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Viruses are a part of our lives
In general, the risk of transmission of SARS-CoV-2 is highest in the near field and during prolonged and close contact. In case of prolonged stay in e.g. small, poorly or not ventilated rooms, where many people are present, a transmission by infectious, small airborne particles (aerosols) can also occur over a distance of more than 1.5 m. Such aerosols can best be reduced or removed by regular ventilation or, in the case of ventilation systems, by exchanging the room air with a supply of fresh air (or by appropriate filtration) indoors (see also “What role do aerosols play in the transmission of SARS-CoV-2?”).
At present, a wide variety of (mobile) devices are offered as possible measures to clean or disinfect indoor air. The use of these devices should prevent indoor transmission of SARS-CoV-2. Further information on the various techniques and concepts, e.g. the use of air filters or fogging or radiation techniques, as well as on the efficiency of air exchange can be found, among others, in the statement “The risk of transmission of SARS-CoV-2 indoors can be reduced by appropriate ventilation measures” of the Federal Environment Agency (UBA). The Commission for Indoor Air Hygiene at the UBA commented on air purifiers in schools on 17.11.2020.
In this context, however, it is important to emphasize that even efficient depletion (reduction) of aerosols in indoor air cannot effectively reduce the risk of transmission in the near field, e.g., in face-to-face contact at a distance of < 1.5 m. In addition, some important questions are still unresolved, such as the actual effectiveness in practical use, the health safety of the substances or processes used, or the sufficient distribution of a disinfecting agent or the filtered/disinfected air throughout the room. Also, the risk of indirect transmission via (droplet) contaminated surfaces cannot be reduced by the use of such devices (see also “Guidance on cleaning and disinfection of surfaces outside healthcare facilities in the context of the COVID 19 pandemic”).
The IDE makes it clear that wearing a mouth-nose covering and observing hygiene and distance rules indoors are only sufficiently effective if adequate air exchange via window ventilation or ventilation technology in the room is ensured at the same time.
Appropriate in the current situation means to provide the highest possible supply of fresh air, which creates an indoor air quality as close as possible to the outdoor air.
SARS-CoV-2 presents our society with unexpected and entirely new logistical challenges. We now recognize that inadequately ventilated indoor environments may be at increased risk of contracting SARS-CoV-2. In addition to observing general hygiene and distance rules  and wearing mouth-to-nose coverings, this risk can be significantly reduced by consistent ventilation and proper use of indoor ventilation techniques, even though this cannot achieve 100% protection against indoor SARS-CoV-2 infections.
Where masks and separators are of little use, air purifiers can reduce the risk of infection via aerosols.
A study confirms that air purifiers with appropriate filters can significantly reduce the risk of indoor infections. This makes them the better alternative to ventilation, and not just in the cold season. But they also have a disadvantage that should not be underestimated.
Scientists at the University of the German Armed Forces in Munich (UniBw M) have tested whether air purifiers are able to filter aerosols and thus reduce the risk of corona infections indoors. Their study found that appliances equipped to do so are not only effective at reducing indoor viral loads, but are often more effective than ventilating through open windows.
The fact that aerosols play a major role in the transmission of covid-19 indoors is now virtually undisputed in the scientific community, and numerous specialists even assume that they are responsible for most infections. This is particularly problematic for the catering trade, cultural institutions and event venues, which are therefore unable to operate at all or only to a very limited extent. However, many offices are also affected and, with the end of the major holidays, schools are increasingly affected.
Ventilation becomes difficult in winter
In summer you can ventilate to reduce the aerosol concentration. But the colder it gets, the more problematic it becomes to open the windows often and long enough: Energy is wasted, heating costs rise and the people inside freeze. Moreover, without measuring devices it is hardly possible to know when and how often ventilation is necessary.
For their study, the scientists used an air purifier that can filter up to 1500 cubic meters per hour and systematically analyzed its effectiveness with laser technology. The device is equipped with a filter combination that separates even very small aerosols with a diameter of 0.1 to 0.3 micrometres (µm) almost completely from the room air. To ensure that the device itself does not become a virus slinger, you can set the filter to heat up to about 100 degrees every 30 minutes, so that all pathogens die off. However, according to the researchers, it is sufficient to perform this procedure once a day.
