The initial list of priority pollutants was based on a 1977 consent decree that settled a legal challenge to the U.S. EPA's program for controlling hazardous pollutants. A relatively small number of revisions to the list have been made by the U.S. EPA administrator since 1977. Decisions to expand the list must take into account the toxicity, persistence, and degradability of the pollutant; the potential presence and the importance of affected organisms in any waters; and the nature and extent of the effect of the toxic pollutant on such organisms.