By Ayesha Rascoe

(Reuters) – A U.S. Supreme Court case examining whether the Obama administration should have considered costs before issuing mercury pollution rules for power plants may come down to the meaning of the word “appropriate,” said Tom Donnelly, counsel for the Constitutional Accountability Center.

The Clean Air Act allows the Environmental Protection Agency to regulate power plants for mercury and other toxic pollutants only if it finds it is “appropriate and necessary.”