The US Court of Appeals for the District of Columbia rejected the new federal rules for smog reduction on Friday, stating that the Environmental Protection Agency "has failed to heed the restrictions on its discretion set forth in the Act."

The smog standards were introduced in 2004 and required roughly 470 counties designated as "non-attainment" areas to reduce the level of smog within a three to seventeen year period. The court said the time period did not align with the federal Clean Air Act and held that EPA enforcement was not strict enough in states where smog levels have increased.

According to EPA spokeswoman Jennifer Wood, the EPA has not yet determined if it will seek an en banc rehearing of the case, stating that the "EPA is committed to ensuring our nation's ozone air quality standards are implemented to protect public health and the environment."