October 15, 2009

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U.S. Environmental Protection Agency Plan for Developing Interim 

Preliminary Remediation Goals for Dioxin in Soil

In May 2009, the U.S. Environmental Protection Agency’s (EPA) Administrator, Lisa P. 

Jackson decided that EPA needs to accelerate work underway to reassess the human 

health risks from exposures to dioxin. EPA’s Science Plan for Activities Related to 

Dioxins in the Environment (2009)1details a plan, with interim milestones, for 

completion of the Agency’s dioxin reassessment. By the end of 2010, EPA expects to 

complete the dioxin reassessment and release it to the public, subject to further 

consideration of the science and the scope and complexity of the revisions that will need 

to be made. 

Several key site-specific decisions involving dioxin may need to be 

made before the dioxin reassessment is finalized. EPA’s Office of 

Solid Waste and Emergency Response (OSWER) will be reviewing 

current dioxin cleanup guidance set by the Agency and other 

entities, with the goal of recommending interim preliminary

remediation goals (PRGs) informed by the latest science. These 

interim recommended PRGs would be used until EPA issues its 

dioxin reassessment and OSWER issues final recommended PRGs 

based on the reassessment. After publication of the dioxin 

reassessment and final PRG guidance, EPA will re-evaluate 

cleanup decisions at Superfund, Federal Facilities, Brownfields, 

and Resource Conservation and Recovery Act (RCRA) sites that 

were based on the interim PRGs to ensure that cleanups are 

protective. 

In developing these interim recommended PRGs, OSWER and EPA’s Office of Research 

and Development (ORD) are reviewing current soil cleanup levels and dioxin toxicity

values used by the states, the Agency for Toxic Substances and Disease Registry, and 

other countries. Based on this evaluation, OSWER may reconsider EPA’s currently 

recommended PRGs for Superfund, Federal Facilities, Brownfields and RCRA sites,2

which are 1 ppb (parts per billion) (or 1,000 ppt (parts per trillion)) for dioxin toxicity

equivalents (TEQs)3in residential soil, and a level within the range of 5 ppb (or 5,000 

ppt) and 20 ppb (or 20,000 ppt) in commercial/industrial soil, where exposure is due to 

direct contact. Three key components of EPA’s current recommended PRGs will be reevaluated: the EPA dioxin toxicity value adopted in 1985, generic exposure assumptions, 

and the cancer risk level. OSWER intends to recommend interim PRGs informed by the 

latest science and the work of state and other agencies. 

The proposed plan for the development of the OSWER interim recommended PRGs for 

dioxin in soil is as follows:

 Open public comment period on this OSWER plan 

for developing interim recommended PRGs October 2009 

What is a PRG?

Preliminary remediation goals 

(PRGs) are chemical-specific 

concentration goals for specific 

media (e.g. soil, sediment, water 

and air) and land use combinations 

at Superfund, Federal Facilities, 

Brownfields and RCRA sites. They 

serve as a target to use during the 

initial development, analysis, and

selection of cleanup alternatives. 

These goals should both be 

protective of human health and the 

environment and comply with all 

applicable, relevant and appropriate 

regulations (ARARs) for all 

exposure pathways being addressed.
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 Interim recommended PRGs available for public comment 

in the Federal Register December 31, 2009

 End of public comment period February 2010

 Issuance of guidance that addresses interim recommended 

PRGs June 2010*

 

*Schedule subject to change based on the extent of public comments received.

EPA will make this information available to the public through the following: 1) the EPA 

Superfund program web site; 2) Clu-in, a web site maintained by EPA that has technical 

information related to cleanups; 3) postings on regional web sites and used at public 

meetings; and 4) distribution generally to the regions.

To comment on the proposed plan for developing interim recommended PRGs for dioxin 

in soil, please send comments to: OSWER_Dioxin_PRGs@epa.gov 

Frequently Asked Questions

How are PRGs Used?

Consistent with the National Contingency Plan,4PRGs generally are initial chemicalspecific concentration goals for specific media (e.g. soil, sediment, water and air) and 

land uses at Superfund, Federal Facilities, Brownfields and RCRA sites. These goals

normally should both be protective of human health and the environment and comply

with all applicable, relevant and appropriate regulations (ARARs) for all exposure 

pathways being addressed. 

Risk-based PRGs generally are developed using estimates for the toxicity of a chemical,

as well as generic exposure assumptions. An example of an exposure assumption is 

exposure frequency, or the number of days in a year that a person is exposed to 

contamination. A generic exposure frequency of 350 days per year is typically used for 

Superfund, Federal Facilities, Brownfields, and RCRA sites for residential settings.

A PRG is usually calculated for cancer and for non-cancer effects. The more 

conservative value typically is then selected as the recommended PRG. PRGs for 

carcinogens commonly are set at levels that correspond to one in a million (1 x 10-6) 

individual excess cancer risk5as a point of departure. However, they may be revised to a 

different risk level within the acceptable risk range of 10-6to 10-4based on consideration 

of exposure factors, uncertainty, or technical issues. PRGs for non-carcinogens are 

typically set at a level corresponding to a hazard quotient6equal to 1.0.

