A peer review panel of nine independent scientists discussed EPA’s draft IRIS Toxicological Review of Hexavalent Chromium on May 12 and by-and-large recommended that the agency update the draft to consider the forthcoming results of the Cr-VI Mode of Action (MOA) Research Project funded by the American Chemistry Council. Researchers from ToxStrategies presented preliminary results from the MOA Research Project to the panel during the meeting.
The MOA Research Project was designed to address data gaps from the 2008 National Toxicology Program (NTP) Cr-VI chronic bioassay drinking water study. EPA used the NTP study as the basis for the oral reference dose for Cr-VI carcinogenicity in the draft IRIS Review.
In comments submitted to the docket for the peer review panel, AMWA suggested that the IRIS Review of Cr-VI be expanded to include the data that supports the reduction of Cr-VI to Cr-III within the human body. AMWA’s letter also recommended that the draft be updated to better consider the “extrapolation of the dose response that would be expected in humans based on the oral exposure to the low levels to be expected (i.e., in drinking water), and the possibility of a practical threshold.”
Peer review panel members discussed at length EPA’s use of a linear low-dose extrapolation model for assessing Cr-VI cancer risk. Most panel members had serious concerns with this approach and suggested that Cr-VI toxicity and carcinogenicity exhibit threshold mechanisms in the human system and gut and also demonstrate non-linearity. In addition, many panel members did not believe that the scientific evidence supported a mutagenic MOA for Cr-VI, as proposed in the draft IRIS Review.
While the panel members commended EPA on the draft IRIS Review, many also recommended that EPA reorganize and edit the draft document to address inconsistencies in the synthesis of the scientific evidence presented in the document as well as in the citations. The panel will submit its final comments to EPA in the coming weeks. EPA staff said the agency will “seriously” consider the peer reviewers’ comments.
Comments submitted on the draft IRIS assessment to date, including preliminary research results from ToxStrategies, may be reviewed online. At the site, select “Environmental Protection Agency” and the keyword “EPA-HQ-ORD-2010-0540" (for the docket ID) to view the comments. AMWA’s comment letter is online. A link to the draft IRIS assessment is also online.