Ventilation
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Ventilation is air circulation of air, typically between a room, a tunnel, etc., and the air outside. When people or animals are present, ventilation is especially necessary to remove the carbon dioxide produced and renew the oxygen used up. It is needed additionally if pollution takes place, e.g. by smoking, painting, etc. Ventilation may also refer to a closed system, such as an airliner, where "stale" air is filtered, mixed with oxygen, temperature-adjusted, and recirculated back to the cabin.
If there is something burning (a fireplace, gas heater, candle, oil lamp, etc.) more oxygen is replaced by carbon dioxide (and possibly other poisonous gases and smoke) and more ventilation is needed. If there is a chimney then the additional need of ventilation just refers to replacing the warmer air that leaves by convection through the chimney with fresh air.
Ventilation in a structure is also needed for removing water vapor, produced by breathing, burning, and cooking, and for removing smells, e.g. from a toilet or kitchen. If water vapor is permitted to accumulate, it may damage the insulation or interior finish. A dehumidifier may also be appropriate for removing moisture in the air.
In firefighting, ventilation refers to the tactic of creating a draft with an opening above or opposite the entry point so that heat and smoke will be released, permitting the firefighters to find and attack the fire. Mechanical fans can be used for such ventilation, as are existing openings such as windows, or skylights, scuttle hatches on the roof. If there is no ready-made hole, then the firefighters use their tools to make one, including saws for cutting a large hole in the roof. If a large fire is not properly ventilated, not only will it be much harder to fight, but it could also build up enough poorly burned smoke to create a smoke explosion, or enough heat to create a flashover. Some modern construction of high-rise building incorporates automatic ventilation equipment to produce a positive pressure in smoke-proof stairwells.