Whitedamp is a noxious mixture of gases formed by the combustion of coal, usually in an enclosed environment such as a coal mine. The most toxic constituents are carbon monoxide and hydrogen sulfide. Coal frequently starts to burn slowly in mines when it is exposed to the atmosphere.
Detection
Traditionally, whitedamp was detected by its effect on canaries, who succumb much more quickly than humans. However, there are gas detectors available now which detect toxic gases at very low levels. The levels of gas detection depend on the methods used.
References
Whitedamp Wikipedia(Text) CC BY-SA