Because there is typically a long period between environmental release of a toxic substance and appearance of chronic adverse health and environmental effects, many industrial and commercial chemicals that initially appeared innocuous have turned out to be dangerous. It is thus not uncommon for companies to have engaged in widespread production of dangerous substances—and for government regulatory agencies to have sanctioned their wide use and disposal—in good-faith ignorance of their potential for harm. Then, when the threat becomes apparent, initiation of effective measures to halt and clean up contamination is likely to prove difficult because of the financial and employment stakes of preserving the status quo, and the uncertain legality and fairness of effectively penalizing a course of conduct that was officially permitted initially.¹