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Risk Assessment

Selecting Exposure Routes and Contaminants of Concern by Risk-Based Screening

Region 3 Technical Guidance Manual, Risk Assessment

Human health risk assessment includes effort-intensive steps which require many detailed calculations by experts. Most baseline risk assessments are dominated by a few chemicals and a few routes of exposure. Effort expended on minor contaminants and exposure routes, i.e., those which do not influence overall risk, is essentially wasted. This guidance is intended to identify and focus on dominant contaminants of concern and exposure routes at the earliest feasible point in the baseline risk assessment. Use of these methods will decrease effort and time spent assessing risk, without loss of protectiveness. This guidance is not intended for other risk assessment activities, such as determining preliminary remediation goals.

Selecting Contaminants and Exposure Routes of Concern

Most samples from hazardous waste sites are analyzed for 103 target compounds and analytes recommended by the EPA Superfund program. Semi-volatile analysis can detect additional tentatively identified compounds not on the target lists. Special analytical services procedures, if used, may find still more contaminants. The combined number of contaminants detected at a site sometimes exceeds one hundred.

While EPA considers it necessary to gather information on many contaminants, very little of this data actually influences the overall quantitative assessment of health risk. For most sites, baseline risk assessments are dominated by a few contaminants and a few routes of exposure. The remaining tens, or hundreds, of detected contaminants have a minimal influence on total risk. This small impact is lost by rounding. Entire environmental media may contain not a single contaminant at a concentration which could adversely affect public health. Quantitative risk calculations using data from such "risk-free" media have no effect on the overall risk estimate for the site.

The EPA baseline risk assessment process at several points requires careful data evaluation by scientific experts. These evaluations, which are contaminant-specific, include: (1) statistical comparisons between site-related and background samples, (2) special handling of undetected contaminants, (3) calculation of toxicity equivalence, (4) evaluation of frequency of detection, and (5) comparison with ARARs. Because overall risk is usually driven by a few contaminants and exposure routes, effort spent in detailed evaluation of minor contaminants and routes of exposure is essentially wasted. For some sites, this wasted effort exceeds 90% of the total.

The baseline risk assessment process can be made more efficient by focusing on dominant contaminants and routes of exposure at the earliest feasible stage. The mechanisms recommended for this are (1) a re-ordering of the process of eliminating contaminants and routes of exposure, and (2) use of a risk-based concentration screen. Appropriately used, this process can dramatically reduce the effort of risk assessment, while not changing the result significantly.

Existing Guidance

Chapter 5 of "RAGS IA" (Risk Assessment Guidance for Superfund, Volume I, Human Health Evaluation Manual (Part A); EPA, 1989) provides a detailed procedure for evaluating data for a baseline risk assessment. This procedure includes steps by which the risk assessor selects contaminants of concern in each exposure medium. These steps are summarized in Table 1.

There are two major limitations to the RAGS procedure. First, the eliminating step (a concentration toxicity screen) comes late in the process. Many of the preceding steps (e.g., evaluation of quantitation limits, comparison with background, calculation of toxicity equivalence, and evaluation of frequency of detection) are contaminant- and medium-specific. They require the sustained attention of an expert, and cannot be automated. If the contaminant is eliminated, this work is wasted.

The second limitation is that the concentration toxicity screen compares only relative risk among contaminants in the same medium. While very efficient at selecting dominant contaminants in each medium, this method does not evaluate significance of total risk for the medium. Thus, the concentration toxicity screen can eliminate contaminants, but not routes of exposure.

Recommended Methodology

This guidance makes two changes intended to remove the limitations in existing guidance. These recommendations are intended for baseline risk assessments.

  1. Re-ordering of steps. The eliminating screen is moved forward in the data evaluation process to a point immediately following data quality evaluation. The new process is shown in Table 2. Effort-intensive steps such as evaluation of quantitation limits and comparison with background now follow the eliminating screen. The steps are divided into four categories: data quality evaluation, initial data set reduction, re-inclusion of special cases, and optional final data set reduction.
    The data quality evaluation steps (evaluating appropriateness of methods and qualifiers, significance of blank contamination, and need for special analyses) should be done as described in RAGS IA, Chapter 5. Next, the risk assessor should consult with the RPM to discuss the use of the Risk-Based Screening Table (described in item [2] below) as a screening mechanism. With the RPM's approval, the risk assessor should reduce the data set and document the rationale for eliminating contaminants and routes of exposure from further analysis.

