sars-cov-2 face-masks, aerosols, droplets, droplet nuclei, 6 … · 2020. 7. 20. · one...

SARS-COV-2 Face-masks, Aerosols,

Droplets, Droplet Nuclei, 6 Foot Social Distancing

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www.barkerstats.com SARS S-CoV-2 Transmission July 12, 2020

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Contents Background .................................................................................................................................................................3

SAR S-CoV-2 virus .......................................................................................................................................................3

Electron Microscope Virus Images .........................................................................................................................3

Nomenclature and Definitions ...................................................................................................................................4

Virus Transmission Overview .................................................................................................................................4

Simulated Exhalation ..............................................................................................................................................4

Schlieren image ..................................................................................................................................................4

WHO Aerosol statements ...................................................................................................................................5

Transmission modes ...............................................................................................................................................5

Fomites ...................................................................................................................................................................5

Field Evidence for Aerosols leading to infections ...................................................................................................5

Airborne droplets ...............................................................................................................................................6

Simulation of Corona Virus Exhalations for people in a Queue .............................................................................8

Sources ...............................................................................................................................................................8

Exhalation Simulations ...........................................................................................................................................8

Gas Clouds ..............................................................................................................................................................8

Airborne transmission ............................................................................................................................................8

239 Experts With One Big Claim: The Coronavirus Is Airborne, New York Times, .....................................................8

Contact Tracing ...........................................................................................................................................................9

Definition – Superspreading “Super spreading event” (SSE), 20/80 rule ..........................................................9

Skagit County Church Choir Superspreading ......................................................................................................9

Hair Salon Infections possibly avoided- Anecdotal evidence CDC report ....................................................... 10

Virus transport and dispersal .................................................................................................................................. 11

Droplet and Virus Exhalation and dispersal .................................................................................................... 11

Measurement of particles ....................................................................................................................................... 11

Medical procedures that generate droplets ....................................................................................................... 13


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Droplet Evaporation ............................................................................................................................................ 13

Movement of Air ............................................................................................................................................. 13

Masks Possible Origin of “6 foot distancing” ...................................................................................................... 14

Droplet Diameter vs time to reach ground ..................................................................................................... 14

Additional corroborating evidence for 6 foot distancing. ................................................................................... 14

Aerosols vs Droplets vs Droplet Nuclei .................................................................................................................... 15

WHO and field experiments ............................................................................................................................ 16

Aerosols ........................................................................................................................................................... 16

Droplet Nuclei ...................................................................................................................................................... 16

Kelvin Effect ......................................................................................................................................................... 17

Sources ............................................................................................................................................................ 17

Inhalation And Deposition of Droplets .................................................................................................................... 17

Mucociliary escalator ...................................................................................................................................... 18

Experimentally (laboratory) produced Aerosols ............................................................................................. 18

Virus Penetration ............................................................................................................................................. 19

Droplets, Viral payload and infection ...................................................................................................................... 19

What is an aerosol? ............................................................................................................................................. 19

How big is an aerosol particle? ............................................................................................................................ 19

Droplets, aerosols, droplet nuclei, Droplet diameter and Aerodynamic diameter ............................................ 20

Super spreader events? ....................................................................................................................................... 20

Stokes Law ........................................................................................................................................................... 20

Blood Splatter ...................................................................................................................................................... 20

World Health Organization - WHO .......................................................................................................................... 21

Social Distancing Potential Implications of the WHO revised guidance ......................................................... 21

Airborne Transmission overview ......................................................................................................................... 22

No Evidence of Effect is not evidence of no effect ................................................................................................. 22

Sources .................................................................................................................................................................... 23

Sources Masks ................................................................................................................................................. 23

Sources WHO ................................................................................................................................................... 23

References All .......................................................................................................................................................... 25

https://www.washingtonpost.com/health/2020/06/13/spate-new-research-supports-wearing-masks-control-coronavirus-spread/ ................................................................................................................................................ 26


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Please note that the “cut and pastes” of excerpts from manuscripts does not de-activate embedded hyper links to citations. These embedded links will not work correctly .


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Background One of many critical issues with the SARS-CoV-2 virus is the transmission. The transmission defines how the virus gets from an infected person to a non-infected person. Not that transmission does not include the “minimal amount of virus” (virion) required to start a new infection

SAR S-CoV-2 virus The virus has the composite name of SARS ( Severe Acute Respiratory Syndrome) , CoV (Corona Virus), -2.

