Symptoms
Good health in the workplace is one of the most important prerequisites for having motivated and productive staff. Humidity has a major influence on this: air that is too dry and below 40 percent relative humidity is not only perceived as unpleasant, it also makes itself felt in the form of physical discomfort and illness.
Respiratory infections, dry mucous membranes, weakened immune defences, hoarseness or dysphonia and eye complaints are among the consequences of air that is too dry. All too often, the direct correlation here to the indoor climate is not recognised.
Respiratory tract
Infections of the respiratory tract are transmitted almost exclusively from person to person indoors.
Voice
Air that is too dry is uncomfortable and dangerous, especially for jobs that involve a great deal of speaking and screen work.
Eyes
Intensive screen work is very taxing on the eyes: swollen eyelids, redness and more are often the consequences.
The skin is the largest organ in the human body, and at the same time an important barrier between the environment and the inside of our body.
Humidity and respiratory infections
Infections of the respiratory tract, such as those caused by influenza or coronaviruses, are transmitted almost exclusively from person to person indoors. Humidity has an impact on the risk of these infections. The most common route of infection is airborne transmission at close range via droplets and at longer range via aerosols: viruses are breathed in by another person and taken up through the mucous membranes in the upper respiratory tract. Depending on the particle size, this is called droplet transmission or aerosol transmission. Because of their small size, aerosols are extremely light. Viral aerosol particles can spread through the air over a considerable period of time. The level of relative humidity has a direct impact on the range of aerosols, their ability to remain suspended in the air and their infectiousness.
Lowest risk of infection at 40 to 60% humidity
Relative humidity plays a major role in the ability of viral aerosol particles to remain suspended in indoor air. Unlike the larger and heavier infectious droplets produced by coughing or sneezing, which fall to the ground after a few seconds, lighter and smaller aerosols can stay suspended in the air for hours at a time. Aerosols essentially consist of water, dissolved salts and proteins. At a relative humidity of under 40%, aerosols lose their water content and dry out. This produces dry aerosols, which are smaller and lighter, and which can remain airborne in the room for longer. Unlike wet aerosols, their lower water content also makes them less ‘sticky’ and so they cannot bond together so readily. Air flows and the movements of people in the room also mean that dry aerosols are raised from surfaces again more quickly, and can therefore spread further.
Viruses survive longer in dry air
Apart from its effect on suspended particles, humidity is also hugely important in terms of the infectiousness of these pathogen-rich droplets. At less than 40% relative humidity, aerosols dry out to the extent that the salts they contain crystallise out. This works to protect the viruses and they remain infectious for longer. When breathed in, the crystallised salts then dissolve again in the moist environment of the respiratory tract. The viral particles, which are still infectious, are released onto the mucous membranes, where they can trigger an infection. If relative humidity is within the optimum range of 40 to 60%, however, aerosols only dry out to an extent where salt concentrations rapidly increase but salts do not crystallise out: this can inactivate the viruses contained in these aerosols.
The survival time of viruses
The immune defence of the mucous membranes
Moist mucous membranes provide better protection
We humans are not entirely defenceless in the face of attacks from viruses and bacteria. The functioning of our immune system determines whether we get sick and how fast the recovery process is. We are protected from infection by the self-cleaning mechanisms used by the mucous membranes in our airways. The surfaces of these mucous membranes are covered by fine motile hairs (cilia), which move freely within a fluid secretion (saline layer). Covering this is a sticky gel layer, to which most of the viral, bacterial and pollutant particles we breath in adhere. As long as the cilia remain highly motile, they can transport the mucus together with these microorganisms towards the larynx, where this mucus can then be swallowed or coughed out. As humidity drops, however, this removal system for pathogens becomes less effective.
At lower levels of relative humidity, the saline layer starts to dry out. This has the effect of collapsing the cilia, which therefore lose their motility. The increasing viscosity of the mucous membrane works to block the flow of mucous and the risk of infection from viruses invading mucous membrane cells increases. Once relative humidity has fallen to 20%, this self-cleaning process comes to a complete stop. Experiments have shown that the fastest pathogen transportation rate – and therefore the lowest risk of infection – is achieved at 45% relative humidity.
Self-cleaning abilities of mucous membranes at different levels of relative humidity
Low immune defence in dry air
When air is too dry, two other mechanisms also have a direct impact on the immune system and hamper the effectiveness of our adaptive immune response. Breathing in air that is very dry damages epithelial cells and therefore impairs the repair processes used by respiratory tract epithelia (lung cells). These form a physical barrier underneath the mucous membrane layer, which prevents viruses penetrating into host cells. Secondly, low relative humidity can also reduce the formation of interferon in lung tissue. As messenger substances, interferons trigger the production of proteins that combat invading viruses and thereby prevent virus multiplication.
Case studies: Humidity and respiratory infections
Influence of humidity on the voice
Air that is too dry is uncomfortable and dangerous, especially for jobs that involve a great deal of speaking and screen work. All too often, the direct correlation here to the indoor climate is not recognised. Dryness in the throat and larynx, clearing of the throat and a persistent cough are always the first alarm signals. At the latest when the voice becomes hoarse, or is lost altogether, performance on the job suffers.
To maintain good voice function and prevent voice disorders, adequate moistening of the mucous membranes is therefore necessary. For professions involving a great deal of speaking, a minimum humidity of 40% is recommended.
