The court had issued a stay on the rule after states, power plant operators and industry groups said complying with the rule, which had been scheduled to enter into force on Jan. 1, 2012, could interrupt electricity service.
Republicans opposing the rules say they will increase power costs by forcing power plant operators to buy expensive equipment. Many also worry the rules will take too many power generators offline, creating possible electricity shortages.
McCarthy admitted she could be wrong about the ruling date considering she has expected a decision for the past couple weeks.
But the result will be the same no matter what date the decision is announced, she said — power plant operators in 28 states will need to better control sulfur dioxide and nitrogen oxide emissions.
One thing that is uncertain is when implementation would start and when enforcement would begin, McCarthy said.