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EPA Non-Cancer Toxicity Value for Dioxin and CERCLA/RCRA Cleanups

On February 17, 2012, EPA released the final non-cancer dioxin reassessment, publishing a non-cancer toxicity value, or reference dose (RfD), for 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in EPA's Integrated Risk Information System (IRIS). The RfD for TCDD is for immediate use at Superfund sites to ensure protection of human health.

The following are questions and answers providing information on the use of the RfD at RCRA and CERCLA sites.

Questions and Answers

1. How will the new 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) Reference Dose (RfD) impact the cleanup of dioxin-contaminated sites?

In all cases, EPA's goal is protection of human health and the environment informed by the best available science as embodied in the RfD. In accordance with existing EPA guidance, the new RfD will be used to develop site-specific risk-based cleanup levels at CERCLA and RCRA sites. At sites that have been previously investigated or cleaned up under Superfund and RCRA, EPA Regions will consult with EPA Headquarters and will coordinate with our state partners to identify, prioritize and evaluate sites to determine if additional response action is needed.

2. Will additional cleanups be needed when the Agency releases the dioxin reassessment cancer results?

Dioxin-contaminated sites cleaned up based on the new non-cancer RfD are not expected to need additional cleanup when a new EPA cancer toxicity value for dioxin is published in EPA's Integrated Risk Information System (IRIS). This is because we anticipate that dioxin cleanup levels based on the new non-cancer RfD will be within the cancer risk range currently used by EPA's Superfund and RCRA cleanup programs.

3. What are the new screening soil dioxin preliminary remediation goals (PRGs), based on the new RfD and national, non-adjusted exposure factors?

Preliminary remediation goals (PRGs) are risk-based concentrations in media based on readily available information, which are used as a starting point in developing final remediation goals (NCP 40 CFR §300.430(e)(2)(i)). With the release of the IRIS RfD for TCDD, the Agency no longer has a need for the PRGs for dioxin in soil recommended in EPA's 1998 Approach for Addressing Dioxin in Soil at CERCLA and RCRA Sites (EPA 1998) or the proposed interim PRGs provided in the December 30, 2009 Draft Recommended Interim Preliminary Remediation Goals for Dioxin in Soil at CERCLA and RCRA Sites.

Instead, consistent with the National Contingency Plan's (NCP's) preamble (see e.g., 55 Fed. Reg. 8666 at p. 8745 (March 8, 1990) and subsequent guidance (EPA 2003), the IRIS RfD is the preferred toxicity value to use as a starting point for establishing exposure screening levels that are protective of human health. Thus, this RfD is now the recommended value "to be considered" (TBC) for use in developing site-specific dioxin PRGs and cleanup levels under CERCLA and the NCP. For example, the PRG calculated using the new RfD of 0.7 pg/kg-day (picogram per kilogram-day) and EPA non-adjusted exposure factors would be 50 parts per trillion (ppt) toxicity equivalence (TEQ) for residential soil and 664 ppt TEQ for commercial/industrial soil. We expect the Regions often will have preferred site-specific data they can use to adjust these goals using site-specific exposure factors instead of the national exposure factors.

The site-specific dioxin PRGs themselves are not the final remediation goals. During the remedial investigation/feasibility study (RI/FS), PRGs are generally modified based on site-specific data (e.g., exposure duration, frequency of exposure, etc. - see Risk Assessment Guidance for Superfund (RAGS) Part B, see as well The Role of Baseline Risk Assessments in Remedy Selection Decision). Bioavailability is a factor to be considered during this process.

Superfund's nine remedy selection criteria (NCP 40 CFR §300.430(f)(5)(i)) are subsequently applied to the site-specific risk-based PRGs to develop final remediation goals (see A Guide to Preparing Superfund Proposed Plans, Records of Decisions, and Other Remedy Selection Documents). The nine selection criteria are: protection of human health and the environment; compliance with ARARs; long-term effectiveness and permanence; toxicity, mobility or volume reduction through treatment; short-term effectiveness; implementability; cost effectiveness; state agency acceptance; and community acceptance.

4. To which environmental media does the new RfD apply?

The new oral RfD may be used to evaluate dioxin-contaminated environmental media, such as soil, dust, sediments, or fish tissue.

Additional information on evaluating dioxin at CERCLA and RCRA sites is found below.

Dioxin Tool Box

The Dioxin Tool Box is composed of several technical documents intended to assist Superfund Project Managers in the sampling and analysis of dioxin contaminated soils.

The Dioxin Tool Box includes:

Supporting Documents

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