Energy experts say drilling can be cleaner

NOAA scientists also say natural gas production has contributed to unusual wintertime smog in the West, particularly in regions surrounded by mountains, and especially in snowy areas.

Ozone, the main component in smog, typically forms when sunlight "cooks" a low-lying stew of chemicals such as benzene and engine exhaust. Normally, the process doesn't happen in cold weather.

But NOAA researchers found that when there's heavy snowfall, the sun passes through the stew, then bounces off the snow and heats it again on the way back up. In some cases, smog in remote areas has spiked to levels higher than those in New York or Los Angeles.

In open regions that are more exposed to wind, the ozone vanishes, sometimes within hours or a day. But in Utah basins, it can linger for weeks, Schnell said.

The EPA has passed new rules on oil and gas emissions that are scheduled to go into effect in 2015, and in 2012 it reached legal settlements that will require companies to spend more than $14 million on pollution controls in Utah and Wyoming. Colorado, Texas and other states have passed more stringent rules, too.

Prasad Kasibhatla, a professor of environmental chemistry at Duke University, said that controlling gas drilling pollution is "technically solvable" but requires close attention by regulators.

"One has to demonstrate that it is solved, and monitored," he said.