The Clean Air Act (CAA) depends heavily on regulations adopted by the Environmental Protection Agency (EPA) to implement its broad and often ambiguous terms. However, proposed regulations extending EPA’s authority to new pollutants or source types, or increasing the stringency of existing requirements, are almost always controversial and often immediately challenged in federal court—sometimes by targets of the regulation, sometimes by parties who believe the regulations did not go far enough, and often by both.

Originally published in Natural Resources & Environment Volume 36, Number 1 - Summer 2021.