] Last week the Environmental Protection Agency issued a
] rule to limit mercury emissions from power plants. But
] rather than requiring across-the-board cuts in emissions
] it gave companies the ability to buy and sell pollution
] allowances. In the short run, some plants could buy
] allowances from cleaner companies and delay making large
] expenditures to cut emissions.

I guess a downside of pollution credit trading schemes is that you can end up with hotspots instead of a uniform reduction.