Building regulations in Britain require you to make sure that new bathrooms have proper ventilation,that is, having enough changes of air per hour. You can not do this simply by opening windows regularly or by mechanical means such as a small extractor fan. You should incorporate an electric relay switch, linking it to the light switch which will start the fan when the light is switched on and continue to run for about fifteen minutes after it is switched off. (This sort of device makes internal bathrooms, which would normally be cut off from adequate light and ventilation, a practical project.)
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