Denying the aerosol transmission route is not going to get the world out of this pandemic
Since February 2020, I have been an advocate for the worldwide recognition of the aerosol transmission of SARS-CoV-2. Aerosols (aero-solutions) are a mix of micro droplets and solid particles with gasses, like the air surrounding us. Through breathing, tiny droplets are released into a gas cloud that travels through airflows. In indoor spaces, aerosols accumulate, saturating the air. This mechanism is known to contribute to the spread of infectious diseases, as is the case with Influenza, measles and SARS-CoV-1 (the coronavirus that was active during the 2003 epidemic). While airborne infection and aerosol transmission of viruses are not controversial among scientists (chemical engineers, biochemists and physicists), policy makers and health authorities fail to recognize the importance of aerosol transmission of SARS-CoV-2.
Instead, policies issued by the government and health authorities are based on the belief that infections only occur through fomites or through direct person-to-person droplet transmission. Remarkably, when droplet and fomite transmission fail to prove the exact transmission routes at work during superspreading events, the aerosol transmission route is rejected in advance, arguing that it is assumed that fomite and direct droplet transmission are the major transmission routes. Even more remarkable, health authorities discriminate between research on aerosol transmission routes of SARS-CoV-2 and research on droplet and fomite transmission. While preprints copied from the preprint server MedRxiv are deemed "supportive evidence" when these seem to confirm the predominance of fomites or direct (droplet) transmission, preprints from MedRxiv on airborne infection are rejected for 'not being peer reviewed yet'. Even more bizarre is that engineers, physicists and biochemists specialized in aerosol science are now pushed aside as "believers". People fail to understand that aerosols are minuscule parts of everyday life. It's not like they are insignificant because the bare human eye does not have the quality to spot them.
Aerosol transmission was reported to be the main transmission route in the Amoy Gardens Housing complex community during the 2003 SARS-CoV-1 epidemic (Evidence of Airborne Transmission of the SARS Virus, NEJM, 22 April 2004) and air distribution played a major role in the largest nosocomial SARS outbreak in Hong Kong (Role of air distribution in SARS transmission during the largest nosocomial outbreak in Hong Kong, Indoor air, April 2005, Volume 15, Issue 2). A more extensive read on cases of aerosol transmission of SARS-CoV-2, as well as insights in sets of genes imposing vulnerabilities on people who inherited these genetic traits, is published in the May edition of Human Genomics (COVID-19 vulnerability: the potential impact of genetic susceptibility and airborne transmission, Human Genomics, 2020; 14 :17). Repeatedly, since the 2003 outbreak of SARS-CoV-1, the global community has been urged to adopt measures to ensure ventilation rates and air filtration and to prevent aerosol generation during (medical and dental) procedures (Detection of Airborne SARS Coronavirus and Environmental Contamination in SARS Outbreak Units, Journal of Infectious Diseases, 1 May 2005, Volume 191, Issue 9).
No rigid divisive line should be drawn between transmission routes
Neither transmission route should be rejected, nor should there be drawn a sharp dividing line between aerosol transmission and other transmission routes. Fomite transmission of faecal particles, urine and saliva contributes to the spread of SARS-CoV-2, as well as direct droplet transmission. There is more to this. Aerosols are even borne out of fomites ("airborne"). It starts with fomites like poop and saliva, which turn into aerosols generated by mechanisms like breathing, talking, using, drilling, flushing the toilet, draining down, ventilating, blowing into the air by mechanical climate control. Pathogens under both classifications (droplet and airborne) have the potential to be transmitted by aerosols, as should be recognized (Recognition of aerosol transmission of infectious agents: a commentary, BMC Infectious Diseases, 31 January 2019; 19; 101).
A recent test estimates that 1 minute of loud speaking generates at least 1,000 virion-containing droplet nuclei (particles) that remain airborne for more than 8 minutes. The droplets that were observed were regarded to be sufficiently small to reach the lower respiratory tract, implying an increased risk for infection (The airborne lifetime of small speech droplets and their potential importance in SARS-CoV-2 infection, Proceedings of the National Academy of Sciences of the United States of America, 13 May 2020).
