strategy. Materials in Group 1 can and should be intelligently cycled between the economy and environment, since they are relatively benign, with many of them being renewable. Materials in the Group 2 should be recycled safely within the economy, and carefully returned to natural systems no faster than they are withdrawn. Materials in Group 3 must be very judiciously handled in use, recycled where possible, and thoroughly treated before being transferred from economic to ecological systems. Materials in the fourth group are inherently risky and destructive enough to have no justification for their use in a sensible economic system; they should be reduced, replaced