The Supreme Court ruled 5-4 against Environmental Protection Agency pollution rules for power plants Monday, in a blow to President Obama’s environmental agenda.

The EPA rules in question regulate hazardous air pollutants and mercury from coal- and oil-fired power plants, known as the MATS regulations. The regulations went into effect April 16. The utility industry argues that the rules cost them billions of dollars to comply and that EPA ignored the cost issue in putting the regulations into effect.

Many of the companies have either made the investments or closed power plants to comply. If the investments necessary to upgrade a plant to comply with the regulation aren’t justified when considering the operational costs, revenues earned and other factors, then the decision is made to retire it.