“Enrichment not possible”
According to the press release of UniBw M, the results showed that the aerosol concentration in a room with a size of 80 square meters (m²) is halved within six minutes. During continuous operation of the device, the measured residence time of the exhaled aerosol in the room is therefore very short, so that an enrichment of the air with infectious aerosols is not possible. The researchers measured a similar effectiveness in a 22-metre-long hallway of around 40 m².
In large rooms, those with many objects or very unfavourable geometries, several air cleaners may be necessary to filter all areas of the room quickly and keep the virus load low everywhere, the scientists write. It also depends on the location. For example, the air purifier should be positioned on the longest side of the room in the middle if possible. In addition, no objects should interfere with the airflow towards the ceiling area.
Effective, but expensive
The Armed Forces researchers not only see air purifiers as a good alternative to ventilation. The technology guarantees a permanently low virus load, which often cannot be guaranteed by open windows, they write. However, masks and distance rules cannot be dispensed with, as there is still a risk of direct infection via droplets.
The scientists emphasize that air purifiers can also greatly reduce the risk of infection from aerosols in schools. However, the device used for the study costs around 3500 euros. In addition, there are electricity and maintenance costs. A separate room air cleaner is required for each classroom. Whether many German schools can afford this is doubtful.
An analysis of international studies on coronavirus supports research that sees aerosols as a major route of transmission of covid-19. If the hypothesis is correct, protective measures for indoor spaces need to be reconsidered, including classrooms and offices.
Already in mid-May, virologist Christian Drosten pointed out that aerosols are probably the most important transmission route for the coronavirus besides droplet infection. These are basically droplets as well. However, at less than 5 micrometers, they are much smaller and can remain suspended in the air in particles for long periods of time.
Since then, there has been growing evidence that aerosols play an important role in the transmission of covid-19, but scientific evidence is still lacking. Although it cannot provide this, a summary analysis by US lung specialist Kevin P. Fennelly does confirm the dominant role of aerosols in corona dispersal. It was published in the renowned medical journal “The Lancet”.
Aerosols more dangerous than larger droplets
Fennelly has analyzed several studies on the spread of viruses via particles emitted during coughing and exhalation. His goal was to find out what masks and other measures could protect healthcare workers from contracting Covid-19.
The pulmonologist concludes that the assumption that most respiratory infections occur via droplets is incorrect. Pathogens such as the coronavirus are emitted by infected persons in particles of various sizes, but many viruses are found in aerosols smaller than 5 micrometres.
From this, Fennelly concludes, among other things, that hospital staff must always be equipped with masks that can filter aerosols (FFP2/3). They should also wear shields to protect their eyes. But his findings also play a role outside the health care system. After all, if aerosols really do play a more important role in the transmission of coronavirus than conventional droplet infection, the current distance rules, for example, make no sense when people meet indoors. This is the case, for example, when schools resume regular operations after the summer holidays.
That Sars-Cov-2 can survive in aerosols for a long time is suggested, among other things, by a study published on 22 June in the journal “Emerging Microbes & Infections”. British scientists had produced aerosols from artificial saliva with a diameter of 2 to 3 micrometers and loaded them with coronaviruses. According to Spektrum.de, the researchers were able to detect live viruses in both high and low humidity conditions even after 90 minutes.
Scientists write open letter to WHO
For Lidia Morawska of the Queensland University of Technology in Brisbane, there is no longer any doubt. She co-authored an analysis of a superspreader event at a choir rehearsal in Washington State. “Aerosol transmission is the only logical explanation for such infectious events,” she told Spectrum.co.uk. Morawska wrote an open letter to the World Health Organization (WHO) and national authorities, which 237 international researchers have signed. They call on the organization to take aerosol transmission as a route of infection seriously and to develop appropriate recommendations for ventilation and decontamination of buildings. The WHO has at least announced that it will seriously review the claims.
