The objectives of an HHRA are two-fold: first, to estimate the level of risk to human health associated with concentrations of environmental contaminants; and second, if that risk is found to be unacceptable, to calculate media-specific cleanup levels that will protect human health.
Risks are estimated for current uses of a site as well as foreseeable future uses. All contaminated media are considered (for example, soil, water) if individuals are likely to be exposed to the media. All relevant routes of exposure are also considered, including direct contact, such as inhalation, ingestion, and dermal exposure, and indirect contact, such as exposure to vegetables that have taken up contaminants through the soil or water.
Cleanup levels are calculated based on the relationship between contaminants and risk as defined in the risk assessment and a policy decision (risk management) about the level of risk that is considered acceptable. As a result, cleanup levels for a single contaminant can vary from one site to another either because the relationship between environmental levels and risk differs or because different policy decisions have been made concerning the level of acceptable risk.
HHRA typically is described as including four steps: hazard identification, exposure assessment, toxicity assessment, and risk characterization. Early in the development of the field of risk assessment, hazard identification referred to determining which chemicals or compounds at a site could lead to risk. Today, the list of chemicals and compounds with associated human health risks are well known, and the first step has changed to data collection and analysis, including collecting data on the characteristics of the site and the chemicals or compounds of concern.
The second step in HHRA involves exposure assessment, including identifying the populations of individuals exposed to hazards at the specific site and how those exposures may occur. For example, the Coeur d’Alene River basin HHRA identifies children as the primary population of concern for lead exposure and identifies the presence of local American Indian populations. Potential pathways of exposure are defined, such as children ingesting soil and house dust contaminated with lead, and American Indian ingestion of locally grown foods contaminated with lead. At other sites, exposures could include scenarios such as inhalation and dermal exposure to volatile chemicals in groundwater while showering. In addition to identifying the potential pathways of exposure, this step may involve defining several parameters (for which there are insufficient measured data) that will