1,4-dioxane: The case of the disappearing tumors

Rachel Shaffer is a consultant.  Richard Denison, Ph.D., is a Lead Senior Scientist.

As we highlighted in a previous post, EDF filed extensive comments on EPA’s draft risk evaluation for 1,4-dioxane. Among the many concerns we raised was a decision by the Trump EPA to completely dismiss female mouse liver cancer data used by EPA’s Integrated Risk Information System (IRIS) program as key inputs to its cancer risk modeling conducted in 2013. The Agency appears to be trying every trick in the trade – such as excluding most exposure sources and routes – in its effort to conclude that the chemical presents few or no risks to human health or the environment. Read on for more on this latest one.

In EPA’s draft 1,4-dioxane risk evaluation, Table 4-12 (p. 126) presents the cancer slope factors (CSF) that EPA uses for its risk characterization.  (CSFs are estimates of the increased cancer risk due to exposure to the chemical over a lifetime.)  Included in this table are data for tumors observed in male and female rats as well as male mice from the key oral carcinogenicity study by Kano et al. (2009).  However, EPA omits any mention of the liver tumors Kano et al. observed in female mice in the same study.

This omission is not only conspicuous but highly problematic, given that in the 2013 IRIS assessment, EPA selected these very tumors as the most sensitive endpoint and the basis for the oral CSF.

EPA’s justification for its decision to ignore the female mouse liver tumors is that “female mouse hepatocellular carcinoma data from Kano et al. (2009) were not modeled due to the difficulties that were previously noted in the IRIS assessment” (p. 334).  However, EPA fails to mention that IRIS was able to resolve these difficulties by “[applying] other … models…to the female mouse liver tumor dataset to achieve an adequate fit” (p. 138 of the IRIS assessment). In contrast to EPA’s decision, the IRIS program’s selection of these tumors as the most sensitive endpoint and the basis for the oral CSF was supported by rigorous internal and external peer reviews.

This omission was also criticized by scientists at the New Jersey Department of Environmental Protection in their July 9, 2019 comments on the draft risk evaluation.

EPA’s decision is highly consequential:  By ignoring the tumors seen at the lowest dose, EPA was able to claim a 5-fold lower risk associated with lifetime exposure to 1,4-dioxane. How convenient.

Once again the Trump EPA’s bias towards the chemical industry is showing. With another draft risk evaluation now out for public comment and several more slated to be released in the next few months, we fully expect to see yet more of these nefarious efforts to distort science and underestimate risk.


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