Ventilation is air circulation 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.

If there is something burning (a geyser, heater, candle, oil lamp, etc.) more oxygen is replaced by carbon dioxide (and possibly other gases and smoke) and more ventilation is needed. If there is a chimney than the additional need of ventilation just refers to replacing the air that leaves through the chimney with fresh air.

Ventilation is also needed for removing water vapor, produced by breathing, burning, and cooking, and for removing smells, e.g. from a toilet or kitchen.

See also Gas exchange, Heating, Ventilation and Air-Conditioning.