Fewer problems with the voice with optimal humidity
There are many factors that work together to produce problems with the voice: aside from causes specific to individuals (e.g. poor speaking technique), these are mainly factors in the working environment such as loud background noise, a poor indoor climate or an incorrect posture while sitting. The climate, and in particular the humidity of the climate, has a critical influence on the voice: to maintain good voice function and prevent voice disorders, adequate moistening of the mucous membranes is necessary. When we speak, air is pushed out of the lungs through the larynx. This causes the vocal folds to vibrate and, like the strings of a guitar, to produce tones. If the humidity is too low, the mucous membranes of the vocal folds lose their optimum moisture level, and hence their elasticity. The glottis can no longer be fully closed by the vocal folds after breathing in. Additional air finds its way into the vocal tract, leading to irritation, inflammation, and in the worst case loss of the voice.
Research shows, for instance, that over 60% of employees in call centres or customer service departments suffer from voice strain. Common symptoms are dryness of the throat and larynx, more frequent clearing of the throat, and hoarseness. “These symptoms distract the listener from what is being said and cause significant disruption to communication with the customer,” says speech therapist and voice coach Thomas Niemann of Sprechfreunde.de. The potential consequences include loss of quality of discussion and lower closure rates. In addition, voice disorders are often associated with absence rates: respiratory problems, which include hoarseness and laryngitis, are one of the main causes of sick leave at call centres. With the number of staff away sick not infrequently in the double digits, this involves significant costs and loss of performance for companies. To maintain good voice function and prevent voice disorders, adequate moistening of the mucous membranes is therefore necessary. For professions involving a great deal of speaking, a minimum humidity of 40% is recommended. By retrofitting humidification systems, companies can make a major contribution to improved health and quality of conversation in the workplace.
Humidity and dry eyes
Intensive screen work is very taxing on the eyes: swollen eyelids, redness, foreign body sensation, burning and high sensitivity to light are often the consequences. One of the most common causes of this is inadequate moistening of the surface of the eye with tear fluid.
Dry eyes are one of the most common forms of eye disease and involve all symptoms caused by reduced moistening of the surface of the eye. Also known as “office-eye syndrome”, the symptoms are also produced by climatic and environmental influences: In addition to dust and draughts, humidity is a variable that directly affects the tear film: a humidity level that is too low reduces the eye’s protective tear film.
Less eye strain with the right humidity
There are many factors that work together to produce problems with the voice: aside from causes specific to individuals (e.g. poor speaking technique), these are mainly factors in the working environment such as loud background noise, a poor indoor climate or an incorrect posture while sitting. The climate, and in particular the humidity of the climate, has a critical influence on the voice: to maintain good voice function and prevent voice disorders, adequate moistening of the mucous membranes is necessary. When we speak, air is pushed out of the lungs through the larynx. This causes the vocal folds to vibrate and, like the strings of a guitar, to produce tones. If the humidity is too low, the mucous membranes of the vocal folds lose their optimum moisture level, and hence their elasticity. The glottis can no longer be fully closed by the vocal folds after breathing in. Additional air finds its way into the vocal tract, leading to irritation, inflammation, and in the worst case loss of the voice.
Two effects of dry air on the tear film
Analysis of scientific work shows that when the humidity level is too low, both the production of tear film and the quality of tear film change. With dry eyes, it is not only the volume of the tear fluid but also its changed composition that causes disruption to the moistening of the surface of the eye. 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 constitutes most of the tear fluid, does not run off and evaporate as quickly as it otherwise would. Studies show that compared to an ideal humidity range of 50 to 80%, the cell density of conjunctival goblet cells decreases sharply at low relative humidities below 30%. The goblet cells of the conjunctiva are responsible in particular for producing mucous, which constitutes an important part of the tear film. Insufficient humidity therefore leads not only to faster evaporation of the tear film, but also to reduced mucous production, which serves to protect and maintain the aqueous part of the tear fluid.
Dry air stresses the skin
The skin is the largest organ in the human body, and at the same time an important barrier between the environment and the inside of our body. Dry room air can substantially reduce this protective function of the skin. If the humidity level is too low, the skin becomes chapped and cracked. Particles that can cause inflammation and dermatosis penetrate through cracks in the skin. Chronic skin complaints such as neurodermatitis or dandruff are aggravated by excessively dry air. Insufficient moisture makes itself felt in the form of uncomfortable tautness and itching of the skin, particularly on the forearms, elbows, hands, lower legs and feet. Especially in winter, when the humidity level drops with low temperatures, complaints about dry skin increase.
When does the skin dry out?
The first signs of excessively dry skin are thin scales and reddish patches. When moisture and fats (lipids) are lost, and there is nothing to make up for that loss, the skin starts to dry out. To prevent this, the skin is composed of three layers: The top layer is the multi-layered epidermis, including its protective horny layer. The skin’s barrier function serves not only to defend against unwanted foreign bodies from the outside, but also to ensure that too much fluid is not lost from the inside. The layers of skin below therefore provide the epidermis with a permanent supply of fluids. One of the prerequisites for this is drinking enough water. In addition, the lower layers of the skin also channel lipids upwards. Another protective layer of fat on the surface of the skin is provided by an oily mixture produced in the sebaceous glands.
Adequate humidity protects the skin
When the moisture and fat content of the skin layers decrease, the protective function is lost. The calloused skin becomes increasingly permeable, allowing particles and foreign substances to penetrate and cause skin irritation and inflammation. Dry room air can encourage this process. Particularly in winter, when heated air is dry, the environment draws more moisture from the skin. The moisture differential between the skin and the indoor space leads inevitably to increased loss of water. Transepidermal water loss (TEWL) is when moisture leaves the body through the epidermis by diffusion and evaporation. If this loss of moisture persists, the skin contracts like parchment, becomes scaled and cracked, and can become inflamed. Series of scientific tests show that below 50% relative humidity, there is a significant increase in transepidermal water loss. At the same time, this increase leads to a marked decrease in the hydrogenation of the skin. Studies confirm that even small increases in relative humidity, from 35% to 43%, result in very positive consequences for skin health and a reduction in any symptoms that occur.