ENVIRONMENTAL legislation has proved to be the great exception over the past decade to Americans' exasperation with their government. Elsewhere they have demanded less regulation and fewer intrusions. But the defense of the environmental laws remained consistently solid through the Age of Reagan, and now Congress is collaborating with President Bush to tighten and extend the Clean Air Act. The present clean air rules are expensive, costing Americans some $30 billion a year, and the bill now moving through Congress will add (as far as anyone can calculate it) more than $20 billion to that. But despite the vociferous resistance to taxes for other purposes, public support for the new clean air requirements remains strikingly broad and durable.

The House version of the bill, passed last week, is similar to the Senate's earlier one. The conference will doubtless be a lively affair, but the general outline of the legislation is now established. It will launch two important innovations.

To reduce acid rain, it will create a market in pollution rights among the big power plants that are the chief source of it. That will give even the cleanest of plants an economic incentive to get cleaner and, by selling the pollution rights that they don't need, permit industrial development within a national pollution quota that remains unchanged.

As for automobiles, the manufacturers for 20 years have borne most of the responsibility for the technology to clean up exhaust emissions and reduce smog. More of that responsibility will now shift to the companies that sell fuel. There are cleaner fuels than the gasoline most cars now use. Ideally, the legislation would only press for a higher standard and remain neutral among the various alternatives. In fact, it appears to be tilted toward ethanol -- a tribute to the influence of the corn growers and the companies that use corn to make ethanol. The advantages of adding ethanol to gasoline are questionable and limited to winter weather. It would be dangerous to let the farm lobby run away with this one.

The emerging legislation is generally conservative, in the sense that it addresses concerns that have been before Congress for many years and, with these few exceptions, addresses them in familiar ways. While the bill pushes for cleaner automobile fuel, it doesn't ask whether the country is not consuming too much fuel for its own health. The bill will certainly mean cleaner smoke from coal-burning power plants, but it doesn't ask whether burning coal in the present volumes won't inevitably damage the atmosphere.

This year's Clean Air Act addresses an economy that runs almost entirely on fossil fuel, sending gigantic amounts of carbon dioxide into the air and threatening to change the climate. Neither Mr. Bush nor Congress have had anything to say about that so far except, "Not yet." But that's the next question, and the United States is now engaged in international negotiations that will require it to make up its mind on carbon dioxide and global warming by the end of the year.