The war against smog is one of those slippery affairs that confounds Americans. We want our government programs to be noble crusades that end with resounding triumphs. Fighting smog doesn't fit the mold. It's a war of attrition. We are making grudging gains, but total victory is nowhere in sight. Don't expect the new Clean Air Act -- now being fashioned with much fanfare by Congress -- to change matters much. Still, it's real progress.

Since the Clean Air Act of 1970, there have been some sweeping environmental successes. Lead has virtually disappeared from the air. Particulates (smoke, soot) are down by more than half. Smog is another story, precisely because it's a consequence of our massive dependence on fossil fuels and petrochemicals. When hydrocarbon and nitrogen oxide emissions react with sunlight and heat, they produce ozone (molecules composed of three atoms of oxygen). This is essentially the haze we call smog.

Unless you're a pollution junkie, it's easy to be misled about how well or poorly we're doing against smog. You'll hear that about 100 metropolitan areas, with more than 110 million people, still violate the government's ozone standard. But you'll also hear that controls have sharply cut tailpipe emissions from new cars since 1970. Hydrocarbon emissions are down 96 percent and nitrogen oxides are down 76 percent.

Which statement is true? Well, both.

Growth of cars, people and production has tended to neutralize cleaner vehicles and factories. There are more than 180 million cars and trucks today, up from 108 million in 1970. The nation's gross national product is about 70 percent higher. But the gains against pollution are not phony. "The real test is what things would have looked like if we hadn't made changes in 1970," says economist Paul Portney of Resources for the Future, a Washington think tank. "The answer is Mexico City."

Indeed, we've achieved something better than a standoff against smog. Flunking the government's ozone standard means being out of compliance for a few days a year. In some cities that don't comply, smog levels have receded. Consider Los Angeles, the country's smog capital. It misses the air quality standards about 160 to 175 days a year (about three times more than Houston, 13 times more than New York and 25 times more than Chicago). But this represents an improvement since the late 1970s, when the standard was violated nearly 200 days annually.

More impressive, the average day in Los Angeles is getting better. The California Air Resources Board has evaluated smog's total severity and duration, including days when the standard is -- and isn't -- met. On this basis, the improvement since the late 1970s is about 50 percent.

Contrary to popular wisdom, tailpipe emissions from cars and trucks aren't the main source of the pollutants that cause smog. Cars and trucks generate about a third of all hydrocarbons, down from 40 percent in 1970, although their share tends to be higher in places where smog is worst. But here's the catch. Only about a third of these emissions now come from the tailpipe. The rest come from gasoline that evaporates while cars are traveling, fueling or simply standing.

The upshot is that the new Clean Air Act will shift the government's tactics against smog. For two decades, the emphasis has been on strict tailpipe standards for cars and new controls for industry. Tailpipe standards will be tightened once again, but the legislation's real focus is on the fuel itself: how gasoline is made, stored and sold.

Oil companies will be required to produce ordinary gasolines that don't evaporate so easily. (In technical jargon, the gasoline will be less "volatile.") This change should produce the bill's biggest reductions in hydrocarbons, far greater than the tightening of tailpipe standards. In areas with high smog levels, gas stations will probably be required to install special nozzles on pumps to recapture fumes while cars are being fueled.

The most controversial measure is "reformulated gasoline." Simply put, that's gasoline whose chemical constituents have been altered so that it pollutes less. A few brands are already being sold that achieve about a 10 percent reduction in emissions. The Senate version of the Clean Air Act specifies a 15 percent cut by 1992 for gasoline sold in the nine smoggiest cities. The House version requires a 15 percent cut by 1995 and raises that to 25 percent by 2000. No one knows precisely what's doable at a reasonble cost.

"Reformulated gas" pits the oil lobby against the farm lobby. Each is correct about the other. As the farm lobby says, the oil companies have come late to less-polluting fuels. But the oil industry accurately accuses the farm lobby of wanting the law written to force the use of ethanol -- which can be made from corn and other crops -- in gasoline. The House-Senate conference committee that will set the law's final language should reject that approach. The Clean Air Act shouldn't be turned into farm relief. The law should set standards and let the oil companies meet them as inexpensively as possible.

Perhaps half the cities violating the ozone standard may ultimately be brought into compliance by changes required by the new law, says Robert Friedman of the Congressional Office of Technology Assessment. The other half still won't make it. They've got too far to go, and gains will be offset by more cars, people and production. As cars get cleaner, the remainder of the smog problem also gets tougher to solve. A quarter of hydrocarbon emissions, for example, come from common chemicals used by businesses and households: paints, solvents, inks and cleansers. OTA doubts that the technology now exists to eliminate all smog.

The "dirty" environment is one price we pay for the benefits of a prosperous economy. Although we can use some of our wealth to limit pollution, obliterating it entirely is often impractical or prohibitively costly. It's an ambiguous exercise, but we should not deny our gains. Our war against smog is not a failure even though it isn't yet a complete success.