Afterdamp

Afterdamp is the toxic mixture of gases left in a mine following an explosion caused by firedamp, which itself can initiate a much larger explosion of coal dust. It consists of carbon dioxide, carbon monoxide and nitrogen. Hydrogen sulfide, another highly toxic gas, may also be present. Afterdamp was the deadly gas which caused the majority of casualties in the many pit disasters of the British coalfields, and elsewhere in the world. Such disasters continue to afflict working mines, especially in mainland China.

Detection

Gas detectors are available now which detect toxic gases at very low levels. The levels of gas detection depend on the methods used.

From http://en.wikipedia.org/