In announcing the ozone policy change, the agency appeared to leave the door open to extending the deadline again. But, officials said, the agency will work with states to help them deliver the needed information.

“Under previous administrations, E.P.A. would often fail to meet designation deadlines, and then wait to be sued by activist groups and others, agreeing in a settlement to set schedules for designation,” Mr. Pruitt said in a statement. “We do not believe in regulation through litigation, and we take deadlines seriously. We also take the statute and the authority it gives us seriously.”

Ozone, a smog-causing gas that is formed when chemical emissions from power plants, factories and cars are exposed to heat and sunlight, has been linked to childhood asthma, lower infant birth weights and lung disease.