Why Ventilation is necessary in buildings

Ventilation is necessary in buildings to remove 'stale' air and replace it with 'fresh' air.

This helps to:

Moderate internal temperatures.
Reduce the accumulation of moisture, odours and other gases that can build up during occupied periods.
Create air movement which improves the comfort of occupants.

Very broadly, ventilation in buildings can be classified as 'natural' or 'mechanical'.

Mechanical (or 'forced') ventilation tends to be driven by fans.
Natural ventilation is driven by 'natural' pressure differences from one part of the building to another. Natural ventilation can be wind driven, or buoyancy driven. For more information, see Natural ventilation.

Whilst natural ventilation may be preferable, mechanical ventilation may be necessary where:

The building is too deep to ventilate from the perimeter.
Local air quality is poor, for example, if a building is next to a busy road.
Local noise levels mean that windows cannot be opened.
The local urban structure is very dense and shelters the building from the wind.
Air cooling or air conditioning systems mean that windows cannot be opened.
Privacy or security requirements prevent windows being opened.
Internal partitions block air paths.
The creation of draughts adjacent to openings.