The Supreme Court appears divided in a dispute over the Obama administration's first-ever regulations aimed at reducing power plant emissions.
The focus of the case is on mercury and other hazardous air pollutants that contribute to respiratory illnesses, birth defects and developmental problems in children.
The justices heard argument Wednesday in a challenge brought by industry groups and Republican-led states to the Environmental Protection Agency's decision to take action against coal- and oil-fired power plants that are responsible for half the nation's output of mercury.
Several conservative justices questioned whether EPA should have taken costs into account when it first decided to regulate hazardous air pollutants from power plants, or whether health risks are the only consideration. The EPA did factor in costs at a later stage when it wrote standards to reduce the toxic emissions.