Medical professionals recommend a relative humidity of between 40 – 60%. The greater the difference between outside and inside temperature in winter, the dryer is the air in the room. It is easiest if you measure the relative humidity with a hygrometer.
What is a hygrometer? A hygrometer is an instrument that measures the water content in the air and shows the relative air humidity on a display or mechanical indicator. As well as the room humidity a thermo hygrometer also shows the room temperature.
Place the hygrometer in the middle of the room, making sure it is not next to any windows or doors. The display will show the air humidity (e.g. Selina Thermo Hygrometer from Stadler Form). Do not place the hygrometer in direct sunlight or the humidity values will be distorted. You need a humidifier if the humidity is below 40 %. If the humidity is above 60 %, using a dehumidifier is recommended so as to prevent mould formation.
This video shows you how Selina and Selina little measure the humdity level of a room.