ABSTRACT
The subject of most risk assessments is an agent that has been or will be imposed on the
environment such as a new pesticide, an effluent, a timber harvest, or an exotic species. In
some cases, the impetus for the assessment is a new source of such agents. These include new
technologies such as a new coal liquefaction technology, facilities such as a highway, activities
such as off-road motorcycle races, plans such as a timber management plan, or even cultural
pressures such as suburban sprawl that will impose multiple agents on the environment. The
assessment must begin with a reasonably complete description of the agent and its source.
That information is typically provided to regulatory agencies by the applicant for a permit,
and the required information is specified in regulations. However, if the source is contamin-
ated media, an established exotic species or equivalently existing ambient source, then it and
its associated agents must be characterized by census, sampling, or analysis. If such data are
not available, the characterization of sources and agents must be included in the analysis plan.
Conventionally the design of a facility, including any regulatory restrictions, determines what
may be released from a facility, how it is released, and how frequently. This information is
commonly known as the source term. Source terms typically estimate the characteristics of
normal operations including variance in concentrations and release rates due to variance in
raw materials, product mix, and conditions. However, they should go further and estimate the
variance due to unintended releases. Chemicals and other agents may be released by spills and
other accidents, by fugitive releases (e.g., leaks around gaskets or during pouring), or by
operation during startup, shutdown, or upset conditions (e.g., when operating temperatures or
pressures are not achieved or when switching between products). In addition, many chemicals
get into the environment even though they are not intentionally released (PCBs) or even not
intentionally produced (chlorinated dioxins).
