Among the many dangers of coal mining—cave-ins and collapses, fire, explosions, various health risks from overwork to pneumoconiosis ("black lung")—is the danger of the buildup of different gases in the mine. This can effect the worker by being poisonous or merely through the replacement of oxygen in the breathing air leading to suffocation.1
While death is at either end of the spectrum, it isn't guaranteed, being dependent on the percentage of whichever gas it is. That is why both adequate ventilation and methods to test the safety of the air are indispensable to coal mining. Without ways to detect and disperse the gases, coal mining would be far deadlier than it has been (over 15,183 killed in US mine disasters between 1832 and 1992—not counting indirect deaths through accidents or respiratory problems, et ceterea).2
Besides "air," which is made up of primarily/approximately 78% nitrogen and 21% oxygen (note: carbon dioxide constitutes about 0.03%), there are four main gases that concern workers in a coal mine. They are carbon dioxide (CO2), carbon monoxide (CO), methane (CH4), and hydrogen sulfide (H2S). The levels and combinations of these gases along with the level of oxygen or "air" determine the danger and what they are known as.
After Damp The combination of gases remaining following an explosion (whether due to "fire damp" or to coal dust—which in fine quantities can result in an explosion under the right conditions). It varies depending on the amount of oxygen that was present as part of the explosion. If methane in the explosion combusts completely, it will produce carbon dioxide and steam. This will leave a form of "black damp." If there is less oxygen, there will be additional amounts of carbon monoxide and hydrogen mixed with the aforementioned gases. This will result if there is either greater amounts of methane or smaller amounts of oxygen.
After damp is particularly dangerous, as carbon monoxide is poisonous, carbon dioxide can cause suffocation, and free oxygen will be necessarily depleted. A further danger is that if air is re-added to the mixture, it can become explosive again (not an unheard of occurrence). According to my source (www.therhondda.co.uk), over 75% of deaths in coal mine explosions in Great Britain were caused by "after-damp alone." The main reason being the carbon monoxide in it. There can be many other gases found in after damp, following the explosion, including hydrogen sulfide, sulfur dioxide, nitrous oxide, ammonia, and others.
Black Damp Also known as "choke damp" or "stythe," it is when levels of carbon dioxide are high and mixed with nitrogen (taking the place of free oxygen). These levels can be from 5% to 20%. My source gives around 13% as an average carbon dioxide level in black damp (15% and higher will put out a flame). While at the low end of the scale it is about the same density as air, as the percentage increases, so does the density, making black damp especially dangerous the lower the level in a mine one is working (workers would sometimes hold their lights near the floor to aid in detection).
Black damp is neither combustible nor poisonous, by itself, but enough in the air mixture can lead to physical effects or even death by suffocation.
Carbon dioxide Besides being a part of both after damp and black damp, as noted above, carbon dioxide levels increase due to human and (in some cases, particularly in the past) animal respiration. Other sources include burning of candles or torches (less common since electricity came to mining), explosions, chemical reactions with certain rocks/minerals, even the decay of timber. One of the key tasks of a mine ventilation system is to get rid of carbon dioxide (hardly the most deadly, but the one most apt to build up in the day to day operation of a mine).
While not combustible or poisonous, by itself, levels of 18% can kill, 25% quickly. If it is combined with a corresponding drop in the percentage of oxygen, those numbers can drop to 9% and 10%, respectively. Levels as low as 3% can make breathing more difficult, symptoms worsening as the percentage rises.
Carbon monoxide As noted, this colorless, odorless, poisonous gas (sometimes referred to as "white damp") is the most dangerous gas to be dealt with in a mine. Unlike carbon dioxide, which as the levels increase the density makes it sink, carbon monoxide is lighter than air and subsequently more deadly because of it. It can be caused by explosions either from fire damp or coal dust (thus being an integral part of after damp). In addition to being toxic, it is also very inflammable.
