Extractor fans give an extra boost to ventilation in spaces such as bathrooms which tend to produce an excess of water vapour. Switch control for the units can be wired into light switches to keep the wall surface as clean as possible. Domestic bathroom units come in 100mm or 150mm sizes, the latter becoming more necessary when a bathroom exceeds nine square metres and does not have any natural ventilation. Timer models allow the fan to continue running after the light or switch control is turned off to ensure optimal extraction of water vapour.