Schematic representation of the proportions of SARS-CoV-2 viruses in particles emitted from the body

Reducing Aerosol Transmission Essential to Preventing COVID-19 Transmission

German working committee recommends concrete countermeasures for indoor areas: masks, ventilation, air purification, and overhead air extraction

Leibniz Institute for Tropospheric Research
Published:Dec 10, 2020
|4 min read
Schematic representation of the amount of SARS-CoV-2 viruses (0,1 μm) in aerosol particle emissions of the nose and the mouth. 
From left to right: (A) Basal respiration, (B): Speaking, singing, and shouting (mouth), (C): Even larger droplets are emitted of the nose and the mouth when sneezing. Suspended viruses are embedded in saliva or dried up lung liquid—a cold and damp climate and darkness extend their activity. The particles A can hover longer than one day in unventilated rooms, the particles B several hours. The largest particles (C and mostly bigger) of the sneezing sink to the ground within few seconds. In contrast to cloth face masks, N95 and FFP2 masks protect against the particles A.
Hartmut Herrmann / Konstanze Kunze, TROPOS

Aerosols and their spread play an essential role in the transmission of COVID-19. However, the risk of transmission could be significantly reduced if more could be done to reduce indoor airborne viruses. The working committee particulate matter (AAF) has therefore issued an statement with concrete recommendations. 

These include window ventilation, exhaust ventilation, air purification systems, and CO2 measuring devices for indoor areas such as classrooms or transportation, and the increased use of N95 and FFP2 masks. These countermeasures could help in the short term to better contain the coronavirus pandemic, especially during winter, until vaccination is effective on a large scale. They could also help in the long term to better control infections, such as seasonal influenza and other pandemics, in the future.

The German AAF brings together experts from the fields of engineering, chemistry, physics, biology, meteorology, and medicine, who are organized in the professional associations ProcessNet (DECHEMA/ VDI-GVC), Gesellschaft Deutscher Chemiker (GDCh), and VDI/DIN Commission Reinhaltung der Luft (KRdL). In its autumn meeting, the AAF discussed the role of aerosol particles in the spread of SARS-CoV-2 and prepared a statement on this topic. On the basis of their expertise, the authors describe in the now published statement different aerosol types with regard to their formation, range, and residence time in the air, and derive recommendations for protection by various measures. The authors expressly support the current recommendations of the Robert Koch Institute (RKI), but suggest that even more should be done to reduce the number of viruses in indoor air.

The AAF advises strict application of the recommendations based on the active aerosol propagation path: masks (especially the use of N95 and FFP2 masks) are helpful and necessary, ventilation is a good immediate measure, and suitable air purifiers should be used.

Furthermore, the panel concludes that more attention should be paid to the type of ventilation in addition to the measures already taken: Especially the smaller aerosol particles in the warm air we exhale that rises and then spreads below the ceiling. The experts on therefore recommend that in ventilation systems, fresh air should not be supplied from top to bottom, as this leads to turbulence between the fresh air and the air we exhale and viruses can then float in the room air for longer. Ceiling fans, which are counterproductive in the current COVID 19 pandemic, would also contribute to this. Instead, care should be taken to ensure that air is actually extracted upwards. In the future, a reversal of the air supply and extraction in aircraft or public transport could help.

Related Article: More Than One-Third of Children with COVID-19 Show No Symptoms 

The panel of experts also advises to install ventilation and especially overhead exhaust suction systems in many areas at short notice, especially in classrooms or in the catering industry. Monitoring the CO2 concentration is a suitable indicator of how well the ventilation works. For cultural facilities, too, monitoring the CO2 content and thus the indoor air could later provide opportunities for normalizing operation. Funds should be made available so that ventilation, extraction, air purification systems, and COmeasuring devices can be installed in school classes. At the local level, it would be helpful if administrative regulations were relaxed and school management teams were given more freedom of action. If these measures were implemented consistently, about 90 percent of all potentially viral aerosols could be removed from classrooms.

"We clearly see the human and technical effort involved in the short and medium term, but we are convinced that the appropriate consideration of the virus spread via the aerosol path can lead to a short-term and also sustainable containment of the current incidence of infection. Such investments would also be beneficial for later, for example for the air quality in classrooms", explains professor Hartmut Herrmann of the Leibniz Institute for Tropospheric Research (TROPOS), who is chairman of the AAF. 

Even if the World Health Organization (WHO) still pays too little attention to this, experts have long been convinced that aerosols, i.e. tiny suspended particles in the air, contribute strongly to the spread of SARS-CoV-2.

Protective measures against the spread of SARS-CoV-2 via indoor aerosols currently pose major challenges for many sectors of society. The risk of infection is particularly high in hospitals and nursing homes because infected and healthy people can remain in the same room for long periods of time. It is particularly important to develop an appropriate strategy to protect healthcare workers from airborne transmission. Their recommendations in the International Journal of Environmental Research and Public Health include regular ventilation, controlling fresh air consumption via CO2 monitoring, and using humidifiers to keep the relative humidity indoors at 40 to 60 percent. If it is not possible to ventilate sufficiently, the use of portable air purifiers are also advisable.

- This press release was originally published on the Leibniz Institute for Tropospheric Research website