THE REAGAN administration is evidently having second thoughts about the revision of the Clean Air Act. Earlier, the Enivironmental Protection Agency seemed to be preparing a sweeping and dismaying relaxation of the present law. But now the EPA says that it has dropped any plans to produce its own bill. Instead, it says that it is offering only a brief and slightly ambiguous list of "basic principles" that the administration would like Congress to follow.

It's an interesting example of the Reagan administration's sense of political priorities and its flexibility. After various people in the administration had made fierce statements about the onerous costs of pollution control, and the need to get at least some of that burden off the back of industry, someone leaked a draft of a bill being written within the EPA that would do just that. In the ensuing row, the White House discovered that it had less support than it had expected -- and less than it needed among the Republicans on the crucial Senate Environment and Public Works Committee. After a bit of thought, the administration has decided to back off, avoid confrontation and leave the initiative with the congressional committees. That decision considerably improves the prospect for the next phase of the country's most extensive environmental legislation.

The EPA's "principles" are, as you would expect, a mixture of the useful and the questionable. Earlier it had considered a legal requirement to apply cost-benefit analysis to health standards. Now, wisely, the agency has decided against it. Cost-benefit analysis would plunge the government into the unacceptable exercise of trying to assign cash values to future deaths and illnesses. But, the EPA adds, standards ought to reflect "real health risks." What, exactly, does the word "real" mean? It is better simply to acknowledge that in an industrial society it is not possible to eliminate all risks to health through air pollution, and Congress is going to have to make a political decision on a reasonable degree of risk.

The country has learned a lot about air pollution since the act was passed in 1970.No doubt some of the rules can safely be relaxed, as the EPA suggests. But why relax the requirement on nitrogen oxide emissions by automobiles, from the level that present technology has already reached? And if some rules can been loosened, there are others -- particularly those involving the very find particles in coal smoke and diesel exhaust -- that clearly need to be tighter.

Clean air is neither cheap nor easy to achieve. But the support for it remains remarkably strong. The Clean Air Act is something of an anomaly -- an exception to the swing of public and political opinion against government regulation. The administration seems to be acknowledging that, and leaving the hard choices to Congress.