The steelworkers have two primary sources of pollution, namely, the blast furnaces for making pig iron and the open-hearth furnaces for changing iron into steel. In both cases, the engineers have decided that the most effective abatement methods are (1) increasing the height of the smokestacks, (2) using filter devices (including gas traps) in the smokestacks, and (3) including cleaner, high-grade material among the fuels for the furnaces. Each of these methods has a technological limit on how heavily it can be used (e.g., a maximum flexibility for using the method at a fraction of its technological limit.