Environment: Pollution Cannot Move

In addition to dealing with auto emissions, the clean air legislation of 1970 specified that each state devise its own scheme for controlling stationary pollution sources. By last spring, however, environmentalists had spotted a gaping loophole. Polluters could simply move their plants to states with cleaner air—provided their emissions stayed under federal limits for air pollutants.

But last week the Supreme Court closed the loophole. The Justices held that no "significant deterioration" of air quality will be allowed in any part of any state.

Although the interpretation of "significant" remains to be thrashed out, the decision was a far-reaching achievement for the Sierra...