Large vs. small droplets: a definition too narrow
The point is, policies force people to maintain (social) distance of of approximately 5 feet, but maintaining distance does not protect against aerosol transmission. The distance provision is based on Wells' 1930 theory, which is now interpreted as a theory distinguishing between large and small droplets. In this view, small droplets would evaporate after exhaling/coughing/sneezing, as the environment is colder and drier than the lungs of the person exhaling the drops. Interestingly, research on airborne dispersion has centered on 'aggressive' forms of exhalation like sneezing and coughing for years, while it was already proven in 1946 that breathing and talking generate large amounts of aerosols (The size and duration of air-carriage of respiratory droplets and droplet-nuclei, Journal of Epidemiology and Infection, J.P. Duguid, September 1946).
The 'large drops vs. small drops'-debate is of relevance when it comes to measures preventing direct droplet transmission: ballistic research makes clear that medium and large droplets are transmitted mainly through the droplet-borne route. After dispersion into the air, aerosols are transmitted via the short-range and long-range airborne route. Regardless of the transmission route (direct droplets or airborne), surgical masks reduce infectiousness (Airborne spread of infectious agents in the indoor environment, American Journal of Infection Control, 2 September 2016, 44(9)).
The turbulent gas cloud does not respect social distance rules of 5 feet
Drops do not fly isolated through the air, but through a turbulent gas cloud. This is why the 1-2 meter/5 ft distance model falls short. The actual mechanism is called "multiphase turbulent gas cloud", which means that respiratory drops, mucus, nearby gases and moisture accumulate in a cloud. Depending on humidity and temperature of the environment and the size of the virus-carrying particles, this cloud can travel up to 8 meters through the air. Finally, particles settle on surfaces ánd remain up in the air (droplet nuclei = aerosols) for about 3 hours after evaporation from droplets to gas (Aerosol and Surface Stability of SARS-CoV-2 as compared to SARS-CoV-1, NEJM, 16 April 2020).
The distance rule is based on the assumption that air does not move though a space. This model created in 1934, called the 'Wells-Riley" model of well-mixed air, hypothesizes that there is an ideal homogeneous mixing of respiratory drops and an ideal homogeneous distribution of humans in any given space. Only ventilation rate is included in the well-mixed air model, while parameters of dose-response, air turbulence, flow direction, flow pattern and spatial heterogeneity should be included (Review and comparison between the Wells-Riley and dose-response approaches to risk assessment of infectious respiratory diseases, Indoor Air, Vol. 20 Issue 1, February 2010). This explains why some people sitting next to an infected person do not get infected, while people sitting at a greater distance can become infected through the movement of viral particles through air flows. An inter-personal distance of 2 m can only be considered to be reasonable when wearing face masks during public activities is compulsory (Airborne Transmission Route of COVID-19: Why 2 Meters/6 Feet of Inter-Personal Distance Could Not Be Enough, International Journal of Environmental Research and Public Health, MDPI, April 2020; 17(8)).
The Open Air Factor (OAF)
Since February 2020, I have been an advocate for the worldwide recognition of the aerosol transmission of SARS-CoV-2. Aerosols (aero-solutions) are a mix of micro droplets and solid particles with gasses, like the air surrounding us. Through breathing, tiny droplets are released into a gas cloud that travels through airflows. In indoor spaces, aerosols accumulate, saturating the air. This mechanism is known to contribute to the spread of infectious diseases, as is the case with Influenza, measles and SARS-CoV-1 (the coronavirus that was active during the 2003 epidemic). While airborne infection and aerosol transmission of viruses are not controversial among scientists (chemical engineers, biochemists and physicists), policy makers and health authorities fail to recognize the importance of aerosol transmission of SARS-CoV-2.