Electron Microscope Virus Images https://www.niaid.nih.gov/news-events/novel-coronavirus-sarscov2-images

This scanning electron microscope image shows SARS-CoV-2 (yellow)—also known as 2019-nCoV, the virus that causes COVID-19—isolated from a patient in the U.S., emerging from the surface of cells (blue/pink) cultured in the lab.

Credit: NIAID-RML

https://www.niaid.nih.gov/news-events/novel-coronavirus-sarscov2-images


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Nomenclature and Definitions

There is compelling evidence that the virus is transmitted by air, due to coughing or sneezing

The transmission mechanism is by aerosols or other airborne vectors.

In the scientific literature of corona virus transmission there are several modes of transmission.

Corona virus transmission mode aerosols Droplets More than> 5 μm Droplet nuclei less than <= 5 μm fomites Objects and surfaces bioaerosols Faecal bioaersols

Virus Transmission Overview There are at least three main steps in virus transmission. First the host (assume a human) has the virus, replicating in cells. The virus must get from the cells, to the lungs or airway passages or situated for the “generation process” expiration (coughing, sneezing, talking, singing) or through bodily fluids (sweat, etc.). Second the expired virus must be in sufficient quantity to cause infection at the target (a different human). Virus particles may be deactivating during the “flight”. Finally, the virus at the target must land in a location to cause further infection and with an adequate “payload” to cause infection.

Simulated Exhalation Schlieren image (visualization using light refraction caused by differences in air density)

https://www.ncbi.nlm.nih.gov/books/NBK143281/figure/annexc.f1/?report=objectonly

Simulated cough spread https://youtu.be/evATiHUejxg



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WHO Aerosol statements

Even in its latest update on the coronavirus, released June 29, the W.H.O. said airborne transmission of the virus is possible only after medical procedures that produce aerosols, or droplets smaller than 5 microns. (A micron is equal to one millionth of a meter.)

Transmission modes

Fomites Virus containing droplets may cause direct transmission from close contact or contribute to contamination of fomites.(4) Fomites are surfaces or objects (e.g. clothing, equipment, furniture) that can become contaminated by virus, where it may remain active for hours to days.

Field Evidence for Aerosols leading to infections

Scientists have not been able to grow the coronavirus from aerosols in the lab. But that doesn’t mean aerosols are not infective, Dr. Marr said: Most of the samples in those experiments have come from hospital rooms with good air flow that would dilute viral levels.

https://www.who.int/publications/i/item/WHO-2019-nCoV-IPC-2020.4

https://www.sciencedirect.com/science/article/pii/S0013935120307143?via%3Dihub


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Airborne droplets Unfortunately, the truth is that we have only a rudimentary knowledge of several aspects of infection spread, including on one critical aspect of the SARS-CoV-2 virus: how THIS virus transmits (Bourouiba, 2020, Brosseau, 2020). In general it is considered that viral respiratory infections spread by direct contact, such as touching an infected person or the surfaces and fomites that the person has either touched, or on which large virus-containing droplets expired by the person have landed (Morawska 2006), and there the virus can remain stable for days (van Doremalen

et al. 2020). The droplets can also be deposited directly on a person in close proximity to the infected person. Therefore, frequent hand-washing and maintaining a distance of at least one meter (arm’s length) are considered the main precautions against contracting the infection (WHO 2020a). One transmission route that is mentioned only in passing, or not at all, is the transport of virus-laden particles in the air. Immediately after droplets are expired, the liquid content starts to evaporate, and some droplets become so small that transport by air current affects them more than gravitation. Such small droplets are free to travel in the air and carry their viral content meters and tens of meters from where they originated (e.g. Morawska et al. 2009), as graphically presented in Fig. 1 .

particles of more than 5 μm as droplets, and those less than 5 μm as aerosols or droplet nuclei (Siegel et al., 2007; WHO,

2014). Conversely, there have been some other postulations, indicating that aerodynamic diameter of 20 μm or 10 μm or less should be reckoned to be aerosols, based on their ability to linger in the air for a prolonged period, and the reachability to the respirable fraction of the lung (alveolar region) (Gralton et al., 2011; Nicas et al., 2005; Tellier, 2009). Small aerosols are more susceptible to be inhaled deep into the lung, which causes infection in the alveolar tissues of the lower respiratory tract, while large droplets are trapped in the upper airways (Thomas, 2013). For easy

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7151430/#b0010







https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7151430/figure/f0005/

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7293495/#bib95








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apprehension, aerosols can be defined as suspensions of solid or liquid particles in the air, which can be generated by either natural or anthropogenic phenomena (Judson and Munster, 2019; Tellier, 2009).




