How were the toxicity values provided in IRIS on Chromium used to calculate Chromium Screening Levels?
It is recommended that valent-specific data for Chromium be collected when Chromium is likely to be an important contaminant at a site, and when Cr6 may exist. In the absence of valent-specific data, screening levels for total Chromium are provided. For Chromium (VI) (Cr6), IRIS shows an air unit risk of 1.2E-2 per (ug/m3). However, the supporting documentation in the IRIS file states that this toxicity value is based on an assumed 1:6 ratio of Cr6:Cr3. Because of this assumption and in an effort to be transparent, RSLs based on this cancer toxicity value are presented as “Chromium, Total (1:6 ratio Cr VI:III)” numbers. In the RSL Table, the Cr6 specific value (assuming 100% Cr6) is derived by multiplying the IRIS Cr6 value by 7. This is considered to be a conservative and protective assumption, and is also consistent with the State of California’s interpretation of the Mancuso study that forms the basis of Cr6’s toxicity values. If you are working on a chromium site, you may want to c
