ABSTRACT
In addition to describing the place where exposure will occur, it is necessary to describe the
circumstances under which it will occur (Oliver and Laskowski 1986). This is particularly
important if an agent acts episodically, has significant temporal variability, or takes different
forms in alternative futures. The scenarios define sets of hypothetical or actual conditions
under which exposure may occur and for which risks will be characterized. Scenarios may be
designed to represent typical cases, reasonable worst cases, the expected range of cases, or a
set of plausible bad cases. If the life cycle of a chemical or product is assessed, scenarios must
be developed for manufacturing, transportation, use, and disposal (Scheringer et al. 2001;
Weidema et al. 2004). In a risk assessment of regional development, the development scen-
arios must be defined (Gentile et al. 2001). For contaminated sites, scenarios must address
future movements of the contaminants, changes in land use, and changes in the biota. For
example, ecological risk assessments of the Oak Ridge Reservation assumed that river otters
and bald eagles would soon become reestablished on the site. The number and type of
scenarios developed should be defined in consultation with the risk manager or should be
based on established policy.