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Simulation of Corona Virus Exhalations for people in a Queue

Simulation of exhalation of virus laden droplets may include many components, listed below (Mathews, 2020).

• Virus Size and Shape • Effect of temperature and Humidity on enveloped viruses • Relative Humidity • Humidity • Temperature • Virus in Human Exhale • Infectious Dose • Bio-aerosols • Excretion of Corona Virus • Waiting time in the Queue • Distance Between People • Consume Factor • Scattering Factor • Height of the person • Evaporation rate • Time per exhale

Sources

Exhalation Simulations https://www.medrxiv.org/content/10.1101/2020.05.16.20104489v2.full.pdf

Mathews, Santhosh Samuel. "A Computer Simulation Study on novel Corona Virus Transmission among the People in a Queue." medRxiv (2020).

Gas Clouds https://jamanetwork.com/journals/jama/fullarticle/2763852

Bourouiba, Lydia. "Turbulent gas clouds and respiratory pathogen emissions: potential implications for reducing transmission of COVID-19." Jama 323.18 (2020): 1837-1838.

Airborne transmission Two hundred and thirty nine experts signed and open letter to the WHO requesting a modification and broadening of the criteria for airborne transmisions.

239 Experts With One Big Claim: The Coronavirus Is Airborne,

https://www.medrxiv.org/content/10.1101/2020.05.16.20104489v2.full.pdf

https://jamanetwork.com/journals/jama/fullarticle/2763852


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New York Times,

https://www.nytimes.com/2020/07/04/health/239-experts-with-one-big-claim-the-coronavirus-is-airborne.html

The link to the open letter

https://academic.oup.com/cid/article/doi/10.1093/cid/ciaa939/5867798

Contact Tracing Definition – Superspreading “Super spreading event” (SSE), 20/80 rule https://wwwnc.cdc.gov/eid/article/26/6/20-0495_article

One of the first known examples of superspreading was Mary Mallon “Typhoid Mary”. She infected 50 or more people.

A pragmatic working definition of superspreading may be a “20-80” type rule, where 20% of the population infect 80% of the population (Woolhouse 1997), and Excerpting (woolhouse 1997).

From an analysis of the distributions of measures of transmission rates among hosts, we identify an empirical relationship suggesting that, typically, 20% of the host population contributes at least 80% of the net transmission potential, as measured by the basic reproduction number, R0. This is an example of a statistical pattern known as the 20/80 rule. The rule applies to a variety of disease systems, including vector-borne parasites and sexually transmitted pathogens. The rule implies that control programs targeted at the ‘‘core’’ 20% group are potentially highly effective and, conversely, that programs that fail to reach all of this group will be much less effective than expected in reducing levels of infection in the population as a whole. Woolhouse et al. (1997) also observed that 20% of the population contributed to >80% of transmission and suggested targeting interventions to the core 20% (12). SSEs have also caused explosive outbreaks of measles, including among vaccinated persons (13).

Skagit County Church Choir Superspreading

https://www.nytimes.com/2020/05/12/health/coronavirus-choir.html

Excerpting

One sick singer attended choir practice, infecting 52 others, two of whom died. A study released by the C.D.C. shows that self-isolation and tracing efforts helped contain the outbreak.

…

The full choir consists of 122 singers, but only 61 made it that night, including one who had been fighting cold-like symptoms for a few days.



https://wwwnc.cdc.gov/eid/article/26/6/20-0495_article

https://wwwnc.cdc.gov/eid/article/26/6/20-0495_article#r12

https://wwwnc.cdc.gov/eid/article/26/6/20-0495_article#r13



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That person later tested positive for the coronavirus, and within two days of the practice, six more members of the choir had developed a fever. Ultimately, 53 members of the choir became ill with Covid-19, the disease caused by the virus, and two of them died.

Hair Salon Infections possibly avoided- Anecdotal evidence CDC report Among 139 clients exposed to two symptomatic hair stylists with confirmed COVID-19 while both the stylists and the clients wore face masks, no symptomatic secondary cases were reported; among 67 clients tested for SARS-CoV-2, all test results were negative. Adherence to the community’s and company’s face-covering policy likely mitigated spread of SARS-CoV-2.

