WHILE POLITICS at this moment is totally preoccupied with spending and taxes, a little way down the road lies the Clean Air Act. The present law expires in September, requiring it to be rewritten and reauthorized this summer. There is no legislation in which the Reagan administration will have to balance more explicitly the exigencies of public health against economic development, and opportunities for industrial investment against the traditional ideas of comfort. The next administrator of the Environmental Protection Agency, not yet chosen by the president, will be a central figure in this process of revision.

Clean air policy is based on a peculiarly uncertain kind of science in which no issue ever seems to be settled definity. But on the record of a decade's experience with the present law, it's possible to say that there's a strong case for fundamental review. Some pollutants are less of a threat than they seemed in the early 1970s -- and some are more so.

Mounting evidence indicates that, of the pollutants controlled under this act, the most dangerous are certain compounds of sulfur and small particles of ash. That amounts to saying that the most dangerous sources of air pollution are coal burning power plants and steel mills. But before Congress gets to the straghtforward political decisions, it is going to have to deal with a series of untricate technical developments.

To take one major example, present law restricts the volume of particulate matter that a smokestack can emit. But it turns out that the relatively large particles are no danger at all, because they can't be inhaled. It's the tiny ones that are sucked down into the lungs, where they can make dire respiratory trouble. Present technology keeps smokestacks within legal limits by eliminating the big particles but not the smallest ones. That standard needs to be revised -- and that means revising the expensive technology developed to enforce it.

At the other end of the scale, there's not much evidence that either ozone or carbon monoxide has serious effects on health in concentration anywhere near the present standards. Until now, the law has been equally categorical in its treatment of all of these pollutants, those that generate high deaths rates and those that merely create an unattractive haze or, at worst, temporarily smarting eyes.

Nitrogen oxides do not seem to affect human health directly. But they contribute to acid rain that falls hundreds of miles from the source, killing fish and vegetation. Congress may be asked to allow some latitude for local discretion in setting standards. But the legislation will have to distinguish between the pollution that says at home and the pollution that blows into another region -- or even another country.

While there are cases in which some of the rules can now safely be relaxed, expereience over the past decade has borne out the wisdom of the principle of controlling air pollution where it is cost-effective. It's difficult to think of more cost-effective regulations than those protecting health by enforcing air quality.