= Solution
\b[“Carbonic acid” in this question means gaseous <carbon dioxide>, not liquid acid poisoning. Dangerous accumulation causes <hypercapnia>, impaired consciousness and potentially death; prevention requires controlling its production and providing effective fresh-air replacement.] At high concentrations, displacement of <oxygen> can contribute, but lack of <oxygen> is not the only mechanism.
Early symptoms can include headache, dizziness, discomfort, restlessness, sweating, rapid or laboured <breathing> and an increased pulse. Greater exposure can cause confusion, loss of coordination, profound drowsiness, collapse, <convulsions>, <coma> and death. In a heavily contaminated space, collapse may occur before a person can recognize the danger or escape. <Carbon dioxide> is colourless and odourless, so absence of an offensive smell provides no reassurance.<Carbon Dioxide by the National Institute for Occupational Safety and Health>[Pocket Guide, physical description, exposure route, symptoms and target organs]{ref}
The physiological point matters. Increased inspired <carbon dioxide> impairs elimination of the body's own <carbon dioxide> and alters the acid–base state of the <blood>. A hazardous atmosphere can therefore remain dangerous even when enough <oxygen> is present to sustain a flame. The modern institutional discussion of <carbon dioxide> fire suppression distinguishes the gas's direct physiological effects from mere dilution of <oxygen>.<Carbon Dioxide as a Fire Suppressant: Examining the Risks by the United States Environmental Protection Agency>[“Life Safety Considerations of Carbon Dioxide”: physiological effects and oxygen concentration]{ref}
The characteristic settings are poorly ventilated spaces in which <carbon dioxide> is continuously generated or accumulates: brewery or wine-fermentation vessels, cellars, pits, wells, vaults and places containing decomposing organic material. Fermentation and <cellular respiration> generate the gas; combustion is another source. In a badly ventilated occupied room, human <breathing> and unflued fires or lamps add to the burden. A dangerous combustion atmosphere may also contain <carbon monoxide>, which is a separate poison and requires a separate assessment. <Carbon dioxide> is denser than <air>, so low or poorly swept spaces deserve attention; currents and mixing mean that one cannot assume a sharp, motionless layer with safe air above it.<Carbon Dioxide by the National Institute for Occupational Safety and Health>[Gas density and properties]{ref}
For prevention, remove or enclose the source where practicable and maintain <building ventilation> that actually sweeps the occupied and low-lying spaces. Exhaust contaminated <air> to a safe place and bring replacement <air> from a clean source. Opening a small window elsewhere in a building does not establish an adequate supply at the worker's position. Fermentation vessels, pits and similar spaces should be arranged for inspection and cleaning without entry where possible.
The nineteenth-century emphasis on fresh air and caution before entering such spaces remains sound. Current practice adds atmosphere testing for <carbon dioxide>, <oxygen> deficiency and other relevant gases, together with controlled entry and rescue arrangements. A burning candle is an inadequate safety test: flame behaviour neither measures <carbon dioxide> nor rules out a different toxic gas. A rescuer who enters an unknown atmosphere without appropriate protection can become a second casualty. Trained rescue with suitable supplied breathing air and retrieval arrangements must be distinguished from simply holding one's breath and climbing in.<Permit-required Confined Spaces by the Occupational Safety and Health Administration>[29 CFR 1910.146, atmosphere evaluation, ventilation and rescue provisions; cited as a present technical comparison, not English law in 1865]{ref}
For ordinary rooms and wards, prevention also means limiting overcrowding, maintaining reliable air inlets and outlets, and removing products of combustion through proper flues. These principles concern continuing air exchange: a large room merely delays contamination if its <air> is never renewed. They explain why the following ward question asks separately about cubic space and the rate of change of <air>.
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