Sources

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7151430/

https://www.nytimes.com/2020/07/14/health/coronavirus-hair-salon-masks.html?referringSource=articleShare


https://www.nytimes.com/2020/07/14/health/coronavirus-hair-salon-masks.html?referringSource=articleShare


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Virus transport and dispersal In general it is considered that viral respiratory infections spread by direct contact, such as touching an infected person or the surfaces and fomites that the person has either touched, or on which large virus-containing droplets expired by the person have landed (Morawska 2006), and there the virus can remain stable for days (van Doremalen

et al. 2020). The droplets can also be deposited directly on a person in close proximity to the infected person. Therefore, frequent hand-washing and maintaining a distance of at least one meter (arm’s length) are considered the main precautions against contracting the infection (WHO 2020a). One transmission route that is mentioned only in passing, or not at all, is the transport of virus-laden particles in the air. Immediately after droplets are expired, the liquid content starts to evaporate, and some droplets become so small that transport by air current affects them more than gravitation. Such small droplets are free to travel in the air and carry their viral content meters and tens of meters from where they originated (e.g. Morawska et al. 2009)


Droplet and Virus Exhalation and dispersal

There are at least two different models of exhalation and virus laden droplet dispersal

The “two droplet” model was published in 19xx. Exhaled virus laden droplet were either large or small.

A more recent model, by Bouriba is a “gaseous cloud dispersal”.

https://jamanetwork.com/journals/jama/fullarticle/2763852

Measurement of particles In the following I cite articles with measurements in microns.

How small is a micron?

A micron is one-millionth of a meter or one twenty-five thousandth of an inch. IN other words, 25,000 microns is an inch and 1,000,000 microns is a meter .

What’s a nanometer?

The corona virus itself is about 250 nanometers.

A nanometer is one billionth of a meter, 0.000000001 or 10-9 meters In other words, one billion nanometers (1,000,000,000 ) is 1 meter. A nanometer is 1/(1000) of a micron.








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IN other words, a thousand nanometers (1,000) is one micron. And 250 nanometers is 0.25 microns

Unit notation

The nanometer unit is nm. The micron unit is µ (Greek “mu”). The meter unit is m.


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Medical procedures that generate droplets

• intubation and related procedures (e.g. manual ventilation, suctioning) • cardiopulmonary resuscitation • bronchoscopy • surgery and autopsy.

Droplet Evaporation

Excerpt (Wells, 1934)

In the classic study of airborne transmission, Wells (1934) was able to identify the difference between disease transmission via large droplets and by airborne routes. Wells found that, under normal air conditions, droplets smaller than 100 μm in diameter would completely dry out before falling approximately 2 m to the ground. This finding allowed the establishment of the theory of droplets and droplet nuclei transmission depending on the size of the infected droplet. The Wells evaporation-falling curve of droplets (see Figure C.2) is important in understanding airborne transmission and transmission by large droplets. Wells' study also demonstrated that droplets could transform into droplet nuclei by evaporation.

Movement of Air Droplet nuclei floating on the air may be carried by the movement of air. Entrainment of air into neighbouring airspaces may occur during the most innocuous daily activities; for example, as a result of people walking, or the opening of a door between a room and the adjacent corridor or space (Hayden et al., 1998; Edge, Paterson & Settles, 2005; Tang et al., 2005, 2006). In addition, the air temperature (and therefore air density) differences across an open doorway will also cause air exchange to occur between the two areas, providing a second mechanism to allow air into other areas (Tang et al., 2005, 2006) (see Figure C.3).

Even a patient simply sitting in or beside the bed will create air temperature differences from their body heat. A higher air temperature directly above the patient's head (or body, if lying down) will create convective air currents that may entrain potentially infectious air from neighbouring spaces into the higher temperature column rising air above the patient (Craven & Settles, 2006). Patients lying in bed, breathing or sleeping, may produce exhaled airflows that can reach the airspace of a patient in the neighbouring bed, and even further in the presence of certain types of ventilation systems (see below) (Qian et al., 2006). In the same way, other mechanical devices, including fans, televisions and medical

Sources

https://www.ncbi.nlm.nih.gov/books/NBK143281/















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Masks Possible Origin of “6 foot distancing”

The WHO, CDC and Dr. Anthony Fauci (NIAID) all recommend 6 foot social distancing. Specific citations for that distance do not appear to be available from the agencies. In a careful search I found an often-cited paper, ( Wells 1934), that discusses droplet size, and time and distance to ground, and specifically notes “2 meters’ (approximately 6 feet).

