To determine the costs & benefits of the Clean Air Act (CAA) enacted before the 1990 amendments, EPA is undertaking a retrospective study, using a sequential, six step methodology. This study will compare the economic, health & environmental conditions that have resulted from the CAA amend. enacted before 1990 with projections of what these conditions would have been without the amend. For the second study EPA will contrast the effects of air quality standards imposed by legislation enacted before 1990 with the effects of all potential post-1990 standards.