THOUGH IT is a fiendishly complex law -- a Washington lawyer's dream -- in many respects, the Clean Air Act has been one of the country's most successful statutes. Since its passage a decade ago, the accelerating deterioration of the nation's air has been halted and then reversed. Sulfur dioxide levels have fallen dramatically. Levels of tiny, health-threatening suspended particulates have been lowered, and carbon monixide levels in polluted central cities have been brought down by one-third.

But now it appears that the law cannot cope with one of the most serious developing pollution problems and, paradoxically, that it may even be making the problem worse. That new -- or more accurately, newly rocognized -- problem is acid rain, caused by emissions of sulfur and nitric oxides largely from coal-burning power plants and automobiles. Rain in the northeastern United States is almost 100 times more acid than normal rainfall, and is 50 times more acid than it was 25 years ago. Scientists estimate that more than 50,000 lakes in the United States and Canada could become "dead" -- devoid of fish and plant life -- in 15 years. Crops, buildings, artworks and human health are all threatened, though to what extent is uncertain.

Acid rain's peculiar attribute is that it is felt miles away from the source of the pollution. Researchers in Sweden were the first to show that acid rain could travel hundreds of miles: Sweden's problem originates in West Germany, as most of Canada's does in the United States. More recent work in the Arctic suggests that acid rain may be transported thousands of miles by winds in the upper atmosphere.

The Clean Air Act, on the other hand, focuses on controlling pollution in the immediate vicinity of power plants. Since the installation of new pollution control technologies on old plants is quite expensive, utilities have turned to building taller and taller stacks -- as high as 800 feet -- that only makes the problem worse.

Moreover, the act assigns to the individual states responsibility for putting clean air standards into effect. Only if a state proves unable to do the job does the fedeal government step in. State plans consider only the pollution generated within state boundaries -- there is no provision for pollution generated hundreds of miles away. Anyway, which state is responsible for acid rain -- the one that generates it or the state on which it falls? And who can tell what comes from where?

Environmental Protection Agency head Douglas Costle has suggested that the only mechanism to control acid rain will be a multi-state regional organization, and he is probably right. But this poses awesome legislative difficulties at a time when economic woes make Congress more amenable to undoing than to strengthening environmental laws. But the problem cannot simply be postponed. Not only does it hold the potential for drastic -- possibly irreversible -- environmental damage; it also portends ugly conflicts between states and regions, with international conflict not far behind.