Droplet Diameter vs time to reach ground

Sources Wells 1934

Additional corroborating evidence for 6 foot distancing. While there are no specific citations for the 6 foot distancing, by careful literature searching there may be corroborating evidence for 6 foot distancing based on research in hospital Tuberculosis wards (Riley, 1957). Tuberculosis is highly infective by airborne mode.


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Aerosols vs Droplets vs Droplet Nuclei In general, infected people spread viral particles whenever they talk, breathe, cough, or sneeze. Such viral particles are known to be encapsulated in globs of mucus, saliva, and water, and the fate/behavior of globs in the environment depends on the size of the globs. Bigger globs fall faster than they evaporate so that they splash down nearby in the form of droplets (Grayson et al., 2016; Liu et al., 2016). Smaller globs evaporate faster in the form of aerosols, and linger in the air, and drift farther away than the droplets do.




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WHO and field experiments

A central point in the WHO’s caution in revising the guidelines is that

….Scientists have not been able to grow the coronavirus from aerosols in the lab. But that doesn’t mean aerosols are not infective, Dr. Marr said: Most of the samples in those experiments have come from hospital rooms with good air flow that would dilute viral levels….

Respiratory particles may often be distinguished to be droplets or aerosols based on the particle size and specifically in terms of the aerodynamic diameter (Hinds, 1999).

Aerosols

Strictly speaking, ‘aerosols’ refer to particles in suspension in a gas, such as small droplets in air. There have been numerous publications classifying droplets using particle sizes over the years [5,6,7,8,9,10]. For example it is generally accepted that: i) small particles of < 5–10 μm aerodynamic diameter that follow airflow streamlines are potentially capable of short and long range transmission; particles of < 5 μm readily penetrates the airways all the way down to the alveolar space, and particles of < 10 μm readily penetrates below the glottis (7) ii) large droplets of diameters > 20 μm refer to those that follow a more ballistic trajectory (i.e. falling mostly under the influence of gravity), where the droplets are too large to follow inhalation airflow streamlines. For these particle sizes, for example, surgical masks would be effective, as they will act as a direct physical barrier to droplets of this size that are too large to be inhaled into the respiratory tract around the sides of the mask (which are not close-fitting); iii) ‘intermediate particles’ of diameters 10–20 μm, will share some properties of both small and large droplets, to some extent, but settle more quickly than particles < 10 μm and potentially carry a smaller infectious dose than large (> 20 μm) droplets.

Source

https://bmcinfectdis.biomedcentral.com/articles/10.1186/s12879-019-3707-y

Droplet Nuclei

Aerosols’ would also include ‘droplet nuclei’ which are small particles with an aerodynamic diameter of 10 μm or less, typically produced through the process of rapid desiccation of exhaled respiratory droplets [5, 6]. However, in some situations, such as where there are strong ambient air cross-flows, for example, larger droplets can behave like aerosols with the potential to transmit infection via this route (see next section below)





https://bmcinfectdis.biomedcentral.com/articles/10.1186/s12879-019-3707-y#ref-CR5











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Kelvin Effect

The effect whereby the vapour pressure over a curved surface (e.g. that of a water droplet) is greater than that over a flat plane. The smaller the radius, the greater the vapour pressure. The effect is particularly important for the process of condensation and for small droplets (such as those in clouds).

Example (excerpt)

The ratio shown in figure 3 applies for droplets of initial diameter as small as approximately 0.5 mm, at which point the Kelvin effect becomes important. Figure 3 shows that at 100% RH, there is no evaporation, and the ratio is 1. At 90% RH, the droplets are dramatically smaller at equilibrium, 0.28 or 0.51 of their initial size for low and high-protein contents (3 mg ml21 and 76 mg ml21), respectively. At 50% RH, these values are 0.19 and 0.41, respectively. For comparison, Liu et al. [70] predicted that dried droplet nuclei would be 0.32 of their original diameter. In fact, at RH below 64% for the low-protein droplets and below 42% for the high-protein droplets, the equilibrium diameter is unchanged because all bulk liquid water has been lost and only the solutes, or

what some have called the droplet nucleus, remain.

