What is relative humidity?

Definition

When discussing air humidity, a distinction is made between relative humidity and absolute humidity. At different temperatures, the air can hold different amounts of moisture. As a general rule, the higher the temperature, the more moisture the air can hold. Furthermore, at every temperature there is a specific point at which the air can no longer absorb any more moisture. This point is known as the saturation point or dew point. From this point onwards, condensation begins to form. In contrast to relative humidity, absolute humidity describes the exact amount of moisture contained in the air in grams of water per kilogram of air (g/kg).

Importance of relative humidity

Heating generally leads to a decrease in relative humidity, although the value of absolute humidity remains the same. The correct humidity and the resulting hygienically sound air are essential for humans, animals, plants and materials, as well as for research and development, production, storage and preservation.

Comfort

As dry air seeks to absorb more moisture, it draws this from its surroundings, including the people within them. This leads to itchy skin, burning eyes, headaches and feelings of tiredness. The optimal range of air condition values for comfort lies between 21°C and 22°C and relative humidity between 40% and 60%.

Example

Let’s assume we have a glass in which the air temperature is 20°C. The glass contains 10 grams of water per kilogram of air, and the relative humidity is 70%. We know, first of all, that the air is very humid and could only absorb 30% more moisture. Furthermore, it is clear that the dew point line would be reached if the absolute humidity were to rise by 5 grams of water per kilogram of air. The air would also reach the dew point line if it cooled by at least 6°C. If water vapour were now introduced into the vessel, thereby increasing the humidity, fog and then condensation would form once 15 grams of water per kilogram of air were reached. Looking at it the other way round, the air would also reach the saturation line and thus form mist and condensation if the temperature were lowered by 6°C.

Health protection

At relative humidity levels below 30%, as occurs due to heating in winter, people often suffer from symptoms of dehydration. This leads to the drying out of the respiratory tract mucous membranes, meaning that dust, dirt and pathogens can no longer be cleared from the respiratory tract quickly enough. As a result of their prolonged presence in the respiratory tract, the risk of respiratory diseases increases. Typical consequences of this process include coughing, bronchitis, the common cold and sinusitis. In the optimal range of between 40% and 60% relative humidity, the risk posed by unwanted microorganisms and the occurrence of specific symptoms of illness is minimal.

When relative humidity falls below 35%, the drying out of clothing, carpets, furniture and similar items promotes dust formation. The smouldering of dust on radiators produces ammonia and other gases, which further irritate the respiratory organs. Furthermore, all types of plastic become electrically charged in dry air and thus attract additional dust particles. The drying out of the mucous membranes of the upper respiratory tract leads to their function being impaired (+Explanation of the protective mechanism of the mucous membranes).

If humidity levels are too high, above 70%, moisture can condense in colder areas. In such cases, it is likely that interior furnishings containing organic materials may emit odours due to mould growth or mustiness. Furthermore, this can lead to structural or material damage (e.g. mould growth on thermal bridges).

Hygroscopic materials / Industry

All hygroscopic substances strive for equilibrium. Consequently, after a certain period of exposure to the humidity of the surrounding air, a hygroscopic substance reaches a state of equilibrium in which it either absorbs or releases water. A large proportion of the materials in our environment contain water in varying quantities. Hygroscopic substances are characterised by the fact that their water content depends on the humidity of the surrounding air.