It is important to note that PRGs are not intended to act as site-specific cleanup levels; 

rather they are intended to serve as initial guidelines for use in scoping characterization 

and remediation alternatives at Superfund, Federal Facilities, Brownfields, and RCRA 

sites. Final cleanup levels for a site typically would be developed by modifying the
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PRGs based on consideration of site specific factors (e.g., exposure frequency or 

acceptable cancer risk level).7

For specific information on how PRGs are calculated, see EPA’s Risk Assessment 

Guidance for Superfund (RAGS) Part B, Chapter 1 and Chapter 3. Available online at: 

http://www.epa.gov/oswer/riskassessment/ragsb/index.htm

What are the current PRGs for dioxin in soil?

For Superfund, Federal Facilities, Brownfields, and RCRA sites, OSWER’s 1998 soil 

dioxin guidance2recommends a PRG of 1 ppb (or 1,000 ppt) for dioxin toxicity

equivalents (TEQs)3in residential soil, and a level within the range of 5 ppb (or 5,000 

ppt) and 20 ppb (or 20,000 ppt) in commercial/industrial soil, where exposure is due to 

direct contact. A range in levels has been recommended for commercial/industrial soil 

due to the greater variability in human exposures associated with these land uses.

The currently recommended PRGs are based on an EPA dioxin toxicity value adopted by 

the Agency in 1985. This toxicity value is considered a Tier 3 toxicity value under the 

2003 OSWER memorandum on the hierarchy of sources of toxicological information8

that should be considered for site-specific risk assessments (the interim recommended 

PRGs will also be based on a Tier 3 toxicity value.) The categories for toxicity values are 

as follows:

 Tier 1- EPA’s Integrated Risk Information System (IRIS)9

 Tier 2- EPA’s Provisional Peer-Reviewed Toxicity Values (PPRTVs) – The 

Office of Research and Development/National Center for Environmental 

Assessment/Superfund Health Risk Technical Support Center develops PPRTVs, 

at the request of EPA’s Superfund program, for chemicals not found in IRIS. 

 Tier 3- Other Toxicity Values – Tier 3 includes additional EPA and non-EPA 

sources of toxicity information. Priority should be given to those sources of 

information that are the most current, the basis for which is transparent and 

publicly available, and which have been peer-reviewed. 

The currently recommended PRGs are also based on generic exposure assumptions and 

cancer risk levels used at Superfund, Federal Facilities, Brownfields, and RCRA sites. 

The 1 ppb (or 1,000 ppt) represents a lifetime excess cancer risk of 2.5 x 10-4for 

residential soil, and 5 ppb (or 5,000 ppt) represents a lifetime excess cancer risk of 1.4 x 

10-4for commercial/industrial soil. 

Why is EPA considering modifying the existing PRGs for dioxin in soil? 

While EPA finalizes its ongoing dioxin reassessment, EPA Administrator Jackson has 

asked that Agency staff consider the newest science to help evaluate the protectiveness of 

current PRGs for dioxin in soil. 
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How will the interim PRGs for dioxin in soil be used?

The interim PRGs will be used to evaluate pending cleanup decisions. These decisions will 

be re-evaluated when the final PRGs for dioxin in soil are developed. As noted above, the 

interim PRGs being developed for dioxin are not site-specific cleanup levels. They are 

intended to serve as recommended PRG levels that can generally be used as a starting 

point in the process used for determining cleanup levels at Superfund, Federal Facilities, 

Brownfields, and RCRA sites. Final cleanup levels for a site would typically be 

developed by modifying the PRGs based on consideration of site-specific information 

(such as exposure frequency or acceptable cancer risk level) obtained as part of the sitespecific baseline risk assessment. 

 

1 U.S. Environmental Protection Agency. May 26, 2009. EPA’s Science Plan for Activities Related to 

Dioxins in the Environment. Press Release. Available online: 

http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=209690.

2 U.S. Environmental Protection Agency. April 13, 1998. Approach for Addressing Dioxin in Soil at 

CERCLA and RCRA Sites. OSWER Directive 9200.4-26. Available at: 

http://www.epa.gov/superfund/resources/remedy/pdf/92-00426-s.pdf.

3 Toxicity equivalents consider the toxicity of the less toxic dioxin-like compounds as fractions of the 

toxicity of the most toxic compound (2,3,7,8-TCDD). Each compound is attributed a specific “Toxic 

Equivalency Factor” (TEF). This factor indicates the degree of toxicity compared to 2,3,7,8-TCDD, which 

is given a reference value of 1.

4

40 CFR 300 Section 430(e)(2)(i)(A)(2)

5 Cancer risks normally are estimated as the incremental probability of an individual developing cancer 

over a lifetime as a result of exposure to a contaminant. This incremental or excess individual lifetime 

cancer risk is generally expressed as the probability (e.g., 1 x 10-6or one in a million) of an individual 

developing cancer.

6

 A hazard quotient generally is the ratio of an exposure level for a single substance to a reference dose, 

which is an estimate of the daily exposure that is likely to be without an appreciable risk of deleterious 

effects during a lifetime.

7

 EPA's Risk Assessment Guidance for Superfund, Part B, Development of Preliminary Remediation Goals 

1991, EPA/540/R-92/003

8 U.S. Environmental Protection Agency. Office of Solid Waste and Emergency Response. December 5, 

2003. Human Health Toxicity Values in Superfund Risk Assessments. OSWER Directive 9285.7-53. 

Available at: http://www.epa.gov/oswer/riskassessment/pdf/hhmemo.pdf.

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IRIS develops human health risk information for an EPA database that addresses potential human health 

effects that may result from long-term exposure to environmental contaminants. Dioxin toxicity values 

established based on the final dioxin reassessment will be incorporated into IRIS.