…The effect of surface tension (the Kelvin effect) on the equilibrium radius of an aqueous solution drop at fractional relative humidity h less than unity is analyzed mathematically….

Sources

. Marr LC, Tang JW, Van Mullekom J, Lakdawala SS. Mechanistic insights into the effect of humidity on airborne influenza virus survival, transmission and incidence. J R Soc Interface 2019; 16(150)

Lewis, Ernie R. "The effect of surface tension (Kelvin effect) on the equilibrium radius of a hygroscopic aqueous aerosol particle." Journal of aerosol science 37.11 (2006): 1605-1617.

https://www.oxfordreference.com/view/10.1093/oi/authority.20110803100033105

Inhalation And Deposition of Droplets

https://www.oxfordreference.com/view/10.1093/oi/authority.20110803100033105


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Speech generated droplets of 20 to 500 microns.

Sources

https://www.nejm.org/doi/pdf/10.1056/NEJMc2007800

Droplets may be deposited in the nasal passages, and upper airway passages. Aerosolized particles can penetrate deep into the lungs and deposited in the alveoli.

Mucociliary escalator

Excerpting

Inhaled droplets are deposited in the upper regions of the respiratory tract, from which they may be removed in nasal secretions or carried upward by the mucociliary escalator, to be expelled or swallowed. In contrast, inhaled aerosolized particles can penetrate to the depths of the lungs, where they may be deposited in the alveoli. Experimentally (laboratory) produced Aerosols source van Doremalen N, Bushmaker T, Morris DH, et al. Aerosol and surface stability of SARS-CoV-2 as compared with SARSCoV-1. N Engl J Med 2020; 382:1564-7.

https://www.nejm.org/doi/pdf/10.1056/NEJMc2007800


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Virus Penetration The Infectious Diseases Society of America (IDSA) has proposed a scheme that is essentially equivalent [7], defining “respirable particles” as having a diameter of 10 μm or less; and “inspirable particles” as having a diameter between 10 μm and 100 μm, nearly all of which are deposited in the upper airways. Some authors have proposed the term “fine aerosols”, consisting of particles of 5 μm or less, but this has been in part dictated by constraints from measurement instruments [8]. Several authors lump together transmission by either large droplets or aerosol-sized particles as “airborne transmission” [9], or use “aerosol transmission” to describe pathogens that can cause disease via inspirable particles of any size [10].

Source https://bmcinfectdis.biomedcentral.com/articles/10.1186/s12879-019-3707-y

Droplets, Viral payload and infection

There has been no discernible evidence on the minimum infectious viral load for COVID-19 pandemic, but many researchers speculate that a few hundreds of SARS-CoV-2 virus would be enough to cause the disease among susceptible hosts (Beggs, 2020; SMC, 2020).

What is an aerosol?

For easy apprehension, aerosols can be defined as suspensions of solid or liquid particles in the air, which can be generated by either natural or anthropogenic phenomena (Judson and Munster, 2019; Tellier, 2009).

How big is an aerosol particle?

First, not everyone will agree on measurement of tiny aerosols and .

Dr. Morawska and others pointed to several incidents that indicate airborne transmission of the virus, particularly in poorly ventilated and crowded indoor spaces. They said the W.H.O. was making an artificial distinction between tiny aerosols and larger droplets, even though infected people produce both.

Respiratory particles may often be distinguished to be droplets or aerosols based on the particle size and specifically in terms of the aerodynamic diameter (Hinds, 1999). One could dispute that, unlike larger droplets, aerosols may pose a greater risk of the spread of the COVID-19 disease among many susceptible hosts positioned far from the point of origin. Nevertheless, it has been proven that viral disease outbreaks via aerosol transmission are not as severe as one would think, because of dilution and inactivation of viruses that linger for extended periods in the air (Shiu et al., 2019). There has been no discernible evidence on the minimum infectious viral load for COVID-19 pandemic, but many researchers speculate that a few hundreds of SARS-CoV-2 virus would be enough to cause the disease among susceptible hosts (Beggs, 2020; SMC, 2020).











https://www.nytimes.com/2020/04/20/health/airflow-coronavirus-restaurants.html

https://news.sky.com/story/coronavirus-circulating-air-may-have-spread-covid-19-to-1-500-german-meat-plant-staff-12014156






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Droplets, aerosols, droplet nuclei, Droplet diameter and Aerodynamic diameter

