in the united states and a number of other countries, atmospheric dispersion modeling studies are required to determine the flue-gas stack height needed to comply with the local air pollution regulations. the united states also requires the height of a flue-gas stack to comply with what is known as the "good engineering practice" (gep) stack height. in the case of existing flue gas stacks that exceed the gep stack height, any air pollution dispersion modeling studies for such stacks must use the gep stack height rather than the actual stack height.