Lead at Superfund Sites: Risk Assessment
The science of risk assessment encompasses the analysis of site data, development of exposure and risk calculations, and preparation of human health and ecological impact reports. For more additional information on the Superfund risk assessment process, visit EPA's Superfund Risk Assessment web page.
EPA's approach to lead risk assessment follows these four steps in the use of the Integrated Exposure Uptake Biokinetic (IEUBK) Model:
EPA's risk assessment for lead is unique because a reference dose (RfD) value for lead is not available. An RfD is typically derived from a concentration below which no adverse effects have been observed. Existing evidence indicates that adverse health effects occur even at very low exposures to lead (e.g., subtle neurological effects in children have been observed at low doses).
Since the toxicokinetics (the absorption, distribution, metabolism and excretion of toxins in the body) of lead (Pb) are well understood, lead is regulated based on blood lead concentration (PbB). EPA and the Centers for Disease Control and Prevention (CDC) have determined that childhood blood lead concentrations at or above 10 micrograms of lead per deciliter of blood (µg/dL) present risks to children's health. EPA's risk reduction goal for contaminated sites is to limit the probability of a child's blood lead concentration exceeding 10 µg/dL (the P10) to 5 percent or less after cleanup.
Blood lead concentration can be correlated with both exposure and adverse health effects. To predict blood lead concentration and the probability of a child's blood lead concentration exceeding 10 µg/dL (the P10) based on a given multimedia exposure scenario, one can apply a model which considers lead exposure and toxicokinetics in a receptor – i.e., a child (using the IEUBK model) or fetus (using the Adult Lead Methodology (ALM)) to derive an exposure level that satisfies the risk reduction goal.
Although the IEUBK model may be run using default values, it is preferable to obtain site-specific data for soil and dust lead concentrations. Site data may also support refined estimates for other model parameters such as bioavailability variables.
For each potential exposure medium (soil, dust, food, water, air) at a site, several strategies may be employed for sample collection. The sampling strategy must be appropriate for its use in the risk assessment. For this reason, risk assessors should be involved in the development of the sampling strategy. The following areas are of major concern: sample size, location and type, temporal and meteorological factors, field analyses, and sampling costs. For more information, see the Risk Assessment Guidance for Superfund.
Consistent with current EPA guidance, the Technical Review Workgroup for Metals and Asbestos (TRW) recommends consideration of the collection of blood lead concentration data when feasible. If such human health monitoring is planned to assess current and/or historical exposures, the Agency for Toxic Substances and Disease Registry (ATSDR) should be consulted to make sure any collected human site-specific data are of sufficiently high quality to be used for human health risk assessment. The Guidance Manual for the Integrated Exposure Uptake Biokinetic Model for Lead in Children provides more information pertaining to the comparison of measured and predicted blood lead concentrations.
Once collected, the entire data set must be evaluated. Data quality may depend on the analytical method used and sample quantitative limits. The evaluation allows for selection of a subset of the data for use in the risk assessment.
Exposure may be defined as the contact of a receptor with a chemical or physical agent. Exposure assessment is the estimation or determination of the magnitude, frequency, duration and route of exposure. The exposure assessment incorporates intake and uptake in quantifying the magnitude of the exposure. Exposure assessments should consider not only the current population but also potential future populations.
For assessing lead exposure for lead risk assessment, EPA currently recommends two models, depending upon the age of the receptor population:
- The IEUBK model: for children, exposure assessments should be performed using the IEUBK model.
- The ALM: For adults, EPA's 1996 ALM (Recommendations of the Technical Review Workgroup for Lead for an Approach to Assessing Risks Associated with Adult Exposures to Lead in Soil, EPA-540-R-03-001, OSWER Dir #9285.7-54) should be used.
Both account for intake and uptake components of lead exposure, and allow the user to input site-specific data (e.g., exposure frequency, sources of lead) and predict blood lead concentrations (PbBs). Predicted blood lead concentrations provide one indication of the associated lead exposure for both current and potential future populations.
The toxicity assessment weighs all available evidence and estimates the potential for the occurrence of adverse health effects. The CDC has identified a blood lead concentration level of 10 µg/dL as the level of concern above which significant health risks occur. For lead, the toxicity assessment is based on exceeding the 10 µg/dL blood lead concentration. Both the IEUBK model and EPA's 1996 ALM generate predicted blood lead concentrations and provide information on the percentage of the population exceeding 10 µg/dL. For lead risk assessment purposes, the IEUBK model and the ALM can provide information necessary to determine the probability of adverse health effects associated with lead exposures.
Risk characterization is the key step in the risk assessment process; it serves as the bridge between risk assessment and the information needs of risk management. Risk characterization is a combination of all information gathered during the three phases of the risk assessment. It relates toxicity and exposure assessments and can include the development of preliminary remediation goals.
It is important to identify and understand all assumptions and uncertainties associated with the risk assessment in order to place conclusions in the proper perspective. Moreover, uncertainty analysis may identify areas at a site where additional data collection could aid in the selection of a more suitable remedy. This is especially true for model parameters that are very sensitive to site-specific input (e.g., bioavailability). When conducting a risk assessment, it is more important to identify the key site-related variables and assumptions that contribute most to the uncertainty than to precisely quantify the degree of uncertainty in the entire risk assessment.