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Risk assessment is a formalized process for characterizing and estimating the magnitude of harm resulting from some condition—usually exposure to one or more hazardous substances in the environment. This chapter addresses what risk assessment is, what it is used for, and how it is done. “Environmental risk assessment” usually refers to human health risks, while “ecological risk assessment” refers to damage to natural or artificial ecosystems, wildlife species, and endangered species. There are some common properties and important differences.1,2 Environmental risk assessment interfaces with environmental toxicology and exposure assessment, while ecological risk interfaces with ecotoxicology. Risk assessments are used in a wide variety of context, for example, to establish no effect concentrations3 which can inform cleanup levels,4 sediment quality standards,5 or comparison of alternative remediation strategies. Ecological risk is also applied to the probability of extinction of species or populations (population viability analysis) due to chance6 or pollution,7 and to the likelihood that exotic species will become invasive.8 Increasingly, governments and the public have realized that it is critical to protect the health and well-being of ecological systems, both for their own value as well as for the ecological services that they provide for humans including safe drinking water, clean air, fertile land for agriculture, unpolluted waters for fisheries, erosion control and stabilization of coastal environments, and places for recreation and other aesthetic pursuits so important to people.9 Ecological risk has been linked with the growing interest in restoring damaged habitats.10 Moreover, changes in ecosystem health can have direct effects on human health by changing human exposure to disease organisms.11 Risk assessment for genetically modified crops bridges human health and ecological concerns.12 Harmonization of ecological and human health risk assessment has been done on a few occasions (see below).13,14

Risk assessments are intended to provide objective information to inform public policy decisions.15 Their utility for individual risk is variable. Risk assessments are used in other walks of life from bridge construction to finance to medical errors,16 and more recently to terrorism.17 Risk assessment is primarily a scientific endeavor, while risk management refers to those actions taken by society to ameliorate risks. Risk management takes into account human values and fiscal concerns and determines what risk assessments need to be done and how they are to be used, but the methods and outcomes of risk assessment should not be biased by these concerns.18 Risk management may involve policy decisions that set particular standards for contaminants in air, water, soil, or food, or they may reflect particular decisions on whether and how much to remediate a hazardous waste site.

There is ongoing controversy as to whether risk assessment can remain value-free or whether that is an illusion. In 1983, the modern environmental risk assessment approach was codified by the National Research Council's “red book” on Risk Assessment ...

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