In discussing the reduction of air pollution in the developing world, Richard Fuller of the Blacksmith Institute, an environmental organization, observed, "It's the 90/10 rule. To do 90 percent of the work only costs 10 percent of the money. It's the last 10 percent of the cleanup that costs 90 percent of the money." Why should it be any more costly to clean up the last 10 percent of polluted air than to clean up the first 90 percent? What trade-offs would be involved in cleaning up the final 10 percent?