Then it allotted a certain number of pollution allowances to each emitter and let the operators of individual plants figure out how they wanted to proceed.

Those governments or private groups that found it too difficult or too costly to meet their limits could buy pollution allowances abroad.

At stake were more than 275,000 pollution allowances, each permitting a utility to emit a ton of sulfur dioxide, the chemical that causes acid rain.

The rule, which will take until the year 2010 to reach its goal, also allows utilities to buy and sell pollution allowances.

The market will allow firms to buy up current and future pollution allowances.

A lot of proposals want to allow trading of pollution allowances if you do anything beyond what's reasonably available.

After false starts beginning in the 1970's, trading in pollution allowances was validated with the Clean Air Act of 1990.

It lets utilities buy and sell credits that give them a pollution allowance.

Indeed, the futures market at the Chicago Board of Trade plans to trade the pollution allowances.

But rather than requiring across-the-board cuts in emissions it gave companies the ability to buy and sell pollution allowances.