There have been numerous disagreements on the average particle size of droplets and aerosols (Shiu et al., 2019). The World Health Organization (WHO) and Centers for Disease Control and Prevention (CDC) postulate that the particles of more than 5 μm as droplets, and those less than 5 μm as aerosols or droplet nuclei (Siegel et al., 2007; WHO,

2014). Conversely, there have been some other postulations, indicating that aerodynamic diameter of 20 μm or 10 μm or less should be reckoned to be aerosols, based on their ability to linger in the air for a prolonged period, and the reachability to the respirable fraction of the lung (alveolar region) (Gralton et al., 2011; Nicas et al., 2005; Tellier, 2009). Small aerosols are more susceptible to be inhaled deep into the lung, which causes infection in the alveolar tissues of the lower respiratory tract, while large droplets are trapped in the upper airways (Thomas, 2013). For easy apprehension, aerosols can be defined as suspensions of solid or liquid particles in the air, which can be generated by either natural or anthropogenic phenomena (Judson and Munster, 2019; Tellier, 2009).

Super spreader events?

Experts all agree that the coronavirus does not behave that way. Dr. Marr and others said the coronavirus seemed to be most infectious when people were in prolonged contact at close range, especially indoors, and even more so in super spreader events — exactly what scientists would expect from aerosol transmission.

Stokes Law One of the standard rules (Stoke’s Law) applied in engineering calculations to estimate the suspension times of droplets falling under gravity with air resistance, was derived assuming several conditions including that the ambient air is still [13,14,15,16,17]. So actual suspension times will be far higher where there are significant cross-flows, which is often the case in healthcare environments, e.g. with doors opening, bed and equipment movement, and people walking back and forth, constantly. Conversely, suspension times, even for smaller droplet nuclei, can be greatly reduced if they encounter a significant downdraft (e.g. if they pass under a ceiling supply vent). In addition, the degree of airway penetration, for different particle sizes, also depends on the flow rate.

Source


Blood Splatter In the field of dentistry and orthopedics, where high-powered electric tools are used, even bloodborne viruses (such as human immunodeficiency virus – HIV, hepatitis B and hepatitis B viruses) can become airborne when they are contained in high velocity blood splatter generated by these instruments [18, 19]. Yet, whether they can cause efficient transmission via this route is more debatable. This illustrates another point, that although some pathogens can be airborne in certain situations, they may not necessarily transmit infection and cause disease via this route.











https://www.nytimes.com/2020/06/30/science/how-coronavirus-spreads.html









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Source


World Health Organization - WHO

WHO Is in the process of revising its guidance about virus aerosol transmission, technically called the “virus transmission vector”. WHO modified its guidance about airborne/aerosolized virus effective today. Possibly more revisions may occur.

Until the announcement yesterday of an open letter to WHO signed by 239 scientists, the virus transmission vector was believed to be coughs, or sneezes of droplets (aka aerosols). That has been the theory since about 1940. WHO has acknowledged the letter and modified its guidance. Note CDC has not yet modified its guidance.

Some of the research efforts for SARS/COV2 (the virus) reveal that the virus may stay airborne. These are based on laboratory, wind-tunnel and similar experiments. Most important is that the virus may be airborne for more than 6 feet "social distance". A human created droplet is typically water or water/mucous. The belief was the droplets descended to the ground in approximately 6 feet. Six feet is an oversimplification in the sense that particles do not simply float to say 5, feet 11.99 inches, then drop directly to the ground.

By airborne, the "virion" can stay airborne and circulate through say, hospital ventilation systems or regular air conditioning systems. An aerosol is a technical term for the virion in a water /mucous droplet with a diameter measured in microns. Airborne can mean the droplet evaporated, or that the virus particle alone can be dispersed through the air. Its a huge difference, and the WHO (in my professional opinion) may be in the process of issuing new guidance about mask wearing and possibly other steps to protect from the virus.

Sources


Social Distancing Potential Implications of the WHO revised guidance

The coronavirus is finding new victims worldwide, in bars and restaurants, offices, markets and casinos, giving rise to frightening clusters of infection that increasingly confirm what many scientists have been saying for months: The virus lingers in the air indoors, infecting those nearby.

