Have you ever thought what the purpose of a dehumidifier is, when will you need it and how to choose one? A dehumidifier is a machine that removes moisture from the interior air of a room and maintains the room’s humidity levels through a humidistat. Ideal relative humidity levels in British homes is 50 to 55%. Air which is too dry can cause dry skin and create static shocks around the house. Too much moisture can encourage mould and allergen growth leading to risks of health problems and can cause damage to the house.