Preliminary Remediation Goals: What's New

Region 9 PRG Topics

What's New

California Human Health Screening Levels (CHHSLs) (PDF)(67pp, 760K)

Regional Screening Level Table

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.

The September 2008 version:

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.

Region 9 Topics and Programs | A-Z Index