The measurement of plant emissions is entering a new phase. The driving force ¾ at least in the U.S. ¾ is the Clean Air Act (CAA) Amendments of 1990, which represent the latest effort to strengthen the laws that protect the nation’s air. The amendments expand the narrow focus of the original 1970 Act into a broader and, at the same time much more specific program for ridding the skies of smog, acid rain and toxic chemicals