By John S. McClenahen Although yesterday's "clean air" cases before the U.S. Supreme Court were widely billed as a test of the EPA's powers to limit smog and particulate matter, actually three major issues are before the justices. One is the question of whether the EPA can revise "clean air" standards, as it did with smog and particulates in 1997. But the court also has before it the question of whether EPA must factor the economic costs of cleanup into equation when setting industrial pollution-control standards. "For the last 30 years, the Clean Air Act consistently has been interpreted by EPA and Congress to prohibit the consideration of costs in setting health-based standards," notes Environmental Defense, a New York-based environment-protection group. The third question before the court is whether the Clean Air Act as interpreted by EPA is an unconstitutional delegation by Congress of its legislative authority. The answer to that question presumably would affect not only the EPA, but several other initialed federal regulatory agencies such as the FCC. Because of the importance of the issues, the Supreme Court is likely to issue its decision near the end of its current term, probably not before May, 2001, says William Brownell, a partner in the administrative law group and head of the environmental team at Hunton & Williams, a Washington-based law firm. The cases argued Nov. 7 are Browner v American Trucking Assns.(No. 99-1257) and American Trucking Assns. v Browner(No. 99-1426).