Table of Contents
What is an aerosol?
Aerosols are mixtures of solid or liquid particles (“suspended particles”) in a gas or mix of gases, such as air. Environmental aerosol particles may differ enormously in size, having diameters ranging from about 1 nanometer (nm) up to several 100 micrometres (µm). Larger particles usually sink to the ground quickly. Particles of less than 10 µm in diameter can remain suspended in the air for hours or even days.
Aerosols are generally unstable and usually change over time depending on humidity, temperature and other physical and chemical processes. Aerosols can also contain bacteria and viruses.
In medical research, a distinction is often made between “droplet infection” and infection “via aerosols” . In this context, the larger liquid aerosol particles between 5 µm and about 500 µm (the bigger ones being barely visible) are often referred to as “droplets”, while those smaller than 5 µm are described as “aerosols”. In physical terms, however, both are aerosols. With respect of their properties, there is no clear boundary between droplets and other aerosol particles, their transition being smooth. For the sake of clarity we will refer to “aerosol particles” or simply “particles” in the following text.
How are SARS-CoV-2 containing aerosols formed?
When exhaling, every individual spreads a bulk of gases and also aerosol particles into his or her immediate environment . Speaking, shouting or singing, but especially coughing, sneezing or physical exertion give rise to the emission of larger numbers of particles. When pathogens such as SARS-CoV-2 viruses are present in the airways, aerosols emitted may contain these pathogens. In the case of SARS-CoV-2, the formation of such aerosols is particularly problematic because even infected individuals without symptoms can emit virus-containing particles.
The spectrum of exhaled particles differs depending on whether the individual is breathing normally, singing, coughing or sneezing. Normal breathing mainly produces small particles (less than 5 µm). When speaking and singing, greater numbers of such particles are exhaled in comparison to breathing, while coughing and sneezing also generate larger particles of up to 100 µm or more in diameter. Moisture-laden speaking produces even larger saliva drops that may be visible to the naked eye.
Coronaviruses have a diameter of 0.12-0.16 µm but are usually emitted as part of larger particles which remain airborne for different lengths of time, depending on their size, and can be transported over varying distances with the air flow.
Exhaled aerosol particles change in size and composition according to the environmental conditions. Particles usually shrink during the transition from the respiratory tract into the ambient air due to the evaporation of their water content. The exact processes that lead to the formation and modification of such aerosol particles depend on a variety of different factors and are difficult to predict in individual cases.
When can individuals catch COVID-19 via aerosols?
COVID-19 can be transmitted via aerosol particles under the following conditions:
- The amount of infectious SARS-CoV-2 in the aerosol is large enough to convey a critical dose to the receiving susceptible person . The exact amount of viruses necessary for infection is not yet known and probably depends on several individual factors.
- The virus-containing aerosol must come into contact with sensitive cells such as respiratory tract cells, or the connective membranes of the eyes of an uninfected person.
- The virus must be able to replicate in these cells.
As far as exposure to virus-containing particles is concerned, there are two opposing effects:
Larger particles can contain more viruses in absolute terms and may thus be more infectious. At the same time, larger droplets will sink to the ground more rapidly and are thus available for airborne infection for a shorter period. Reducing the risk of infection by such larger particles was a rationale to recommend the minimum social distance of 1.5 metres.
Smaller aerosol particles tend to contain fewer viruses but can remain airborne for longer. This means that they pose a risk of infection over distances greater than 1-2 metres and for longer periods . A report of infections during a choir rehearsal lasting several hours, in which social distance rules were being observed, suggests that the increased infections observed in that context were caused by the transmission of smaller particles which remained suspended for longer in the ambient air .
How can the risk of infection via aerosol particles be reduced?
On the one hand, measures can be taken to reduce the exhalation of aerosol particles. These include hygienic sneezing practices (sneezing into the crook of the arm or a tissue) and wearing a mask that covers the nose and mouth . Wearing such a face cover significantly reduces the amount of aerosol particles released. The protective effect increases with the size of the particles involved [5, 6]. Smaller particles are less likely to be retained by a face cover than larger ones.
