Under the rule, some utilities will be able to buy allowances rather than cleaning up emissions.

They will then have to buy and surrender allowances each year equal to their annual emissions or face financial penalties.

Plants that burn coal, which emits far more carbon dioxide, may have to buy additional allowances.

This means you can buy fewer allowances and save money.

In the short run, some plants could buy allowances from cleaner companies and delay making large expenditures to cut emissions.

The number of single companies that would have to buy allowances depends on how the initial allocation is made.

And even environmentalists got into the act by buying allowances and withholding them.

They will also have to buy allowances at a fixed price of £12 per tonne to cover their emissions.

In fact, experts say that most utilities that buy allowances will meet only a portion of their needs that way.

So Congress set up a trading system in which power companies can buy and sell "allowances" to emit sulfur dioxide.