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ENVIRONMENTAL TOXICOLOGY

On the other hand, WHO (2004a) provides a TDI of 0.6 µg/kg bw/day for uranium in drinking water that is based on an identical rationale as provided by Health Canada (1999). Overall, the Health Canada TDI for uranium was considered to be appropriate for use in the development of a PSQGHH. In the cases of the ATSDR (1999) and U.S. EPA (2005a), these international health agencies developed toxicity reference values that were considerably less conservative than the Health Canada value while WHO (2001a) offered a value that was only slightly more conservative, and likely insignificantly so given the uncertainties in the determination of toxicological reference values. Finally, WHO (2004a) offers a TDI value that is identical to Health Canada (1999). Based on the above, the Health Canada TDI of 0.6 µg/kg bw/day was considered to be appropriate for the purposes of establishing soil quality guidelines. Summary of Various Toxicity Reference Values (TRVs) for Uranium Health Agency Animal Species Used to Estimate TRV Assumed LOAEL (µg/kg bw/day) Endpoint Uncertainty Factor Used to Estimate TRV Estimated TRV (µg/kg bw/day) Health Canada (1999) Rat 60 Renal toxicity 100 0.6 ATSDR (1999) Rabbit 50 Renal toxicity 30 2 U.S. EPA (2005a) Rabbit 2,800 Decreased body weight, renal toxicity 1,000 3 WHO (2001a) Rabbit 50 Renal toxicity 100 0.5 WHO (2004a) Rat 60 Renal toxicity 100 0.6 In the case of inhalation exposures, ATSDR (1999) and WHO (2001a) are the only major international health agencies that have toxicity reference values for this pathway. ATSDR (1999) recommends a MRL of 0.0003 mg U/m3 (or 0.3 µg U/m3 ) for protection of chronic continuous exposures to soluble uranium and 0.008 mg U/m3 (or 8 µg U/m3 ) for protection of intermediate exposures to insoluble uranium. The ATSDR MRL of 0.3 µg U/m3 for soluble uranium was based on a NOAEC of 0.05 mg U/m3 in dogs in the uranium tetrachloride study by Stokinger et al. (1953) (where minimal microscopic lesions in the renal tubules were observed at a LOAEC of 0.20 mg U/m3 ) and then application of a 30-fold uncertainty factor to account for interspecies and intraspecies variability and additional factors to account for continuous exposures and differences between dog and human respiratory dynamics. The ATSDR MRL of 8 µg U/m3 for insoluble uranium was based on a NOAEC of 1.1 mg U/m3 in dogs in the uranium dioxide study of Rothstein et al. (1949) (where minimal microscopic lesions in the renal tubules were observed at a LOAEC of 8.2 mg U/m3 ) and then application of a 30-fold uncertainty factor to account