On the other hand, the now-familiar social distancing measures can be adopted to prevent exhaled aerosol particles from passing undiluted from one person to another . However, in situations where smaller particles may accumulate in the air, this measure may be not sufficient .
Indoors, due to limited air volume, the probability of accumulation of infectious particles is generally higher than outdoors. It follows that there is an increased risk of infection when two or more people gather in indoor spaces. Many factors play a role in the probability of infection in indoor spaces, and these can vary greatly from case to case: the number of persons present, their type of activity, the volume of the indoor space, the rate of air exchange, the kind of air flow, the type of ventilation available (window ventilation, ventilation technology) and any filters that may be in use.
Small rooms such as toilets, shared kitchens, changing rooms and lifts or small offices are problematic when used simultaneously by several people. Plans for the shared use of such spaces, must be drawn up to ensure that users can either be kept physically separate or use the spaces during different time slabs. In meeting rooms, care must be taken to ensure that they are not used by too many people at the same time and that the number of persons allowed in the space or the length of their stay takes into account the characteristics of the space (e.g. room size, air exchange). Furthermore, adapted ventilation concepts can contribute to the reduction of particle concentrations (see question below).
During indoor gatherings, any activities should be avoided that increase the exhalation of aerosol particles and, subsequently, the concentration of potentially infectious particles in the corresponding indoor spaces. Such actions include occasional sneezes and coughs (not necessarily associated with an infectious disease); anyone who coughs or sneezes should do so into the crook of the arm. Singing, shouting and screaming also lead to an increase in particles which accumulate in indoor spaces. Aerosol particles can also be produced while playing wind instruments. It should be borne in mind that sports activities associated with an elevated respiratory rate also lead to an increased exhalation of aerosol particles. If activities releasing increased aerosol concentrations cannot be avoided, the recommendation is to ventilate more intensively (see below) or, where possible, to hold such activities outside.
Models to mathematically simulate the dispersion of virus-containing particles and infection are currently under development to predict the infection probability from indoor aerosol particles; these models take into account various factors regarding the number of people in the group, their activities (speaking, singing, etc.), their duration of stay, the room characteristics (room volume) and ventilation (ventilation rate) [8, 9]. Such models, which are currently still being tested by UBA for their practical suitability, may provide assistance in the future when it becomes necessary to estimate the maximum duration of meetings in an indoor space for the case an infected person is in the room. Since many other factors, such as correct compliance with social distancing and hygiene measures, the dose of infection and individual sensitivities, can influence the likelihood of infection , it is not yet clear whether a reliable assessment of the risk of infection based on such models will be possible in the near future.
Which indoor hygiene measures can minimise the concentration of infectious aerosol particles?
Effective ventilation (exchange of indoor air with outdoor air) can reduce the concentration of infectious particles in the indoor air.
In the case of window ventilation, cross-ventilation which quickly exchanges indoor air with fresh ambient air by means of a draught is optimal but, unfortunately, not always practical. Brief but intense ventilation for a few minutes with a wide-open window (ideally several open windows at a time) is considered most effective. Keeping windows partially open, even permanently, is not effective.
For effective infection protection, rooms where many people gather should be ventilated as well and as often as possible. Classrooms in schools should be ventilated frequently to reduce increased concentrations of carbon dioxide during lessons . In the case of new buildings and complex renovations, the best long-term solution is to ensure that a mechanical ventilation system is implemented for densely-populated rooms from the outset. For schools, this is regarded as the desirable regulatory standard for the future. Mechanical ventilation may also become increasingly necessary for residential buildings as building envelopes become increasingly airtight.
It is worth to note that efficient ventilation alone cannot in general prevent the transmission of SARS-CoV-2 viruses from an infected person to another. This would require very high air exchange rates that cannot be implemented in practice. In indoor spaces used by two or more people, additional measures such as the wearing of face coverings, maintaining social distancing and an adapted usage plan remain essential.
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