Regional Screening Levels (Formerly PRGs)

Regional Screening Table Resources

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Region 9 RSLs Home

Region 9-Specific Information

The updated Region 9 PRG table (now called Regional Screening Level (RSL) table) should be used in the same way that the PRGs have been used in the past. Both a User’s Guide and FAQ are provided.

The PRG screening table is a living document that reflects the current state of the science of toxicology and risk assessment.

May 2012

The RSL Table lists Superfund human health screening values for soil, air, and tapwater. The most significant developments for the Spring 2012 update appear below.

PCE (Perchloroethylene, "Perc", Tetrachloroethylene). PCE is one of the most common industrial contaminants we have at Superfund sites and despite its environmental footprint and potential to cause adverse health effects, it is still a popular dry cleaning agent. After more than 20 years of scientific studies and debate, EPA has completed its health risk assessment of this chemical which is now posted on EPA’s IRIS website. Chemical assessments on IRIS are used by various regulatory programs to set guidelines and standards for environmental media. The most immediate impact is that Superfund has established new screening levels for this chemical to evaluate potential for vapor intrusion as well as groundwater contamination. (Note: The Maximum Contaminant Level [MCL] standard set by the Office of Water has not changed but will be undergoing review as a result of the new IRIS assessment of PCE).

For each environmental media (soil, air, and tapwater), there are actually two health-based RSLs for PCE, one that screens for potential cancer risk (at one in a million [10-6] increased chance of cancer) and another that screens for potential non-cancer health hazards (hazard index = 1). The summary table lists the lower or more stringent value, but both the cancer and non-cancer RSLs can be found in the media-specific supporting tables.

Taken together, the cancer and non-cancer RSLs define EPA's health-protective range. For indoor air, EPA's new health range for residential buildings is 9.4 to 42 micrograms PCE per cubic meter of air (ug/m3) and for commercial / industrial buildings the new range is 47 to 180 ug/m3 PCE. Unlike its cousin, TCE, the cancer RSLs for PCE have actually increased (are less conservative) by about 20X. You may want to let your California State counterparts know about this new development, as the RSLs were previously calculated using California toxicity values.

Dioxin. Despite the recent attention focused on dioxin, there is actually little to report with this Spring update to the RSLs. IRIS has announced a new non-cancer toxicity value for dioxin (2,3,7,8-TCDD), but this only changes the non-cancer RSLs slightly (less than 2X). For residential soil as an example, the non-cancer RSL changes from 72 to 51 parts per trillion (ppt). However, the cancer RSL for residential soils of 4.5 ppt dioxin remains unchanged and at least for now, EPA will continue to use the California cancer toxicity values to estimate cancer RSLs for dioxin.

Lead. For those of you working on lead sites, you may have noticed that in previous RSL tables, there were no air screening values listed for this heavy metal. With this latest version, the National Ambient Air Quality Standard (NAAQS) for lead of 0.15 ug/m3 has been adopted as the residential air RSL. For more on lead RSLs, please see the lead FAQ.

Methylene chloride (aka Dichloromethane). EPA's IRIS toxicity database has re-evaluated the health risks posed by this volatile organic chemical (VOC) that may be detected in indoor air due to various sources. Methylene chloride is now listed as a mutagen in the RSL table because it is thought to cause cancer via DNA damage. As a result of this new status, an additional safety factor is applied to take into account increased cancer susceptibility of children. Given this new development, you might expect that the RSLs would be more stringent (that is, set at lower concentration levels) for this chemical. However, this is actually not the case. For indoor air concerns, the cancer RSL is raised from 5.2 to 96 ug/m3 in residential buildings and raised from 26 to 1200 ug/m3 in commercial / industrial buildings.

The next revision to the RSL Table is scheduled for release sometime in November, 2012.

November 2011
Click to show/hide the November 2011 updates.

November 2011

TCE (Trichloroethylene). TCE is one of the most common industrial contaminants we have at Superfund sites. After more than 20 years of scientific studies and debate, EPA has completed its health risk assessment of this chemical which is now posted on EPA’s IRIS website. Chemical assessments on IRIS are used by various regulatory programs to set guidelines and standards for environmental media. The most immediate impact is that Superfund has established new screening levels for this chemical to evaluate potential for vapor intrusion as well as groundwater contamination. (Note: The Maximum Contaminant Level [MCL] standard set by the Office of Water has not changed but will be undergoing review as a result of the new IRIS assessment of TCE).

For each environmental media (soil, air, and tapwater), there are actually two health-based RSLs for TCE, one that screens for potential cancer risk and another one that screens for potential non-cancer health hazards. Although the summary table only lists the lower or more stringent value, the media-specific supporting tables present both values which outline the health-protective range for TCE. For example, the protective range for indoor air quality concerns due to TCE vapor intrusion is: 0.43 - 2.1 micrograms TCE per cubic meter of air (typically abbreviated as ug/m3) for residential buildings and 3.0 - 8.8 ug/m3 TCE for industrial / commercial buildings.

Vapor Intrusion Update. The EPA RSLs and California Human Health Screening Levels (CHHSLs) for air can be used to evaluate indoor air quality concerns associated with vapor intrusion. They can also be used to evaluate the potential for vapor intrusion below ground surface. However, they must first be divided by an attenuation factor that accounts for the reduction in VOC concentrations as the contaminant moves from the subsurface to indoors. California Department of Toxic Substances Control (DTSC) recently published its final Vapor Intrusion Guidance (PDF) that contains recommended attenuation factors for various scenarios (see Table 2). The recommendations in this Guidance should be followed in the State of California. Since this Guidance draws upon national guidance as well as regional considerations for the Pacific Southwest, other states in Region 9 may find this document to be useful as well.

