The EPA said it determined that 45 areas across the United States didn't meet 2008 standards for smog. The EPA set its smog standard at 75 parts per billion in 2008.
"Almost all of these areas already have programs in place to improve air quality because they did not meet the 1997 smog standards," the EPA said in a statement. Only three new areas were identified in its latest review.
The EPA said smog can aggravate existing respiratory conditions and lead to premature death for people with heart and lung disease.
The EPA in a ruling last year required states, mostly in the eastern United States, to control air pollution that crosses their borders by wind and weather. The smog-cutting initiative would cost the energy sector billions of dollars but EPA Administrator Lisa Jackson had said it was worth the health benefits.
Some U.S. lawmakers said the measure was an assault to domestic energy production and would undermine economic growth.
The EPA said fewer areas in the United States weren't meeting the air quality standards, which it said reflected a steady improvement in air quality.