The major polluting states have argued that they would bear an inordinate share of the multibillion-dollar cost of controlling sulfur pollution to achieve uncertain benefits elsewhere.

Those arguing for natural gas also observe that it produces significantly less sulfur pollution in the atmosphere than gasoline or heating oil.

The sulfur pollution is so pervasive as to have an extraordinary side effect that is helping the rest of the world, but only temporarily: It actually slows global warming.

China contributes one-sixth of the world's sulfur pollution.

This is the same kind of trading system that has sharply - and without great cost - lowered emissions of sulfur pollution from power plants over the last decade.

That is, the scrubber turns sulfur pollution into industrial sulfates.

The utility industry and its supporters contend that mandated controls are unnecessary because sulfur pollution has already declined and that clean coal technology will assure future gains.

One benefit of natural gas is its cleaner burning, he said; it produces less sulfur pollution in the atmosphere than gasoline or heating oil.

Dry nitrogen and sulfur pollution also turn to acid when deposited in waterways or mixed with moisture in the soil.

Most legislation called for cutting sulfur pollution by at least half of the more than 20 million tons spewed into the atmosphere each year.