risk to humans caused by environmental exposure to the same substance may be uncertain and controversial. Exposure levels for certain chemicals may be measurable and reproducible for populations in some locations but available only as modeled estimates in others. Investigators may have data to show that a pollutant is present in the soil at certain locations but may not have data to determine whether residents are exposed to it and, more importantly, whether residents are exposed at potentially harmful levels. Sophisticated methods for quantifying uncertainty may be available for some data categories but untested and of uncertain utility for others. Gathering additional information would require time and resources. As a result, uncertainty is always present in data and analysis, and decision making is invariably based on a combination of well-understood and less-well-understood information.

Consistent with its mission to “protect human health and the environment”1 (Box 1-1) (EPA, 2011), the U.S. Environmental Protection Agency (EPA) estimates the nature, magnitude, and likelihood of risks to human health and the environment; identifies the potential regulatory actions for mitigating those risks and best protecting public health2 and the environment; and considers the need to protect the public along with other factors when deciding on the appropriate regulatory action. Each of those steps has uncertainties that can be estimated qualitatively, quantitatively, or both qualitatively and quantitatively. A challenge for EPA is to determine how to best develop and use those estimates of uncertainty in data and analyses in its decision-making processes.


Human health risk assessment3 is one of the most powerful tools used by EPA in making regulatory decisions to manage threats to health and the environment. Historically, the analysis and consideration of uncertainty in decision making at EPA has focused on the uncertainties in the data and the analysis used in human health risk assessments, including the underlying sciences that comprise the field, such as exposure science, toxicology, and modeling.


1 Many of the principles discussed in this report apply also to ecological risk assessment and decision making, but because the committee’s charge focuses on human health, the report addresses issues relating only to the human health component of EPA’s mission.

2 Throughout this report the committee uses the term public health when referring to EPA’s mission. The committee includes in the use of that term the whole population and individuals or individual subgroups within the whole population.

3 Human health risk assessment is a systematic process within which scientific and other information relating to the nature and magnitude of threats to human health is organized and evaluated (NRC, 1983).

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