Wood Burning Fumes
Wood was the original fuel used by mankind, both for cooking and heating, so it is no surprise that everyone is familiar with woodsmoke, and its characteristic smell and appearance.
Smoke arises due to the fact that wood (and other solid biofuels) is difficult to burn completely, especially under the conditions present in relatively simple appliances. Part of the problem is that the fuel is not a standard commodity - the species of tree, size of the logs and the moisture content all effect how it burns. One method of reducing the inherent variability is to form the wood into pellets or briquettes before burning, which enables the burning conditions to be better optimised, nevertheless even in the most advanced appliances, the combustion of solid biofuels is never perfect.
The result is that flue gases of solid biofuel stoves and boilers contain some substances which can be categorised as pollutants. These include gases, liquids and even solids, as fine particulates suspended in the gases. 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 (phenols) 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 buoyancy of the hot gas but eventually fall out as dust. This problem may be worse where an extractor fan is used to enhance combustion.