The U.S. Supreme Court case Environmental Defense, et al. v. Duke Energy Corporation, et al., 2006 revolved around the interpretation of the Clean Air Act by Duke Energy and its implications on air pollution control measures. The Environmental Defense Fund (EDF) sued Duke Energy for modifying several coal-fired power plants without obtaining permits required under the New Source Review (NSR) provisions of the Clean Air Act which mandate that any modification leading to increased emissions should be accompanied by new permits and installation of modern pollution controls. However, Duke Energy argued that it was not necessary as their modifications did not increase hourly emission rates but only allowed plants to operate longer hours thereby increasing total annual emissions. The Supreme Court ruled in favor of EDF stating that an increase in annual emissions is a valid trigger for NSR requirements irrespective of changes in hourly emission rates.

In the dissenting opinion for Environmental Defense, et al. v. Duke Energy Corporation, et al., Justice Thomas argued that the majority's interpretation of "modification" in the Clean Air Act was incorrect and inconsistent with previous court rulings. He contended that a change in an existing source should only be considered a modification if it increases the total amount of pollutants emitted by that source annually. The majority’s decision to consider any physical or operational changes as modifications would lead to absurd results such as classifying routine maintenance activities as modifications requiring new permits under Prevention of Significant Deterioration (PSD) program regulations. Furthermore, he criticized their reliance on Chevron deference - allowing administrative agencies to interpret ambiguous statutes they administer - stating this case did not meet its criteria since Congress had directly spoken on how 'modifications' should be interpreted within PSD regulations.