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Risk Assessment for Dioxin at Superfund Sites

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Toxicity values for dioxin

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A. Non-cancer toxicity value for TCDD

On February 17, 2012, EPA released the publishing an oral 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 for TCDD at Superfund sites.

Frequently Asked Questions and Answers

1. How will the 2012 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 2012 RfD for TCDD will be used to develop site-specific risk-based cleanup levels at Superfund sites. At sites that have been previously investigated or cleaned up under Superfund, EPA Regions will consult with EPA Headquarters and will coordinate with 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 cancer dioxin reassessment results?

Dioxin-contaminated sites cleaned up based on the 2012 non-cancer RfD are not expected to need additional cleanup when a new EPA cancer toxicity value for TCDD is published in EPA's Integrated Risk Information System (IRIS). This is because we anticipate that dioxin cleanup levels based on the 2012 non-cancer RfD will be within the cancer risk range currently used by EPA's Superfund cleanup program. The status of EPA’s dioxin cancer reassessment is described on this page. 

3. What are the soil dioxin screening levels (SSLs)/soil dioxin preliminary remediation goals (PRGs), using the RfD and national, non-adjusted exposure factors?

Soil screening levels (SSLs) are contaminant concentrations below which no further action or study is generally warranted under the Comprehensive Environmental Response, Compensation and Liability Act (CERCLA) (EPA’s Soil Screening Guidance: User’s Guide).

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)).

Soil screening levels can be used as PRGs, provided appropriate conditions are met (i.e., conditions found at a specific site are similar to conditions assumed in developing the SSLs) (EPA’s Soil Screening Guidance: User’s Guide).

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 Resource Conservation and Recovery Act (RCRA) Sites 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) preamble (see e.g., 55 Fed. Reg. 8666 at p. 8745 (March 8, 1990)) and subsequent guidance (EPA’s Human Health Toxicity Values in Superfund Risk Assessments), 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 SSLs, PRGs and cleanup levels under CERCLA and the NCP. Information on EPA Regional Screening Levels (RSLs), using the RfD and either national non-adjusted (i.e. default) exposure factors and/or site-specific exposure factors, can be found at:

Using this information, 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.). For reference, see the following:

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.

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; implement ability; cost effectiveness; state agency acceptance; and community acceptance.

4. To which environmental media does the 2012 RfD apply?

The 2012 oral RfD may be used to evaluate dioxin-contaminated environmental media, such as soil, dust, sediments, or fish tissue.

B. Cancer Toxicity Value for TCDD

EPA’s Human Health Toxicity Values in Superfund Risk Assessments provides a hierarchy of tiers of human health toxicity values generally recommended for use in risk assessments at Superfund sites. This document provides guidance, as in the case of dioxin, on selecting toxicity values to use in the absence of IRIS toxicity values, designated as Tier 1 toxicity values.

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Exposure factors

A. Standard Exposure Factors

In the absence of site-specific exposure factors, default, or standard, factors can be used. The Human Health Evaluation Manual, Supplemental Guidance: Update of Standard Default Exposure Factors provides guidance on the use of default exposure factors to characterize exposures to human populations for human health risk assessments at Superfund sites.

B. Bioavailability of Dioxin in Soil

Relative bioavailability (RBA) refers to differences in the bioavailability of dioxin in a medium, such as soil, and in the test medium used in the critical study(s) on which toxicity values were based.

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Toxicity Equivalence Factors (TEFs) / Toxicity Equivalence (TEQs)

Toxicity Equivalence Factors (TEFs).  2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and dioxin-like compounds (DLCs), including polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), and dioxin-like polychlorinated biphenyls (PCBs), typically occur as mixtures in environmental media. The evaluation of TCDD and DLCs at Superfund sites includes consideration of the toxicity of these contaminants. In the absence of DLC toxicity values, the toxicity of DLCs can be addressed by considering their toxicity relative to TCDD. EPA recommends using updated TEFs to assess human health risks from exposure to dioxin-like compounds.

A TEF for a DLC is a measure of the compound’s toxicity relative to TCDD, which is assigned a TEF of 1. For example, 1,2,3,4,7,8- hexachloro-dibenzo-p-dioxin is considered one tenth as toxic as TCDD and has therefore been given a TEF of 0.1.

Toxicity Equivalence (TEQ). For a single DLC, dioxin toxicity equivalence (or TCDD TEQ) is the product of the concentration of the DLC in an environmental mixture and its corresponding TEF; total TEQ for the mixture is the sum of the individual TCDD TEQs across the DLCs. The total TEQ for the mixture, rather than DLC concentrations measured in media, is used for site evaluation purposes.

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