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Analysis Methods for Design Decisions 185

1. Chronic risks are the result of long-term exposure to low lev-
els of toxic or hazardous agents, which may cause adverse
health effects in humans, or may harm fish, wildlife, and
other environmental resources. An example might be traces
of carcinogenic substances in drinking water.
2. Acute risks are the result of sudden, episodic exposure to
substantial levels of toxic or hazardous agents that may cause
immediate harm to humans or other living things. An exam-
ple might be a hazardous chemical spill during transport.
Risk can generally be measured in two dimensions: 1. the likeli-
hood that an adverse outcome will actually occur, expressed as a prob-
ability, and 2. the magnitude of the consequences expressed in terms
that depend on the nature of the outcome (e.g. cost of ecological dam-
age, incidence rate of disease). Acute risks are frequently assessed in
terms of a risk profile, which assigns a probability distribution to the
range of risk magnitude. Chronic risks are usually assessed in terms
of the expected incidence of specific outcomes over a given time
period. In principle, however, these two classes of risk are part of a
continuum and can be assessed within a common framework.
Environmental risk analysis is a widely accepted approach where-
by risks can be anticipated, understood and controlled. While there
are many versions of this approach, a number of fundamental pro-
cess steps are commonly practiced (see Table 9.1):
• Vulnerability assessment is the process of identifying and char-
acterizing potential sources of risk (sometimes called hazards
or vulnerabilities) in terms of their nature, mechanisms of
action, and possible outcomes; this provides a basis for prior-
ity-setting to determine which risks merit greater attention.
• Quantitative risk assessment is the process of estimating the
likelihoods and/or magnitudes of selected risks that have
been identified; this generally includes assessment of the
uncertainty associated with the “best” estimates of risk.
• Integrated risk evaluation is the process of assigning relative
importance to risks that have been identified and/or quanti-
fied, based on regulatory, economic, social, or other factors
that influence their acceptability to stakeholders.
• Risk management is the process of deciding how to avoid, mit-
igate, or otherwise control those risks that are deemed unac-
ceptable; it generally involves a balancing of risks against the
costs and benefits of mitigation alternatives.
• Risk communication is the process of understanding the con-
cerns of stakeholders regarding identified risks and explain-
ing the results of risk assessment, evaluation, and management
decisions in terms that are meaningful to them.