CONGRESS IS considering an interesting "first" in the 20-year history of environmental cleanup. Last year, the emissions standards for all three of the chief automobile air pollutants--hydrocarbons, carbon monoxide and nitrogen oxides (NOx)--were finally met. One year later, the question now being debated is an auto industry proposal that would undo this considerable achievement by rolling back the carbon monoxide and NOx standards to twice their current levels.

Congress' original 1970 goals for cleaning up auto air pollution eventually proved to be--as Detroit had predicted--overly ambitious. There were numerous delays and relaxations. But though light- and heavy-duty trucks are still unregulated, the auto emissions standards have already resulted in measurable--and in some cases major--improvements in the nation's air quality, especially in the big cities. Though performance on the road falls below the official standard, new cars must emit 90 percent less carbon monoxide and hydrocarbons and 75 percent less NOx than did pre-regulation cars. Contrary to expectations, this remarkable improvement has been made without cost in fuel efficiency. In fact, the sophisticated controls that were required actually increased mileage.

Why turn back the clock? Cleaner exhaust has not come free. Current industry estimates peg the cost of emissions control hardware at $250-$425 per car. If the standards are relaxed, some of the hardware can be removed. But the sticker price reduction will not be nearly as big as the cost, since the capital investments that have already been made must still be amortized. The saving for the consumer is likely to be in the neighborhood of $100. Another major rationale for the rollback is that the higher NOx standard is just the level current diesel engines can achieve without additional controls. General Motors, in particular, is counting on diesels to meet fuel economy standards. In fact, a number of studies--most recently one made by the Congressional Research Service--have concluded that only GM and Japanese and European manufacturers are likely to benefit at all from the rollback, while the ailing American companies would be further weakened.

Any proposal to relax health-related environmental standards should have to demonstrate pretty convincingly that the standard is unnecessary, technically wrong or impossible to meet. The criteria should be correspondingly higher in a case--such as this one-- where a standard has been met and been shown to be effective, and where a sizable capital investment has already been made. The proposal to roll back auto emissions standards does not fit the bill.