text: It does sound odd, but here is an example:\n\n"The 1990 Clean Air Act allows the EPA to grant "emissions permits" for certain pollutants. These are, in effect, rights to pollute that can be traded among polluters. Imagine a giant bubble that encloses all existing sources of air pollution. Within that bubble some emitters may pollute over the control level as long as other polluters compensate by polluting less. The government or some other authority decides on the desired level of pollution and the initial distribution of pollution rights within an industry or for a geographic region—the "bubble" that encloses these sources. Purchases and sales of permits within the "bubble" should reduce the total level of pollution to the allowable limit at the lowest total cost. \n\nFor example, a St. Louis study found that the cost of reducing particulate emissions for a paper-products factory was $4 per ton, while the cost to a brewery was $600 per ton. The Clean Air Act could require St. Louis to reduce its emissions by a certain amount. Under the traditional approach, the brewery and the paper factory would each be required to cut emissions by, say, ten tons. The cost to the paper factory would be only $40, while the cost to the brewery would be $6,000. But with tradable permits, the brewery could pay the paper factory to cut emissions by twenty tons so that the brewery could continue to operate without reducing emissions at all. The net result is the same emission reduction of twenty tons as under the command-and-control approach, but the total cost to society of the reduction is only $80 instead of $6,040."