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HH: Risk Characterization
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In this step, the risk assessor combines the information on exposure (average daily intake) and toxicity (RfD, slope factor) to predict the types of non-cancer and cancer effects that may occur and provide information on the probability and/or severity of the effects. Resources and guidance documents are provided at the end of this discussion under Resources.
For most chemicals, the potential for non-cancer effects is evaluated by comparing the estimated daily intake of the chemical over a specific time period with the RfD for that chemical derived for a similar period of exposure. This comparison results in a non-cancer Hazard Quotient (HQ), as follows:
HQ = DI/RfD
HQ = Hazard Quotient
DI = Daily Intake (mg/kg-day)
RfD = Reference Dose (mg/kg-day)
If the HQ for a chemical is equal to or less than one (1E+00), it is believed that there is no appreciable risk that non-cancer health effects will occur. If the HQ exceeds 1E+00, there is some possibility that non-cancer effects may occur, although an HQ above 1E+00 does not indicate an effect will definitely occur. This is because of the margin of safety inherent in the derivation of all RfD values. The larger the HQ value, the more likely it is that an adverse effect may occur.
If an individual is exposed to more than one chemical, a screening-level estimate of the total non-cancer risk is derived simply by summing the HQ values for that individual. This total is referred to as the Hazard Index (HI). If the HI value is less than 1E+00, non-cancer risks are not expected from any chemical, alone or in combination with others. If the screening level HI exceeds 1E+00, it may be appropriate to perform a follow-on evaluation in which HQ values are combined only if they affect the same target tissue or organ system (e.g., the liver). This is because chemicals which do not cause toxicity in the same tissues are not likely to cause additive effects.
Excess Cancer Risk
The excess risk of cancer from exposure to a chemical is described in terms of the probability that an exposed individual will develop cancer because of that exposure by age 70. For each chemical of concern, this value is calculated from the daily intake of the chemical from the site averaged over a lifetime (DIL) and the slope factor (SF) for the chemical, as follows:
Excess Cancer Risk = 1 – exp(–DIL × SF)
In most cases (except when the product of DIL × SF is larger than about 0.01), this equation may be accurately approximated by the following:
Excess Cancer Risk = DIL × SF
Excess cancer risks are summed across all chemicals of concern and all exposure pathways that contribute to exposure of an individual in a given population.
The level of total cancer risk that is of concern is a matter of personal, community, and regulatory judgment. In general, the USEPA considers excess cancer risks that are below about 1 chance in 1,000,000 (1×10-6 or 1E-06) to be so small as to be negligible, and risks above 1E-04 to be sufficiently large that some sort of remediation is desirable. Excess cancer risks that range between 1E-06 and 1E-04 are generally considered to be acceptable (see Role of the Baseline Risk Assessment in Superfund Remedy Selection Decisions (Memorandum from D. R. Clay, OSWER 9355.0-30, April 1991), although this is evaluated on a case-by-case basis and EPA may determine that risks lower than 1E-04 are not sufficiently protective and warrant remedial action.
Risks from Lead
In the case of lead, human exposure and risk are characterized using a somewhat different approach than described above. This is because lead exposure is evaluated using a biokinetic model, and risk is interpreted in terms of blood lead concentration rather than a Hazard Quotient. At present, the maximum level of lead in the blood that is considered to be acceptable is 10 μg/dL, and EPA has established a goal that the risk of exceeding 10 μg/dL should not exceed 5 percent.
EPA has developed a specialized computer model, referred to as the Integrated Exposure Uptake Biokinetic (IEUBK) model, for evaluating lead risks to children. For adults, the EPA recommends a somewhat simpler but conceptually similar model called the Adult Lead Model (ALM). Both models use data on the magnitude of lead exposures from multiple sources to estimate the average blood lead level that would occur in a population of exposed individuals and the probability that any random individual will have a blood lead level greater than 10 μg/dL. Resources for evaluating risks from lead are presented on a separate page, Evaluation of Risks from Lead.
Risk Assessment Guidance for Superfund (RAGS) I Part A: Human Health Evaluation Manual (EPA/540/1-89/002, December 1989)
Guidance on Risk Characterization for Risk Managers and Risk Assessors (Memorandum from F. H. Habicht II, February 1992)
Role of the Baseline Risk Assessment in Superfund Remedy Selection Decisions (Memorandum from D. R. Clay, OSWER 9355.0-30, April 1991)
RAGS I Part D: Standardized Planning, Reporting, and Review of Superfund Risk Assessments (OSWER 9285.7-47, December 2001)