The Supreme Court declined Monday to hear a business group’s claim that the Environmental Protection Agency’s (EPA) 2008 rules for ground-level ozone are too strict.

The Monday decision leaves intact the 75 parts-per-billion ozone standard put in place by the Bush administration, lowered from the previous 80 parts per billion.

ADVERTISEMENT

Ground-level ozone is a product of reactions of various gases from fossil fuel burning. The EPA said it can cause respiratory problems and harm vegetation, and is often a component in smog.

Areas that exceed the standard must take action to try to lower their concentrations.

The Utility Air Regulatory Group, backed by mining and power generation companies, had appealed a lower court’s decision last year that also upheld the standard.

The Environmental Defense Fund (EDF) said the rejection is critical to protecting clean air.

“Millions of Americans are harmed by smog pollution and the court’s decision is a welcome end to efforts by large coal-based power companies to derail vital clean air protections for our communities and families,” EDF senior attorney Peter Zalzal said in a statement.

The EPA is in the process of deciding whether to further restrict the standard, a move that the National Association of Manufacturers said could be the most costly regulation ever.

The agency's staff and science advisers have asked it to reduce the standard to between 60 and 70 parts per billion. The agency will make a decision on whether to propose new restrictions by December.

Like the other decisions Monday to reject cases, the Supreme Court did not explain its denial.