Symptoms caused by excessively dry air and the impact of humidity on health

Health in the workplace is one of the most important factors for productivity and motivation. Humidity plays a major role in this: air that is too dry – with a relative humidity below 40 per cent – is not only perceived as unpleasant, but also manifests itself in physical complaints and illnesses.

Respiratory infections, dry mucous membranes, a weakened immune system,voice disorders, and eye problems are also consequences of air that is too dry. All too often, the direct link to the indoor climate is not recognised.

Air humidity plays a major role in the behaviour of virus-laden aerosols as they remain suspended in the air. Unlike larger and heavier infectious droplets, which fall to the ground within a few seconds of coughing or sneezing, the lighter and smaller aerosols can remain suspended in the air for hours. Aerosols consist mainly of water, dissolved salts and proteins. At a relative humidity of less than 40%, aerosols lose their water content and dry out. This results in dry aerosols, which are smaller and lighter and can remain airborne for longer. Compared to moist aerosols, they are also less sticky due to their lower water content and stick together less. As a result, air currents and the movements of people in the room cause dry aerosols to be stirred up from surfaces more quickly and spread further.

In addition to how long they remain airborne, humidity also has a significant impact on the infectivity of respiratory droplets. Below 40% relative humidity, the aerosols dry out to such an extent that the salts they contain crystallise. This preserves the viruses, allowing them to remain infectious for longer. When inhaled, the crystallised salts dissolve again in the moist respiratory tract. The viruses, which are still infectious, are released onto the mucous membranes of the respiratory tract and can cause infections. If the relative humidity is within the optimal range of 40–60%, the water content of the aerosols evaporates only to the extent that the salt concentration increases significantly without crystallisation, and the viruses contained within can be inactivated.

Humans are not defenceless against attacks from viruses and bacteria. The effectiveness of our immune system determines whether we fall ill and how quickly we recover. In the respiratory tract, the mucous membranes protect us from infections through their self-cleaning function. The surface of the mucous membranes is covered with fine cilia that move freely in a thin secretion (sol layer). Over this lies a sticky gel layer, to which the majority of inhaled viruses, bacteria and airborne pollutants adhere. As long as the cilia can move freely, they transport the mucus, together with the microorganisms, towards the larynx, where it can be swallowed or coughed up. However, as humidity falls, the removal of pathogens becomes less effective.

In low humidity, water is drawn from the sol layer. The cilia become increasingly flattened and lose their mobility. The increasing viscosity of the mucous membranes leads to a blockage of the mucus flow, and the risk of infection from viruses penetrating the mucous membrane cells rises. If the relative humidity drops to 20%, the self-cleaning process comes to a complete standstill. Studies show that the highest transport speed – and thus the lowest risk of infection – is achieved at 45% relative humidity.

A variety of factors contribute to the development of voice disorders: in addition to individual causes (e.g. poor speaking technique), the main factors are those related to the working environment, such as loud background noise, poor indoor air quality or incorrect sitting posture. The indoor climate, and in particular humidity, has a decisive influence on the voice.

To maintain good vocal function and prevent voice disorders, it is necessary to ensure that the mucous membranes are sufficiently moisturised : When speaking, air is forced from the lungs through the larynx. The vocal folds vibrate and, like the strings of a guitar, produce sounds. If the air humidity is too low, the mucous membranes of the vocal folds lose their optimal moisture and, consequently, their elasticity. After inhalation, the glottis can no longer be completely closed by the vocal folds. Air leaks into the vocal tract, leading to irritation, inflammation and, in the worst case, loss of voice.

Analyses of scientific studies show that if humidity is too low, both tear film production and tear film quality are affected. In dry eye syndrome, it is not only the quantity but also the altered composition of the tear fluid that causes the lubrication problems on the eye’s surface. The tear film is made up of several layers. A mucous layer lies directly on the surface of the eye. This ensures that the aqueous layer, which makes up the majority of the tear fluid, does not run off and does not evaporate as quickly.

Studies show that, compared to an ideal humidity range of 50–80%, the cell density of the conjunctival goblet cells decreases significantly at low relative humidity levels below 30%. The goblet cells of the conjunctiva are primarily responsible for producing mucus, thereby forming an important part of the tear film. Insufficient humidity therefore not only leads to faster evaporation of the tear film, but also to reduced mucus production, which is intended to protect and maintain the aqueous component of the tear fluid.

The first signs of excessively dry skin are fine flakes and reddish patches. The skin begins to dry out when moisture and lipids are lost without this loss being compensated for. To prevent this, the skin is made up of three layers: the outermost layer is the multi-layered epidermis, which includes the protective stratum corneum. The skin’s barrier function consists not only of warding off unwanted foreign bodies from the outside, but also of preventing excessive fluid loss from within. The underlying skin layers therefore constantly release fluids to the epidermis. Drinking enough fluids is one of the prerequisites for this. In addition, the lower skin layers transport lipids upwards. A further protective lipid film on the skin’s surface is provided by a mixture of lipids produced in the sebaceous glands. If the moisture and lipid content of the skin layers decreases, the protective function is lost. The stratum corneum becomes increasingly permeable, allowing particles and foreign substances to penetrate and trigger skin irritation and inflammation. Dry indoor air can exacerbate this process.