When authors of the Clean Air Act Amendments of 1990 tick off its many far-reaching provisions, the "permit hammer" rarely gets mentioned.

But congressional conferees spent days fighting over the hammer, an enforcement tool to assure that industrial plants install devices to curb airborne toxics -- even if the Environmental Protection Agency fails to set standards.

The hammer is one of many fail-safes built into the law to translate its grand design into practical programs. Whether the act signed last week becomes the environmental milestone touted by sponsors or just another well-intentioned failure to clean up the air will be determined by such obscure enforcement language.

Nearly 800 pages in length, the act is filled with specific requirements and specific timetables to cut pollution, reflecting efforts by Congress to avoid past failures by limiting the discretion of the EPA to implement the provisions.

Virtually every debate on every section featured arguments over the words "may" or "shall" to define duties of the EPA, which has failed to clean up whole categories of air pollution in cases where its authority was broad but vague. Without specific mandates, industry has delayed or derailed regulations by filing lawsuits or appealing to the Office of Management and Budget, which has veto power over EPA decisions.

"We weren't willing to let this bill be high sounding but a sham in terms of making sure the job was done," said Rep. Henry A. Waxman (D-Calif.), one of the principal architects who pushed for hard-and-fast requirements.

The original Clean Air Act of 1970 was ambitious, even radical in scope but so lacking in specificity -- it was only 65 pages long -- that whole sections were virtually ignored. The EPA, for example, was required to set standards for hazardous pollutants, which are so toxic that even small doses can kill or seriously injure. But 20 years later only seven of the hundreds of pollutants were regulated.

Today's law not only requires technological controls for industrial sources of 189 airborne toxics, it sets deadlines for standards to be set and control devices installed.

And resisting heavy lobbying by industry, Congress even defined the stringency of controls, requiring technology at least as advanced as that in an average of the top 12 percent of the cleanest plants in each polluting industry.

The permit hammer is to be used in case the EPA fails to comply with the regulatory schedule because of OMB intervention or red tape. Any industry that escapes regulation will have to apply to its state government for a permit and justify the level of technology it plans to use -- an expensive, time-consuming process.

"It may be that EPA won't do better than in the past," said Sen. Dave Durenberger (R-Minn.), an author of the provision. "But if the EPA fails, the permit hammer assures toxic reductions anyway."

Some of the biggest fights over "wiggle room," as the issue of discretion is called, involved auto regulations. One of the few successes of the 1970 law was the limit placed on tailpipe emissions -- specific cutbacks mandated in a specific model year.

But auto makers were able to slip out of proposals for onboard canisters to trap smog-forming emissions during refueling at the gas pump. In 1977, Congress directed the EPA to require either the canisters or gas pump nozzles to capture the fumes. After years of study, the EPA decided on canisters but was blocked by OMB.

This time, Congress ordered the canisters phased in for new cars starting in late 1995. And the law even specifies how much the fumes have to be reduced -- 95 percent.

"If we didn't set a minimum standard for emissions capture, when EPA put regulations in place and the auto companies thought they were too costly, they could sue or go to OMB," said Greg Wetstone, a Waxman aide who helped draft the bill. "Then everything is held in abeyance while you have this adjudication or OMB weakens the regulation."

"Courts are less likely to throw out regulations that are mandatory," he added.

Wetstone cited dozens of major provisions with mandatory requirements, ranging from tonnage limits on acid rain emissions by individual units of utilities to the regulations of emissions from such consumer products as nail polish.

Richard Ayres, chairman of the Clean Air Coalition of environmental and civic groups, said that while the law is more explicit and thorough than past efforts, "nobody should believe it's self-executing." Many key portions of the law remain discretionary, he said, including the system to monitor compliance with acid rain provisions and the definition of industrial categories regulated for air toxics.

"Any law in this area depends crucially on the vigor of enforcement by the EPA," said Ayres. "It also depends on the approach regulated industry takes. If they decide to do what they can to keep it from working, they'll find this law can be tied up like the one before it."

William Fay, administrator of the pro-industry Clean Air Working Group, stressed that big business favors clean air laws as long as they are "affordable" and fair.

But asked to describe the impact on industry of the latest law, Fay may have foreshadowed the coming battles over implementation.

"The legal industry will flourish," he said.