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
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