Protecting Children's Health. Tapwater "non-cancer" RSLs have been modified to incorporate child-specific exposure factors for a 1-6 year old, similar to the RSL approach used for soils. Previously, non-cancer tapwater screening levels were based on adult exposure assumptions. This focus on children, results in a lowering of the non-cancer tapwater RSLs by a factor of 2 or 3. Cancer-based RSLs are not affected, as they already incorporate both child and adult exposure factors.

Dermal Exposure and Tapwater. Tapwater RSLs now consider dermal exposures as per Risk Assessment Guidance for Superfund (RAGS), Part E, for chemicals for which data are available. You can identify which chemicals have been evaluated for dermal exposure by looking the chemical up in the "Residential tapwater supporting" tables at the Region 9 PRG page.
Learn more about "RAGS Part E"

Looking Ahead. RSLs are "evergreen", incorporating the latest exposure and toxicity information twice-yearly, in the Fall and Spring. EPA's Exposure Factors Handbook (2011 addition) could influence the exposure parameters used to calculate RSLs. As a result, we may see changes to the RSLs to reflect these new exposure recommendations as early as May 2012, with the release of the Spring update to the RSL Table.

Also, IRIS is finalizing a human health risk assessment of PCE (perchloroethylene), a common contaminant of concern for vapor intrusion and groundwater contamination issues at Superfund sites. So, expect new RSLs for this compound in the Spring 2012 update.

May 2011
Click to show/hide the May 2011 updates.

Vapor Intrusion
Added the New York non-cancer reference level for TCE in air which narrows the acceptable health risk range (residential air range changes from 1.2 - 120 to 1.2 - 10 ug/m3 TCE) when both cancer and non-cancer health endpoints are considered.

Recreational Screening
New RSL Calculator scenario, The "recreator," which will calculate recreational screening numbers for surface water and sediments. First applied during BP's Gulf spill, this tool is now available. The EPA provides very few default exposure parameters for the recreational scenario, so users are encouraged to have a strong understanding of their receptor prior to using it. It is best to consult with one of the regional toxicologists regarding this site-specific scenario.

Provisional Peer Reviewed Toxicity Values (PPRTVs)
Now available online for chemicals. According to U.S. EPA's hierarchy of toxicity databases, PPRTVs represent the second tier after IRIS. In California, consider California human health numbers as a first tier, followed by IRIS and then PPRTVs.

September 2008

(transition from PRG table to RSL table)
Click to show/hide the September 2008 updates.
- incorporates new inhalation exposure equations per RAGS Part F (2008/2009),
- presents industrial air screening values in addition to residential screening values,
- discontinues the use of route-to-route extrapolations to derive toxicity values, and
- includes an additional safety factor for cancer-causing substances that are determined to damage DNA through a mutagenic mechanism of action.
Online Screening Level Calculator

Another added feature with this update is the online screening level calculator. The online calculator enables a risk assessor to generate risk-based screening levels tailored to an individual site. All inputs, including toxicity values and exposure assumptions can be modified with the calculator. In general, use of non-default values in the calculator should be accompanied by a discussion that identifies the specific changes and presents a rationale for those changes. Ultimately, the use of non-default inputs to the calculator should be approved by the regional risk assessor assigned to your site.

Cal-Modified PRGs

In addition to the above changes, “Cal-modified PRGs” are no longer presented in the PRG screening table because California now has its own set of screening values known as California Human Health Screening Levels (CHHSLs) (PDF) (67pp, 760K, About PDF). At last count, there are CHHSLs for approximately 60 chemicals and the list is expected to grow per California Senate Bill 32, the California Land Environmental Restoration and Reuse Act. “Cal-modified PRGs” for specific chemicals can also be derived using the online calculator above.

Vapor Intrusion Pathway

Vapor intrusion screening levels for volatile contaminants in soil gas and/or groundwater may be estimated from either residential or industrial air PRGs listed in the table. Where volatile contaminants are present, it is incumbent on the risk assessor to consider the vapor intrusion pathway. To calculate generic screening values for soil gas, approaches similar to those used to derive Cal/EPA CHHSLs (134pp, 782K, About PDF) may be appropriate for EPA Region 9 sites.

Toxicity Hierarchy

Not all PRGs are created equal. For some chemicals, a robust data set exists upon which the toxicological criteria are based whereas for others, there may be relatively few studies that form the basis of the PRG / RSL calculation. EPA has established a hierarchy for the selection of toxicity values (OSWER Directive 9285.7-53), which has been further refined for the sake of the PRG screening table. Please see Section 2.3 of the User’s Guide for more information. It should be noted that Cal/EPA’s Office of Environmental Health Hazard Assessment (OEHHA) also derives peer-reviewed toxicity values that should be used in California, particularly if they are more stringent than EPA’s toxicity values.

Potential Problems

As with any risk-based tool, there exists the potential for misuse. We try to highlight potential problems in the User’s Guide. However, the use of PRGs at an individual site becomes the responsibility of the user.

Contact Information

If you find an error or have a question that is not addressed in the supporting documents (User’s Guide and FAQ sheet), please send the PRG / RSL workgroup a note.