Int. J. Environ. Res. Public Health 2012, 9 2001 EPA’s Office of Pesticide Programs and Office of Solid Waste and Emergency Response conduct cumulative risk assessments to aid in decisions about pesticide regulation and Superfund sites, respectively. However, these offices’ cumulative risk assessments generally do not involve considerations of non-chemical stressors. Because risk assessments are based on methodologies that necessarily involve a series of assumptions, which are estimated to best reflect the understanding of real-world conditions, the analysis will inevitably contain some amount of uncertainty. On its website, EPA states, “In the ideal world, all risk assessments would be based on a very strong knowledge base (i.e., reliable and complete data on the nature and extent of contamination, fate and transport processes, the magnitude and frequency of human and ecological exposure, and the inherent toxicity of all of the chemicals). However, in real life, information is usually limited on one or more of these key data needed for risk assessment calculations. Consequently, risk assessors often have to make estimates and use judgment when performing risk calculations, meaning that all risk estimates are uncertain to some degree. For this reason, a key part of all good risk assessments is a fair and open presentation of the uncertainties in the calculations and a characterization of how reliable (or how unreliable) the resulting risk estimates really are” [39]. 1.2.2. How EPA Uses Risk Assessment in Decision-Making EPA often supports or justifies its decision-making by estimating risks associated with various pollutants or stressors. EPA uses risk assessment to characterize the nature and magnitude of health risks to humans and ecological receptors (e.g., birds, fish, wildlife) from chemical pollutants and other stressors that may be present in the environment [39]. Typically, EPA uses risk assessments in setting health-based standards for ambient levels of environmental contaminants (e.g., National Ambient Air Quality Standards (NAAQS)), in the registration of pesticides, in permitting the manufacture of new products that use toxic substances, and in governing the reduction or concentration levels of such substances. EPA also uses these assessments in siting new polluting facilities such as waste disposal and hazardous waste facilities, in guiding cleanup levels for sites contaminated by hazardous substances, and in evaluating brownfield sites (abandoned or underutilized commercial and industrial properties). More generally, EPA may use a risk assessment whenever an environmental impact statement or environmental permit is required, in prioritizing public health concerns, and in setting priorities for research and funding ([13], p. ix; [11], p. 342). EPA policy and decision-makers use the information developed through these assessments to help decide how to protect humans and the environment from stressors or pollutants. Although the value and relevance of risk assessments have been questioned, the NRC asserts that risk assessment remains an appropriate method for measuring the relative benefits of the many possible interventions available to improve human health ([13], p. 15). Generally, EPA uses risk assessments that calculate expected health effects in three different statutory approaches to environmental protection: health-based provisions that typically do not tolerate “any” significant risk to public health or welfare; technology-based provisions that do not tolerate risks that can be “feasibly” eliminated; and risk-benefit provisions that find intolerable technologies, substances, or processes that pose “unreasonable” risk [43].