The irreversibility effect implies that a decision maker who neglects the p
rospect of receiving more complete information at later stages of a sequent
ial decision problem will in certain cases too easily take an irreversible
decision, as he ignores the existence of a positive option value in favour
of reversible decisions. This option value represents the decision maker's
flexibility to adapt subsequent decisions to the obtained information. In t
his paper we show that the economic models dealing with irreversibility as
used in environmental and capital investment decision making can be extende
d to emergency response decisions that produce important irreversible effec
ts. In particular, we concentrate on the decision whether or not to evacuat
e an industrial area threatened by a possible nuclear accident. We show in
a simple two-period evacuation decision model that non-optimal conclusions
may be drawn when evacuation is regarded as a 'now or never decision'. The
robustness of these results is verified by means of a sensitivity analysis
of the various model parameters. The importance of 'options thinking' in th
is decision context is illustrated in an example.