Why don the Chromium VI toxicity values match the IRIS profile?
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 an overly conservative assumption by some within the Agency. However, this calculation 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 contact the appropriate regulatory officials in your region to determine what their position is on this issue. Gathering speciated chromium data is also preferred where Cr6 may be an issue.