Instead, policies issued by the government and health authorities are based on the belief that infections only occur through fomites or through direct person-to-person droplet transmission. Remarkably, when droplet and fomite transmission fail to prove the exact transmission routes at work during superspreading events, the aerosol transmission route is rejected in advance, arguing that it is assumed that fomite and direct droplet transmission are the major transmission routes. Even more remarkable, health authorities discriminate between research on aerosol transmission routes of SARS-CoV-2 and research on droplet and fomite transmission. While preprints copied from the preprint server MedRxiv are deemed "supportive evidence" when these seem to confirm the predominance of fomites or direct (droplet) transmission, preprints from MedRxiv on airborne infection are rejected for 'not being peer reviewed yet'. Even more bizarre is that engineers, physicists and biochemists specialized in aerosol science are now pushed aside as "believers". People fail to understand that aerosols are minuscule parts of everyday life. It's not like they are insignificant because the bare human eye does not have the quality to spot them.
Aerosol transmission was reported to be the main transmission route in the Amoy Gardens Housing complex community during the 2003 SARS-CoV-1 epidemic (Evidence of Airborne Transmission of the SARS Virus, NEJM, 22 April 2004) and air distribution played a major role in the largest nosocomial SARS outbreak in Hong Kong (Role of air distribution in SARS transmission during the largest nosocomial outbreak in Hong Kong, Indoor air, April 2005, Volume 15, Issue 2). A more extensive read on cases of aerosol transmission of SARS-CoV-2, as well as insights in sets of genes imposing vulnerabilities on people who inherited these genetic traits, is published in the May edition of Human Genomics (COVID-19 vulnerability: the potential impact of genetic susceptibility and airborne transmission, Human Genomics, 2020; 14 :17). Repeatedly, since the 2003 outbreak of SARS-CoV-1, the global community has been urged to adopt measures to ensure ventilation rates and air filtration and to prevent aerosol generation during (medical and dental) procedures (Detection of Airborne SARS Coronavirus and Environmental Contamination in SARS Outbreak Units, Journal of Infectious Diseases, 1 May 2005, Volume 191, Issue 9).
No rigid divisive line should be drawn between transmission routes
Neither transmission route should be rejected, nor should there be drawn a sharp dividing line between aerosol transmission and other transmission routes. Fomite transmission of faecal particles, urine and saliva contributes to the spread of SARS-CoV-2, as well as direct droplet transmission. There is more to this. Aerosols are even borne out of fomites ("airborne"). It starts with fomites like poop and saliva, which turn into aerosols generated by mechanisms like breathing, talking, using, drilling, flushing the toilet, draining down, ventilating, blowing into the air by mechanical climate control. Pathogens under both classifications (droplet and airborne) have the potential to be transmitted by aerosols, as should be recognized (Recognition of aerosol transmission of infectious agents: a commentary, BMC Infectious Diseases, 31 January 2019; 19; 101).
A recent test estimates that 1 minute of loud speaking generates at least 1,000 virion-containing droplet nuclei (particles) that remain airborne for more than 8 minutes. The droplets that were observed were regarded to be sufficiently small to reach the lower respiratory tract, implying an increased risk for infection (The airborne lifetime of small speech droplets and their potential importance in SARS-CoV-2 infection, Proceedings of the National Academy of Sciences of the United States of America, 13 May 2020).
Large vs. small droplets: a definition too narrow
The point is, policies force people to maintain (social) distance of of approximately 5 feet, but maintaining distance does not protect against aerosol transmission. The distance provision is based on Wells' 1930 theory, which is now interpreted as a theory distinguishing between large and small droplets. In this view, small droplets would evaporate after exhaling/coughing/sneezing, as the environment is colder and drier than the lungs of the person exhaling the drops. Interestingly, research on airborne dispersion has centered on 'aggressive' forms of exhalation like sneezing and coughing for years, while it was already proven in 1946 that breathing and talking generate large amounts of aerosols (The size and duration of air-carriage of respiratory droplets and droplet-nuclei, Journal of Epidemiology and Infection, J.P. Duguid, September 1946).
