Blackdamp

Blackdamp (also known as stythe or choke damp) is an asphyxiant, reducing the available oxygen content of air to a level incapable of sustaining human or animal life. It is not a single gas but a mixture of unbreathable gases left after oxygen is removed from the air and typically consists of nitrogen, carbon dioxide and water vapour. The term is etymologically and practically related to terms for other underground mine gases such as fire damp, white damp, stink damp, and afterdamp.

Etymology
The meaning of "damp" in this term, while most commonly understood to imply humidity, presents evidence of having been separated from that newer, irrelevant meaning at least by the first decade of the 18th century, where the original relevant meaning of "vapor" derives from a Proto-Germanic origin, dampaz, which gave rise to its immediate English predecessor, the Middle Low German damp (with no record of an Old English intermediary). The proto-Germanic dampaz gave rise to many other cognates, including the Old High German damph, the Old Norse dampi, and the modern German Dampf, the last of which still translates as "vapor".

Hazards
Blackdamp is considered a particularly pernicious type of damp (especially in a historical context), due to its omnipresence where exposed coal is found, and slow onset of symptoms. It produces no obvious odor (unlike the hydrogen sulfide of stinkdamp), is constantly being reintroduced to the air (instead of being released in pockets from actively mined sections), and does not require combustion in order to be released (unlike whitedamp or afterdamp). Many of the initial symptoms of oxygen deprivation (dizziness, light-headedness, drowsiness and poor coordination) are relatively innocuous and can easily be mistaken for simple fatigue, given the physically strenuous job of coal mining. The time between the onset of initial symptoms and the start of frank asphyxiation (and rapid unconsciousness) can be as short as seconds. Thus, if the warning signs are missed, a large number of miners can be rapidly incapacitated in the same short period of time, leaving no one to summon help.

In addition to the danger inside the mine, blackdamp can be "exhaled" in large quantities from mines (especially long-abandoned coal mines with few outlets for escaping gas) during sudden changes in atmospheric pressure, potentially causing asphyxiation on the surface.

Disasters
The gas mixture has been responsible for many deaths among underground workers, especially miners—for example, the Hartley Colliery disaster, when 204 men and boys were trapped when the beam of an engine suddenly broke and fell down the single shaft, damaging the ventilation system and blocking it with debris. Despite rescuers' efforts, they could not be reached before they suffocated in the blackdamp atmosphere.

Detection and countermeasures
Historically, the domestic canary was used as an early warning against carbon monoxide.

In active mining operations, the threat from blackdamp is addressed with proper mineshaft ventilation as well as various detection methods, typically using miner's safety lamps or hand-held electronic gas detectors. The safety lamp is merely a specially designed lantern with a flame that is designed to automatically extinguish itself at an oxygen concentration of approximately 18% (normal atmospheric concentration of oxygen is c. 21%). This low detection threshold gives miners an unmistakable warning and allows them to escape before any potentially incapacitating effects are felt.