    After the initial data set reduction, the risk assessor and RPM should consider re-including specific contaminants on the basis of historical data, toxicity, mobility, persistence, bioaccumulation, special exposure routes, special treatability problems, or exceedance of ARARs. These activities should proceed as described in Section 5.9 of RAGS IA.
    Finally, optional further reductions in the data set may be justified, based on the status of a contaminant as an essential nutrient, low frequency of detection, or no statistical difference between site and background levels. These evaluations, the most complicated and contaminant-specific, are saved for last.
  2. Screening by risk-based concentrations. The screening method is changed from the relative concentration toxicity screen of RAGS IA to an absolute comparison of risk. This is done by means of a table of risk-based concentrations (Appendix I). This table contains levels of nearly 600 contaminants in air, drinking water, fish tissue, and soil, which correspond to a systemic hazard quotient of 0.1 or a lifetime cancer risk of 10-6. The risk-based concentrations were developed using protective default exposure scenarios suggested by EPA (1991) and the best available reference doses and carcinogenic potency slopes (see the table for sources), and represent relatively protective environmental concentrations at which EPA would typically not take action.

    The risk-based concentration screen is used as follows:
    1. The risk assessor extracts the maximum concentration of each substance detected in each medium.
    2. If the maximum concentration exceeds the risk- based concentration for that medium, the contaminant is retained for risk assessment, for all routes of exposure involving that medium. Otherwise the contaminant is dropped for that medium.
    3. If a specific contaminant does not exceed its risk- based concentration for any medium, the contaminant is dropped from the risk assessment.
    4. If no contaminant in a specific medium exceeds its risk-based concentration, the medium is dropped from the risk assessment.
    5. All contaminants and exposure routes which are dropped are kept on a sub-list and considered for re-inclusion, based on special properties.
    6. If the risk assessor wants to include a route of exposure not covered in the Risk-Based Screening Table, the equations provided in Appendix I can serve as the basis for new risk-based concentrations. Similarly, the risk assessor can use the same equations to calculate alternate risk levels (i.e., other than a systemic hazard quotient of 0.1 and lifetime cancer risk of 10-6) to be the basis for screening.


The process by which contaminants and exposure routes are selected in quantitative risk assessment can be made less effort-intensive by two simple changes. First, high-effort steps should be postponed until later in the selection process, because performing these operations on trivial contaminants and exposure routes is pointless. Second, changing from a relative concentration toxicity screen to an absolute risk-based concentration screen improves the risk assessor's ability to focus on dominant contaminants and exposure routes at an earlier stage.


  • EPA, 1991. Human Health Evaluation Manual, Supplemental Guidance: "Standard Default Exposure Factors". OSWER Directive 9285.6-03, Office of Emergency and Remedial Response, March 25, 1991.
  • EPA, 1989. Risk Assessment Guidance for Superfund, Volume I, Human Health Evaluation Manual (Part A). Office of Emergency and Remedial Response, December, 1989. EPA/540/1-89/002.

For additional information, call 215-597-6682.

Approved by:
Thomas C. Voltaggio, Director
Hazardous Waste Management Division

Table 1. Summary of existing EPA guidance on selecting contaminants of concern (EPA, 1989, chapter 5)

Section 5.1: Combining data from site investigations

  • Determine if methods are appropriate
  • Evaluate quantitation limits
  • etermine if qualifiers are appropriate
  • Determine if significant blank contamination exists
  • Determine if special analyses for tentatively identified compounds are needed
  • Compare site samples to background

Section 5.9: Further reduction in the number of chemicals (optional)

  • Consult with RPM
  • Document rationale for eliminating chemicals
  • Examine historical information
  • Consider exceptional toxicity, mobility, persistence, or bioaccumulation
  • Consider special exposure routes
  • Consider special treatability problems
  • Determine if contaminants exceed ARARs
  • Group chemicals by class, evaluate toxicity equivalence
  • Evaluate frequency of detection
  • Evaluate essentiality
  • Use a concentration toxicity screen

Table 2. EPA Region III guidance on selecting contaminants and exposure routes of concern

a. Data quality evaluation

  • Determine if methods are appropriate
  • Determine if qualifiers are appropriate
  • Determine if significant blank contamination exists
  • Determine if special analyses for tentatively identified compounds are needed

b. Reduce data set using risk-based concentration screen

  • Consult with RPM
  • Use Risk-Based Screening Table to screen contaminants and exposure routes of concern
  • Document rationale for eliminating chemicals and exposure routes

c. Consider re-including eliminated chemicals and routes, based on:

  • Historical information
  • Exceptional toxicity, mobility, persistence, or bioaccumulation
  • Special exposure routes
  • Special treatability problems
  • ARARs exceedance
  • Toxicity equivalence of chemical class (e.g., CDD/CDFs, PAHs)

d. Make further specific reductions in data set (optional)

  • Evaluate essentiality
  • Evaluate frequency of detection
  • Compare site samples to background