[3] The Act prohibits construction of major air polluting facilities that are not equipped with "the best available control technology" (BACT).

The EPA opposed the permit and objected that the ADEC had identified SCR as the best available control technology but failed to require it as BACT.

The ADEC issued a second report reinforcing the original findings but conceded the lack of cost data from Teck Cominco made it impossible to evaluate the impact of SCR on the mine's profitability.

Under the Clean Air Act, can the EPA block construction of a new major pollutant emitting facility permitted by a state environmental regulator when the EPA finds the state environmental regulator's determination to be unreasonable?

The Supreme Court held in its majority opinion that the Clean Air Act authorized the EPA to bar the construction of the polluting facility in Alaska.