The Environmental Protection Agency (EPA) has issued an interpretive policy ruling¹ under which construction of major² new or expanded stationary sources of air pollution could in certain circumstances be allowed in areas that have not attained the national ambient air quality standards. Approval of such construction would be subject to a number of conditions, most notably that the additional emissions from the new or expanded sources be more than offset by reductions at existing facilities within the region. This position has caused considerable controversy because it contrasts sharply with the widely held view that the Clean Air Act as currently written forbids approval of any additional sources in nonattainment areas. One of the first major policy issues facing the new EPA Administrator thus promises to be reconsideration of the interpretation, which became effective when promulgated.