The NRC Risk Assessment Paradigm
Because cancer and noncancer health impacts associated with environmental exposures generally cannot be directly isolated and measured, EPA scientists and others have spent more than two decades developing an extensive set of risk assessment methods, tools, and data to estimate environmental health risks. Although significant uncertainties remain, this risk assessment methodology has been extensively peer-reviewed, is widely used and understood by the scientific community, and continues to expand and evolve as scientific knowledge advances.
EPA's framework for assessing and managing risks reflects the risk assessment and risk management paradigm set forth by the National Academy of Sciences (NRC) in 1983 [ , ], shown in Figure 1 below. The NRC concluded that risk assessment and risk management are "two distinct elements" between which agencies should maintain a clear conceptual distinction. The 1983 NRC report identified four steps integral to any risk assessment: 1) hazard identification, 2) dose-response assessment, 3) exposure assessment, and 4) risk characterization. The NRC paradigm for risk assessment serves as the basis for OAQPS risk assessments under the air toxics program.
Figure 1. Diagram of NRC risk assessment/risk management paradigm.
Within the NRC paradigm, the evaluation of toxicity in a risk assessment is based on two sequential analyses. The first is the hazard identification, which identifies contaminants that may pose health hazards at environmentally relevant concentrations, and qualitatively describes the effects that may occur in humans. The second analysis is the human health dose-response assessment, which characterizes the relationship between the exposure to a pollutant and the resultant health effects.
Hazard Identification. The types of effects relevant to each chemical (e.g., cancer, or effects other than cancer) are determined as part of the hazard identification. Factors such as the experimental route of exposure, the type and severity of the effects, the biological plausibility of findings, and the consistency of findings across studies all contribute to the hazard identification statement.
Dose-Response Assessment. Generally, the dose-response assessment consists of two parts: the evaluation of data in the observable range, and the extrapolation from the observable range to low doses/risks. Recent terminology refers to the result of analysis in the observable range as the "point of departure" from which extrapolation begins. The approaches used for evaluation in the observable range are similar for all types of effects, but EPA's current extrapolation methods differ considerably for cancer and noncancer effects. Further details of dose-response assessment are provided in the web pages on risk assessment for carcinogens and non-carcinogens.
The nature of the dose-response assessment typically varies among pollutants. Sufficient data may exist for some criteria air pollutants, such as ozone or carbon monoxide, so that relatively complete dose-response relationships can be characterized. In such cases, there is no need for extrapolation to lower doses because adequate human health effects data are available at environmentally relevant levels. However, this has not often been the case for air toxics. Epidemiologic and toxicologic data for air toxics have typically resulted from exposure levels that were high relative to environmental levels.
- National Research Council. 1983. Risk assessment in the federal government. Managing the process. National Academy Press, Washington, DC.
- National Research Council. 1994. Science and Judgment in Risk Assessment. National Academy Press, Washington, DC."