The 'large drops vs. small drops'-debate is of relevance when it comes to measures preventing direct droplet transmission: ballistic research makes clear that medium and large droplets are transmitted mainly through the droplet-borne route. After dispersion into the air, aerosols are transmitted via the short-range and long-range airborne route. Regardless of the transmission route (direct droplets or airborne), surgical masks reduce infectiousness (Airborne spread of infectious agents in the indoor environment, American Journal of Infection Control, 2 September 2016, 44(9)).
The turbulent gas cloud does not respect social distance rules of 5 feet
Drops do not fly isolated through the air, but through a turbulent gas cloud. This is why the 1-2 meter/5 ft distance model falls short. The actual mechanism is called "multiphase turbulent gas cloud", which means that respiratory drops, mucus, nearby gases and moisture accumulate in a cloud. Depending on humidity and temperature of the environment and the size of the virus-carrying particles, this cloud can travel up to 8 meters through the air. Finally, particles settle on surfaces ánd remain up in the air (droplet nuclei = aerosols) for about 3 hours after evaporation from droplets to gas (Aerosol and Surface Stability of SARS-CoV-2 as compared to SARS-CoV-1, NEJM, 16 April 2020).
The distance rule is based on the assumption that air does not move though a space. This model created in 1934, called the 'Wells-Riley" model of well-mixed air, hypothesizes that there is an ideal homogeneous mixing of respiratory drops and an ideal homogeneous distribution of humans in any given space. Only ventilation rate is included in the well-mixed air model, while parameters of dose-response, air turbulence, flow direction, flow pattern and spatial heterogeneity should be included (Review and comparison between the Wells-Riley and dose-response approaches to risk assessment of infectious respiratory diseases, Indoor Air, Vol. 20 Issue 1, February 2010). This explains why some people sitting next to an infected person do not get infected, while people sitting at a greater distance can become infected through the movement of viral particles through air flows. An inter-personal distance of 2 m can only be considered to be reasonable when wearing face masks during public activities is compulsory (Airborne Transmission Route of COVID-19: Why 2 Meters/6 Feet of Inter-Personal Distance Could Not Be Enough, International Journal of Environmental Research and Public Health, MDPI, April 2020; 17(8)).
The Open Air Factor (OAF)
The "open-air factor" OAF refers to the influence of natural air flow and UV on the destabilization of viruses and other infections. Modern hospitals are generally worse at limiting the spread of viruses than hospitals dating from before 1950. Old-fashioned large windows that can be opened are conducive to limiting the spread of virus particles in the hospital (The open-air factor and infection control, Journal of Hospital Infection, April 9, 2019). In other words, being indoors is not ideal, poor ventilation is completely disastrous. A 2013 study confirms the suspicion that modern hospitals with small windows (that cannot be opened) and poor air conditioning accelerate the spread of SARS. Mechanical ventilation is not necessarily worse than natural airflows: the point is to achieve 'cross-ventilation', where air flows from the inside out (Roles of sunlight and natural ventilation for controlling infection: historical and current perspectives, Journal of Hospital Infection 84, June 20, 2013). A lack of SARS transmission among public hospital workers in Vietnam in 2003 was associated with a health policy of cross-ventilation, as well as a face mask policy (Lack of SARS Transmission among public hospital workers, Vietnam, Emerging Infectious Diseases, Februari 2004; 10(2)). A massive body of knowledge on the clinical management of SARS outbreaks is to be found in the Review 'Clinical management and infection control of SARS: lessons learned', Antiviral Research, Volume 100, Issue 2, November 2013.
But cross-ventilating a room is not sufficient to minimize the risk of aerosol transmission. Larger exhaled droplets can be moved upwards by heat, then spread and dispersed horizontally. Ventilation is determined by factors ventilation rate, flow direction and flow pattern (Ventilation control for airborne transmission of human exhaled bio-aerosols in buildings, Journal of Thoracic Disease, July 2018; 10(suppl. 19)). The spatial distribution of viral particles is dependent on airflow pattern. Ventilation dilution depends on ventilation rate. This explains why natural airflow, which has an effective ventilation rate, dilates viral particles.
