After-damp is the mixture of gases resulting from an explosion of fire-damp or coal dust, and its composition varies according to the nature of the gases and the amount of oxygen taking part in the explosion. The complete combustion of methane produces carbon dioxide and steam, whilst the nitrogen remains unchanged. In that case the after-damp would really be a black-damp whose properties would be those due to the presence of carbon dioxide and a shortage of oxygen.

If there is insufficient oxygen available, carbon monoxide and hydrogen are formed as well as carbon dioxide and water. The greater the proportion of methane or the smaller the proportions of oxygen taking part in the explosion, the greater the percentage of carbon monoxide which is present in after-damp. It may be mentioned that if air be readmitted to such a mixture a new explosive mixture could be formed, and such explosions have been known to occur.

The after-damp just mentioned will consist of carbon monoxide, carbon dioxide, nitrogen, and hydrogen. The extremely poisonous effects of carbon monoxide combined with the effects of carbon dioxide and a shortage of oxygen make such an after-damp very deadly indeed. In fact, over 75 % of the deaths caused by explosions in the coal mines of Great Britain have been directly due to the after-damp alone. Carbon monoxide is the chief agent causing death after mine explosions. No doubt traces of other gases are found in after—damp, since there are a number of substances which could be ignited and burnt during an explosion. Among these may be mentioned hydrogen sulphide, sulphur dioxide, nitrous oxide, nitric oxide, ammonia, and other nitrogen compounds and distillation products of coal and timber in general.