A key element that is preventing the EPA from devising new standards to further clean the air and prevent acid rain is the varying ability of different areas of the country to absorb or tolerate airborne pollutants.
The Clean Air Act demands that EPA's regulations be national in scope and be "neither more nor less stringent than necessary."
So a standard that is necessary for a particularly vulnerable watershed could be legally challenged as burdensome by another area of the country "that is naturally resistant to the effects of acid rain," the report states.
The inability to find that just right middle ground has stymied EPA's efforts to promulgate new standards to be enforced.
The EPA declined to make an official available to discuss the GAO's conclusions. It issued a statement that several key pollutants "are at their lowest recorded levels," and that the EPA's new fuel economy rules for cars to take effect in 2025 "will provide significant reductions of NOx well into the future."