The content of gaseous water in the air is commonly referred to as humidity. The gaseous water, the water vapour, is invisible because it is not the vapour clouds of small droplets that form the clearly visible mist. How much water vapour the air can absorb depends strongly on the local temperature.
To precisely determine the humidity in a room, a distinction must be made between relative, maximum and absolute humidity. Absolute humidity describes the water vapour density, i.e. the mass of water vapour contained in a defined volume of air. It is usually given in g/m³ and ranges between zero and the maximum water vapour content that the air can reach with a specified volume at a specified temperature. Relative humidity, on the other hand, describes the relationship between the water vapour density and the maximum air humidity.
In this context, the dew point also plays an important role. This describes the temperature below which water vapour condenses into dew or mist. Since the air can hold less water vapour at lower temperatures, the excess becomes liquid above the dew point.
With regard to absolute humidity , no limit values have yet been formulated for indoor spaces.
Probably the greatest danger in rooms with too high humidity is the formation of mould. The threshold for this is a relative humidity of 60 %. In a room with a temperature of 20 °C, this corresponds to an absolute humidity of 10.4 g/m³.
The varying temperatures within the room are particularly problematic. Depending on the insulation, a relative humidity of 50 % can already be sufficient to contribute to mould growth by falling below the dew point near colder exterior walls. The building materials used also play a significant role here. These have a great influence on how quickly water vapour diffuses.
Impairments of health in connection with the water vapour content of the air are strongly dependent on the relative humidity. If this is too low, the immune system is weakened. In addition, it only leads to dehydration of the mucous membranes, skin irritation and reduced respiratory performance. Especially in winter, the risk of too dry air increases. The cold outdoor air can only absorb a small amount of water vapour. If this air enters indoor rooms and is heated there, e.g. by heaters, the relative humidity decreases while the absolute humidity remains the same.
Excessive humidity can not only lead to discomfort and a strain on the circulatory system as well as impaired productivity. It also increases the likelihood that mould will form indoors. If its spores enter the body via the respiratory tract, this can lead to complaints such as conjunctivitis, gastrointestinal complaints, joint pain, asthma or migraines.
Furthermore, house dust mites also multiply at a high relative humidity of 60 % or more, which can be particularly problematic for allergy sufferers.
At water surfaces, individual water molecules pass into the air with the help of their thermal energy. Water molecules in the air also sink back into the water surface if they hit it. The more water molecules there are in the air, the more frequently this happens. From a relative humidity of 100 %, just as many molecules sink back into the water surface as emerge from it. The air is saturated. If you heat the air, however, this equilibrium only occurs with a higher number of water molecules in the air. Their absorption capacity has increased.
Since the relative humidity of the air depends very much on the temperature, because the absorption capacity of the air for water is temperature-dependent, it is difficult to make statements as to whether active humidification of the air is efficient. If you cover the heating with a damp cloth, for example, the temperature in the room decreases because the heating capacity decreases. This decreasing temperature alone already leads to an increase in relative humidity. However, the absolute humidity does not change due to the temperature change alone, because if the air does not absorb any moisture, the mass of water per cubic metre of air does not increase. Only when the absolute humidity increases can one say with certainty that the damp cloth is contributing to the increase in humidity.
The absolute humidity is measured in the air-Q by a sensor for the relative humidity with very high accuracy. The absolute humidity is derived from the measured value. The air-Q reacts somewhat more sluggishly to changes in humidity than an absolutely exposed humidity sensor.
As an air analyser and measuring device, the air-Q also has a sensor that can measure absolute humidity and other room air parameters in real time. By the way, absolute humidity is also called water vapour density.