The result is that flue gases of wood burning stoves contain some substances which can be categorised as pollutants. These include gases, liquids and even solids. Some of the gases are responsible for the characteristic smell of burning wood, which is not always welcome, especially in congested cities. Liquid pollutants comprise tars and creosote which may condense on the internal walls of the chimney where they will gradually build up and constitute a fire hazard. Finally, solid particles may be carried some distance by the hot air currents in the flue gas but will eventually come to Earth as dust and soot.