IN some ways, the truth about clean-air laws and their impact is about as clear as the Los Angeles basin on a bad air day.
The number of Americans regularly breathing dirty air has declined dramatically in the past five years. But Americans are creating more smog - driving bigger cars longer distances at faster speeds - as the population rises.
Meanwhile, there is evidence that air-quality regulations may not adequately address health issues - especially for children.
But there are also indications that the costs of making air even cleaner may outweigh the benefits.
In the middle of this potentially confusing scene is the Clean Air Act, which dates back to 1963 but saw reforms that made the act substantially stronger in 1970 and 1990.
The future of this landmark legislation is now tied up in political maneuvering by federal regulators, congressional deregulators, and state and local officials. Advocates on both sides have weighed in on this issue that affects nearly every American.
Since passage of the 1990 amendments, the number of people living in areas where smog levels are deemed unhealthy (mainly due to ground-level ozone from auto exhaust and manufacturing-plant waste) has dropped by 50 million, or more than one-third.
''Almost without exception, every area is seeing reduced pollution levels, which means that threats to public health are being reduced, even if all areas do not yet meet air-quality standards,'' observed Mary Nichols, assistant administrator of the Environmental Protection Agency, recently.
That still leaves 90 million Americans living in areas where the acceptable smog level is exceeded. (A safe smog level does not exceed 0.12 parts per million of ozone during a one-hour period on more than one day a year.)
Under pressure from a lawsuit brought by the American Lung Association, the EPA is studying whether the current smog standard (first set in 1979) takes into account the most up-to-date science on air pollution.
More recent studies, for example, show that ozone hangs in the air longer than was earlier presumed. This could cause adverse effects even though the level measured was within acceptable limits. As a result, some clean-air proponents have suggested the acceptable level should be lowered to 0.08 parts per million measured over eight hours.
Environmental groups have joined health advocates in suspecting that the standard may need to be updated. ''If people are actually suffering at current [smog] levels, then something is wrong,'' says Frank O'Donnell, executive director of Clean Air Trust, an advocacy organization headed by former Sens. Edmund Muskie (D) of Maine and Robert Stafford (R) of Vermont.
The cost of cleaner air
Not everyone agrees. Looking at a variety of smog studies on human populations and laboratory animals, researchers at the Center for the Study of American Business at Washington University in St. Louis recently concluded that ''incapacitating or irreversible health effects in humans remains highly speculative.''
In their report, titled ''Smog in America: The High Cost of Hysteria,'' center director Kenneth Chilton and Christopher Boerner also assert that making the Clean Air Act amendments of 1990 even tougher could cost between $3 and $5 for every $1 of identifiable benefits. Cutting smog emissions 40 percent over the next decade, they warn, would cost nearly $11 billion a year in lost business revenues and increased expenses. ''To be certain, no definitive conclusion should be drawn from such crude estimates,'' they write, ''but average cost-to-benefit ratios substantially above 1.0 should raise a warning flag.''
In an earlier study, the conservative Cato Institute called for ''legislative action to alleviate unneeded and costly regulatory mandates under the 1990 Clean Air Act Amendments.''
In response to these and other concerns (including state and local complaints about ''unfunded federal mandates''), the GOP-dominated Congress has eagerly responded. House Republican whip Tom DeLay (R) of Texas introduced a bill that simply stated the Clean Air Act ''is hereby repealed.''
Sen. Lauch Faircloth (R) of North Carolina, chairman of the Senate environment subcommittee dealing with air quality, has drafted the ''Clean Air Simplification and Efficiency Act.'' This is designed to relieve some of the regulatory burden of the Clean Air Act and give much of the enforcement authority to the states.
Critics note that much of Mr. Faircloth's proposal is virtually identical to ideas distributed by petroleum industry representatives last March. In a point-by-point analysis, the EPA's Ms. Nichols warned that the Faircloth amendments ''would take clean-air policy back to the failed approaches of the 1970s, a retreat that would result in significant damage to public health and environmental protection.''
Despite these contradictions, the Clean Air Act may escape this year without major change because of public support for the environment.