Preliminary Remediation Goals
Risk-based preliminary remediation goals (PRGs) are calculated by taking the forward going risk equation and rearranging the equation to solve for the concentration value that corresponds to a specified level of risk.
Forward Going Cancer Risk Equation
For the case of estimating cancer risk to an individual exposed for 30 years, starting at birth (the standard scenario for an RME resident), from a carcinogenic chemical operating with a mutagenic mode of action that does not have chemical-specific data to develop cancer slope factors (i.e., ADAF adjustments are applied to the cancer slope factor), the forward going cancer risk is calculated as follows:
where the cancer risk for age interval "i" is given as:
C = Concentration of the chemical in the contaminated environmental medium (soil or water ) to which the person is exposed. The units are mg/kg for soil and mg/L for water. IRi = Intake rate of the contaminated environmental medium for age bin "i". The units are mg/day for soil and L/day for water. CF = Conversion factor (if needed). BWi = Body weight of the exposed person for age bin "i" (kg). EFi = Exposure frequency for age bin "i" (days/year). This describes how often a person is likely to be exposed to the contaminated medium over the course of a typical year. EDi = Exposure duration for age bin "i" (years). This describes how long a person is likely to be exposed to the contaminated medium during their lifetime. AT = Averaging time (days). This term specifies the length of time over which the average dose is calculated. For quantifying cancer risk, "lifetime" exposure employs an averaging time of 70 years (i.e., 70 years × 365 days/year). SF = Cancer slope factor (mg/kg-day)-1 ADAFi = Age-Dependent Adjustment Factor for age bin "i" (unitless)
Re-arranging the equation to factor out constants (i.e., C, CF, AT, and SF), the forward going cancer risk equation simplifies to:
a = 0 - <2 years b = 2 - <6 years c = 6 - <16 years d = 16 - <30 years Note: Alternate age bins may be considered to address site-specific scenarios.
 The risk calculation for air is slightly different. For more information see Methods for Derivation of Inhalation Reference Concentrations and Application of Inhalation Dosimetry (EPA/600/8-90/066F, October 1994).
Solving the above equation for C for a specified target level of risk yields:
Preliminary Remediation Goal Example
See the following file for an example of this calculation for benzo[a]pyrene for exposure from the ingestion of soil and ingestion of water exposure pathways:
For an example of a PRG calculation derived for a medium with more than one exposure pathway, see the following file that presents risks from exposure to benzidine from the ingestion and dermal exposure pathways:
Note: The case studies utilize the exposure factors and age intervals for a resident under current Superfund Guidance (EPA 1989, 1991, 2004). In applying the Supplemental Guidance to risk assessments, risk assessors should decide if a more refined, age-specific exposure assessment is warranted and use the exposure parameters/age grouping bins that make the most sense for their site. To assist with the computation of age-specific values in exposure estimates, exposure parameters from current EPA guidance (including EPA's Exposure Factors Handbook, Child-Specific Exposure Factors Handbook, Age-Grouping Guidance) were compiled. These values can be downloaded from the Exposure Assessment page of this Handbook.References
EPA. 1989. Risk Assessment Guidance for Superfund (RAGS). Volume I. Human Health Evaluation Manual (Part A). EPA/540/1-89/002, December.
EPA. 2004. Risk Assessment Guidance for Superfund Volume I: Human Health Evaluation Manual (Part E, Supplemental Guidance for Dermal Risk Assessment). OSWER 9285.7-02EP, July.