The exhaust gas vented from combustion , a chemical reaction, or other physical process, which passes through a duct into the atmosphere . Exhaust air is usually captured by an enclosure and brought into the exhaust duct through induced or forced ventilation. Induced ventilation is created by lowering the pressure in the duct using fans at the end of the duct. Forced ventilation occurs when exhaust air is forced into the duct using high pressure inlet air. Flues are valuable because they not only direct polluted air to a pollution control device, but also keep the air pollutant concentrations high. High concentrations can be important if the air pollutant removal process is concentration dependent.
See also Air pollution control