Mine Atmosphere

(mine air), atmospheric air pumped or blown into underground mining excavations; as it passes through the excavations, the air undergoes changes in composition, temperature, and humidity. In comparison with surface atmospheric air, mine air often contains less O₂ and more N₂ and CO₂ because of oxidizing processes and the release of CO₂ from rocks. In addition, admixtures of poisonous and explosive gases, vapors, and dust may be present. In normal mine atmospheres, the content of O₂ is not less than 20 percent and the content of weakly toxic CO₂ is not more than 0.5–1 percent.

The poisonous gaseous vaporous impurities in mine atmospheres include carbon monoxide, nitrogen oxides, hydrogen sulfide, sulfur dioxide, aldehydes, and the gaseous decay products of radioactive substances. Carbon monoxide is formed in blasting, fires, and methane and dust explosions and is present in the exhaust gases of diesel equipment; the maximum permissible concentration in mine atmospheres is 20 mg/m³ (0.0016 percent by volume). Nitrogen oxides are formed in blasting and are present in the exhaust gases of diesel equipment; the maximum permissible concentration is 5 mg/m³ (0.0001 percent) relative to N₂O₅. Hydrogen sulfide is formed in the decay of wood and the decomposition of pyrites and gypsum by water and is released from deposits of rock salt and pyrite ores; the maximum permissible concentration is 10 mg/m³ (0.00066 percent). Sulfur dioxide is formed when rocks containing sulfur are blasted, when sulfur and sulfide dusts explode, and when fires break out in sulfur and chalcopyrite mines; the maximum permissible concentration is 10 mg/m³ (0.00035 percent). Aldehydes are present in the exhaust gases of diesel equipment; the maximum permissible concentration is 0.7 mg/m³ (0.00008 percent) for acrolein and 0.5 mg/m³ (0.00040 percent) for formaldehyde. Gaseous decay products of radioactive substances—radon and, to a minor extent, thoron and actinon—are released in uranium mines; the maximum permissible concentration of radon is 1.10^-11 curie per liter. In addition to these admixtures, mercury and gasoline vapors, heavy hydrocarbons, ammonia, and oil gas are also encountered.

Explosive gaseous and vaporous admixtures may be present in mine atmospheres. Methane, which is released mainly in coal shafts, has a maximum permissible explosion limit of not more than 0.5–2 percent. Hydrogen, which is found mainly in potassium mines, has a maximum permissible content of not more than 0.5 percent. As stated above, gasoline vapors, carbon monoxide, and hydrogen sulfide are both explosive and poisonous, and thus their maximum permissible content is not greater than that permitted by health regulations.

The principal way of maintaining the purity of mine atmospheres is ventilation. During the cold period of the year, atmospheric air introduced into shafts is heated. To reduce the temperature of mine atmospheres, artificially cooled air is used.