficient because the use of a larger total UF would yield AEGL-2 values below or approaching the AEGL-1 values, which are considered no-observed-effect levels and were based on a threshold for inflammation in humans. Furthermore, humans were exposed to HF repeatedly at up to 8 parts per million (ppm) with only slight nasal irritation; that is stoichiometrically equivalent to a UF6 exposure at 28.8 mg/m3, a concentration equivalent to the 10-min AEGL-2. The concentration-exposure time relationship for many irritant and systemically acting vapors and gases may be described by Cn×t=k (C =concentration, t=time, and k is a constant), where the exponent n ranges from 0.8 to 3.5 (ten Berge et al. 1986). To obtain protective AEGL values in the absence of an empirically derived, chemical-specific scaling exponent, temporal scaling was performed using n=3 when extrapolating to shorter time points and n=1 when extrapolating to longer time points. (Although a chemical-specific exponent of 0.66 was derived from a rat lethality study in which the end point was pulmonary edema, the default values were used for time-scaling AEGL-2 values, because the end points for AEGL-2 [renal toxicity] and for death [pulmonary edema] involve different mechanisms of action.)



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