The RKI has already largely acknowledged the importance of aerosol transmission. The “fact sheet” on Covid-19 states that “prolonged residence in small, poorly ventilated or unventilated rooms may increase the probability of transmission by aerosols even over a distance greater than 2 metres, particularly if an infectious person emits a particularly large number of small particles (aerosols) and exposed persons inhale particularly deeply.” And, “Due to the accumulation and dispersion of aerosols, keeping the minimum distance may no longer be sufficient under these conditions.”
The global development of SARS-CoV-2 infection illustrates that the pandemic is only just beginning and cannot be stopped. Even if an effective and well-tolerated vaccine were available, large-scale vaccination of the world’s population to combat the spread of the virus would not be feasible. It is therefore necessary to establish technical solutions to contain the pandemic. Mouth-to-nose coverings are now generally accepted technical aids to reduce the direct risk of infection when breathing, speaking, singing, coughing and sneezing. Indirect infection via infectious aerosol particles that accumulate in the room over time cannot be prevented with mouth-to-nose coverings . This requires tight-fitting particle-filtering respirators. Alternatively, it is possible to separate the aerosol particles in the room by means of filtration or to discharge them via window ventilation. Indoor air systems that reliably separate aerosol particles with a diameter of less than 1 μm are rare. Free ventilation through windows is often not efficient and, at the latest in winter, no longer possible without wasting energy and endangering people’s health and well-being. The question is therefore whether mobile room air cleaners are in principle suitable for making a meaningful contribution to reducing the risk of infection?
The possibility of using mobile air purifiers to protect against SARS-CoV-2 viruses is repeatedly reported in the media. Different methods are offered for air purification, some of which are also combined with each other. These include high-performance filters, UV-C radiation and plasma technology.
With regard to the effectiveness of devices with filter technology, reference is often made to a study by the University of the Federal Armed Forces in Munich. In this study it was shown that the aerosol concentration can be halved in 6 – 15 minutes, depending on the volume flow, by using air cleaners with high-performance filters even in rooms with an area of 80 m2. However, such an efficiency cannot be transferred to the conditions in operationally used rooms without further ado. On the one hand, this was achieved with a very powerful unit at a volume flow in the range of 600 to 1500 m³ air/hour. Moreover, the investigation took place in a largely empty room. The cleaning performance in practice depends strongly on the fact that the filtered air can flow through the room as unhindered as possible or that polluted air can be sucked in from the entire room as far as possible. Furniture, partition walls or anything else impede even ventilation. The air flow can also be deflected or influenced in its propagation by heat sources such as people, PCs, lamps or others.
A critical point is the range at which air can be drawn from the room. The flow velocity of the air drawn in decreases considerably even at a short distance from the unit, so that, depending on the power of the unit, hardly any air movement can be detected after a distance of one metre. If there are chairs, tables or other furniture in the room, the air intake and even ventilation will be impeded.
As evidence of effectiveness, reference is often made to filter performance,e.g., the use of H14 filters that ensure that aerosol particles with a diameter of 0.1 to 0.3 μm are separated from indoor air by 99.995%. Whether the device as a whole provides this cleaning performance or was tested as a whole is not always evident from the descriptions. In addition, the information only applies to the proportion of the room air that was sucked through the filter. Therefore, the test protocols should always be requested. The same applies to air purification devices that work with UV-C radiation, plasma technology or other decontamination processes or use a combination of such processes. Again, it should be noted that the effectiveness of the air purification device as a whole must be demonstrated. The effectiveness of UV-C radiation for the decontamination of surfaces has been proven. Whether the UV-C emitters or the plasma are sufficient to decontaminate the upstream air volumes of several hundred cubic metres per hour is decisive for the application in practice.
It can be stated that general statements on the effectiveness of air purification devices are not possible. In addition to the device-specific criteria, the respective room design and use must also be taken into account. Air purifiers also cannot replace protective measures such as keeping your distance, as they do not provide protection against droplet infections. It should also be borne in mind that air purification devices cannot replace the replacement of stale air with fresh air.
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