The vapors are an important source of chemicals that form urban smog.

It is a main ingredient of urban smog.

Even so, cars and trucks still account for nearly half of all urban smog.

The nation has fought a losing battle against urban smog since 1970.

"We hope that the Northeast will finally see improved air quality with reduced urban smog, acid rain and respiratory disease," he said.

One is ozone, the main component of the urban smog that afflicts millions.

Finally, a third example concerns the contribution of imported pollution to an urban smog.

He fought efforts to strengthen controls on urban smog and toxic emissions.

Pollution from fossil fuels also contributes to urban smog and acid rain.

Many of those differences have to do with tougher requirements in the House bill to reduce urban smog.