Mechanical fans or pumps are exposed to the pollutant gases, so they will need maintenance (cleaning, replacement). This and the energy needed to run the fan/pump cost money.

Narrowing the smokestack's opening forces the smoke out as a faster streaming, narrower jet. This has the advantage of not requiring any moving parts, and it can be retrofitted to existing smokestacks very easily. Backpressure from the smaller opening may reduce the efficiency of the flow of smoke out of the chimney, however. This may require modifications to the activity that produces the pollution, and possibly more pollution production that will partially offset the effect of increasing the plume momentum.