chokedamp

Chokedamp is an accumulation of carbon dioxide, CO2, in confined spaces underground, particularly in mines. Unlike firedamp (methane), it does not burn or explode; instead, it asphyxiates by displacing breathable air. A miner entering a space thick with chokedamp loses consciousness within seconds and will die if not removed immediately. The gas is colorless, odorless, and denser than air, so it pools in low spots, sumps, and poorly ventilated workings.

The gas forms naturally in several ways. Decomposing vegetation and coal seams release CO2 over time. In deep workings, it seeps from surrounding rock. Incomplete combustion in coal dust explosions or pit fires can also generate large quantities. Because chokedamp builds up silently and gives no warning signs like smell or taste, miners historically relied on canaries or mice to detect it: the bird or animal would collapse before CO2 reached lethal levels for humans, giving workers seconds to evacuate.

Recognition and response

Spotting chokedamp is difficult since it is invisible. Practical indicators include a lighted candle or lamp dimming or guttering as oxygen depletes and CO2 rises. Modern mines use portable gas monitors. The danger zone typically begins above 2 to 3 percent CO2 by volume; unconsciousness can occur above 8 percent. Mines combat chokedamp through constant ventilation, either natural draft through shafts or mechanical fans, and by avoiding work in dead-end passages without fresh air circulation.

The term chokedamp entered mining vocabulary because the gas literally chokes: it replaces oxygen in the lungs without triggering the body's panic response to suffocation, which is triggered by CO2 buildup in the blood, not its presence in the lungs. A miner breathing chokedamp feels calm and drowsy moments before collapse. In contrast, damp is old mining slang for any noxious gas, alongside blackdamp (carbon dioxide and nitrogen) and stythe (carbon monoxide). Chokedamp remains the standard term in English mining nomenclature.