Blackdamp

Blackdamp (also known as stythe or choke damp) is a mixture of unbreathable gases formed when oxygen is removed from an enclosed atmosphere and largely replaced by nitrogen, argon, carbon dioxide and water vapour. The gas displaces oxygen in the air, lowering the available oxygen content to a level which is incapable of sustaining human or animal life; it is thus an asphyxiant.

The name black damp is believed to derive from the German word for vapors ("dampf"). The word damp is used in similar mining terms such as white damp (carbon monoxide), fire damp (typically methane) and stink damp (hydrogen sulfide).

Sources

Blackdamp is encountered in enclosed environments such as mines, sewers, wells, tunnels and ships holds. It occurs with particular frequency in abandoned or poorly ventilated coal mines. Coal, once exposed to the air of a mine, naturally begins absorbing oxygen and exuding carbon dioxide and water vapor. The amount of blackdamp exuded by a mine varies based on a number of factors, including the time of year (coal releases more carbon dioxide in the summer months), the amount of exposed coal, and the type of coal, although all mines with exposed coal produce gas.

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 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, a far from unlikely diagnosis in 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.

Detection and Countermeasures

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 ~21%). This low detection threshold gives miners an unmistakable warning and allows them to escape before any potentially incapacitating effects are felt.

From http://en.wikipedia.org/