Carbon monoxide is so dangerous to humans because it is so readily absorbed by the blood—even more than just oxygen. Making it worse, the body is slow to "give it up," making treatment that much more difficult. Also, because the body continues to absorb the gas (death comes at 80% saturation), even low levels can build up in the body causing death. It is at about 0.02% that one begins feeling the effects—in this case, "slight giddiness, headache and breathlessness" (below that number, there are no real symptoms, though it's still indicative of a very real danger). If the level gets as high as 0.2%, death will take place in one to two hours. The amount of time before a fatality (from there on up), is dependent not only on the percentage of carbon monoxide, but the amount of exertion by the person.
Because, unlike carbon dioxide, flames won't extinguish or die down in the presence of carbon monoxide, one of the chief tests was done by bringing small animals, usually birds (the proverbial "canary in a coal mine") and mice. They would succumb much sooner than a full grown man or even a boy. It was not a perfect system, though, as a great deal of physical exertion could sometimes result in the human being affected before the animal.
Fire Damp Made up mainly by methane, an inflammable gas which when mixed with the right proportion of oxygen, can be explosive, fire damp is also quite dangerous in a coal mine. Methane, which makes up between 90% and 100% of the inflammable part of fire damp (generally around 70% to 80% of the "air," when the nonflammable gases are accounted for). Like carbon monoxide, it is lighter than air. It is a product of the mine, itself, being given off when it is released from the rock where it was trapped ages ago (methane is, like coal, a product of decomposition of organic matter). When the rock is removed or a seam is opened or cracked, methane can escape.
Besides the danger of the gas through fire/explosion and breathing it in (while not poisonous, it cannot support life, having no oxygen as part of the molecule—fire damp has very little oxygen, anyway), the gas trapped within the rock can have built up enormous pressure that when released, can cause injury and even death when the rock flies into the miner.
Depending on the amount of oxygen and the percentage of methane, the mixture can be merely inflammable (where the gas will continue to burn on its own after the ignition source is removed) or explosive. At inflammable levels, rather than exploding, the methane will burn around the ignition source in a sort of halo effect, known as a "cap."
In addition to methane, fire damp may also have small amounts of ethane and/or propane (anywhere from 0% to 2%)—both of which are more inflammable and ignite at a lower temperature). In an important way, the degree of other gases present affects the ignition temperature; higher or lower depending on the gas and the percentage present in the air. Another gas, sometimes found in trace amounts, is ethylene, another highly inflammable gas. Despite the low percentages of each, their presence makes fire damp even more volatile and liable to explode, then methane alone.
Hydrogen Sulfide Also known as "stink damp," the gas is caused by the decomposition of iron pyrites in a mine due to the dampness or presence of water. While considerably more deadly than carbon monoxide (and inflammable), amounts of this gas are usually only trace. Another thing in favor of the miners is that the gas has a foul, disagreeable odor (like rotten eggs) which serves as a usually adequate warning.
Like carbon dioxide, it is heavier than air and sinks. Animals could also be used to test for it by placing them near the floor of the mine (1% could cause death rather quickly and as little as 0.07%, over time, could also kill) but the smell and relative nonabundance of the gas make it less of a concern than the others.
While proper ventilation and methods to test for gases have come a long way since they actually used animals to test, the danger of gas buildup in a coal mine is a constant and important concern. Awareness of the potential problem and knowledge of the safety procedures are an important part of keeping men alive who spend so much of their lives digging underground for coal.
(Sources: primary source was www.therhondda.co.uk/working%20conditions/mine%20gases/after%20damp.htm the other pages can be accessed from there; fact-checking done with various sites, including E2; statistics for disasters are from the Mine Safety and Health Administration and were found at www.cdc.gov/niosh/mining/data/disall.html; information for footnote two was from the United Mine Workers of America site at www.umwa.org)
1Buildup of certain gases can also lead to the explosions mentioned; see "fire damp." Also, fire uses up the oxygen, making the percentage of various gases in the air change, causing either suffocation or poisoning (or a combination). The fire at the Avondale, Pennsylvania mine in 1869, burned off all the oxygen, killing over a hundred men and boys.
2Just over two months prior to writing this, on 23 September 2001, two explosions at a mine in Brookwood, Alabama, took the lives of thirteen workers—twelve who had rushed in to help others following the initial explosion. It was the largest US coal mine disaster since 1984.