The recommended air humidity is 40-60%. Basically, it can be said that the air in a room is dry if the difference between the outside and inside temperature is large. Under these circumstances a humidifier is recommended. However, if the air humidity is above 60%, a dehumidifier should be used, otherwise mould may form on the walls. The hygrometer shows you exactly whether a room is too dry or too humid. In addition, a thermo-hygrometer can also show you the temperature.