Using ventilation as means to reduce the risk of airborne transmission of SARS-CoV-2
I've mentioned that natural ventilation is preferable to ensure natural airflow. The ventilation rate of natural airflow is effective when it comes to the dilation of droplet nuclei. As a result, the concentration of viral nuclei in air is minimized. However, objects should not get in the way of the airflow. Another consideration is that people should avoid the course of the airflow, to reduce the risk of becoming infected with viral nuclei.
Recirculation of air should be avoided, as air recirculation increases the risk of airborne transmission. The Open Air Factor is recommended: replace recirculation with Open Air if possible.
Disinfection agents and radiation can speed up destabilization of SARS-CoV-2. UV-C is known to inactivate SARS-CoV-1 within 15 minutes; this might be similar with regards to SARS-CoV-2 (Inactivation of the coronavirus that induces SARS, SARS-CoV, Journal of Virological Methods, October 2004; 121(1)).
But cross-ventilating a room is not sufficient to minimize the risk of aerosol transmission. Larger exhaled droplets can be moved upwards by heat, then spread and dispersed horizontally. Ventilation is determined by factors ventilation rate, flow direction and flow pattern (Ventilation control for airborne transmission of human exhaled bio-aerosols in buildings, Journal of Thoracic Disease, July 2018; 10(suppl. 19)). The spatial distribution of viral particles is dependent on airflow pattern. Ventilation dilution depends on ventilation rate. This explains why natural airflow, which has an effective ventilation rate, dilates viral particles.
Using ventilation as means to reduce the risk of airborne transmission of SARS-CoV-2
I've mentioned that natural ventilation is preferable to ensure natural airflow. The ventilation rate of natural airflow is effective when it comes to the dilation of droplet nuclei. As a result, the concentration of viral nuclei in air is minimized. However, objects should not get in the way of the airflow. Another consideration is that people should avoid the course of the airflow, to reduce the risk of becoming infected with viral nuclei.
Recirculation of air should be avoided, as air recirculation increases the risk of airborne transmission. The Open Air Factor is recommended: replace recirculation with Open Air if possible.
Disinfection agents and radiation can speed up destabilization of SARS-CoV-2. UV-C is known to inactivate SARS-CoV-1 within 15 minutes; this might be similar with regards to SARS-CoV-2 (Inactivation of the coronavirus that induces SARS, SARS-CoV, Journal of Virological Methods, October 2004; 121(1)).
Further reading
1. Judson SD, Munster VJ. Nosocomial transmission of emerging viruses via aerosol‐generating medical procedures. Viruses. 2019;11(10). doi:10.3390/v11100940
2. Tran K, Cimon K, Severn M, Pessoa‐Silva CL, Conly J. Aerosol generating procedures and risk of transmission of acute respiratory infections to healthcare workers: A
systematic review. PLoS One. 2012;7(4). doi:10.1371/journal.pone.0035797
systematic review. PLoS One. 2012;7(4). doi:10.1371/journal.pone.0035797
3. Li Y, Huang X, Yu ITS, Wong TW, Qian H. Role of air distribution in SARS transmission during the largest nosocomial outbreak in Hong Kong. Indoor Air.
2005;15(2):83‐95. doi:10.1111/j.1600‐0668.2004.00317.x
2005;15(2):83‐95. doi:10.1111/j.1600‐0668.2004.00317.x
4. Grosskopf K, Mousavi E. Bioaerosols in health‐care environments. ASHRAE J. 2014;56(8):22‐31.
5. Lindsley WG, Blachere FM, Thewlis RE, et al. Measurements of airborne influenza virus in aerosol particles from human coughs. PLoS One. 2010;5(11).
doi:10.1371/journal.pone.0015100
doi:10.1371/journal.pone.0015100
6. Moriyama M, Hugentobler WJ, Iwasaki A. Seasonality of Respiratory Viral Infections. Annu Rev Virol. 2020:1‐19. doi:10.1146/annurev‐virology‐012420‐022445