If airborne transmission is a significant factor in the pandemic, especially in crowded spaces with poor ventilation, the consequences for containment will be significant. Masks may be needed indoors, even in socially-distant settings. Health care workers may need N95 masks that filter out even the smallest respiratory droplets as they care for coronavirus patients.


https://www.nytimes.com/2020/07/04/health/coronavirus-neanderthals.html

https://www.nytimes.com/2020/07/09/health/virus-aerosols-who.html

https://www.nytimes.com/2020/07/06/podcasts/the-daily/coronavirus-science-indoor-infection.html


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Ventilation systems in schools, nursing homes, residences and businesses may need to minimize recirculating air and add powerful new filters. Ultraviolet lights may be needed to kill viral particles floating in tiny droplets indoors.

Airborne Transmission overview

Source

In summary, despite the various mechanistic arguments about which organisms can be potentially airborne and therefore aerosol-transmissible, ultimately, the main deciding factor appears to be how many studies using various differing approaches: empirical (clinical, epidemiological), and/or experimental (e.g. using animal models), and/or mechanistic (using airflow tracers and air-sampling) methods, reach the same consensus opinion. Over time, the scientific community will eventually form an impression of the predominant transmission route for that specific agent, even if the conclusion is one of mixed transmission routes, with different routes predominating depending on the specific situations. This is the case for influenza viruses, and is likely the most realistic.

Some bacterial and viral infections that have more than one mode of transmission are also anisotropic, like anthrax, plague, tularemia and smallpox: the severity of the disease varies depending on the mode of transmission [37, 89]. Older experimental infection experiments on volunteers suggest that this is the case for influenza, with transmission by aerosols being associated with a more severe illness [14, 90], and some more recent field observations are consistent with this concept [57]. For anisotropic agents, even if a mode of transmission (e.g. aerosols) accounts for only a minority of cases, interruption of that route of transmission may be required if it accounts for the most severe cases.


No Evidence of Effect is not evidence of no effect The distinction between “no evidence of effect” and “no evidence of effect” is not “evidence of no effect” needs to be understood.

“There is no incontrovertible proof that SARS-CoV-2 travels or is transmitted significantly by aerosols, but there is absolutely no evidence that it’s not,” said Dr. Trish Greenhalgh, a primary care doctor at the University of Oxford in Britain.

Multivariate probability distribution of virus transmission

There are many ‘uncertainties” about the virus transmission. Simplified this includes and is not limited to








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• Probability distribution of exhalation droplets (volume • Distributions of virus per exhaled droplets • Exhaled Droplet size • Exhaled Droplet composition • Amount of virus in each exhaled droplet • Viability of virus in exhaled droplet • Distance travelled by exhaled droplet • Degradation of virus in exhaled droplet during transmission

Sources Hinds W.C. second ed. John Wiley & Sons; Hoboken, New Jersey, USA: 1999. Aerosol Technology: Properties, Behavior, and Measurement of Air Borne Particles. [Google Scholar]


Judson S.D., Munster V.J. Nosocomial transmission of emerging viruses via aerosol-generating medical procedures. Viruses. 2019;11:940. doi: 10.3390/v11100940.

Tellier R., Li Y., Cowling B.J., Tang J.W. Recognition of aerosol transmission of infectious agents: a commentary. BMC Infect. Dis. 2019;19 doi: 10.1186/s12879-019-3707-y.

Chartier, Y., and C. L. Pessoa-Silva. Natural ventilation for infection control in health-care settings. World Health Organization, 2009.

Sources Masks https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(20)31142-9/fulltext

Chu, Derek K., et al. "Physical distancing, face masks, and eye protection to prevent person-to-person transmission of SARS-CoV-2 and COVID-19: a systematic review and meta-analysis." The Lancet (2020).

Sources WHO

https://scholar.google.com/scholar?q=Hinds+W.C.+Aerosol+Technology:+Properties,+Behavior,+and+Measurement+of+Air+Borne+Particles+second+ed.+1999+John+Wiley+&+Sons+Hoboken,+New+Jersey,+USA+

https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(20)31142-9/fulltext


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https://www.who.int/emergencies/diseases/novel-coronavirus-2019/question-and-answers-hub/q-a-detail/q-a-on-covid-19-and-masks

https://www.who.int/publications/i/item/advice-on-the-use-of-masks-in-the-community-during-home-care-and-in-healthcare-settings-in-the-context-of-the-novel-coronavirus-(2019-ncov)-outbreak



https://www.who.int/publications/i/item/advice-on-the-use-of-masks-in-the-community-during-home-care-and-in-healthcare-settings-in-the-context-of-the-novel-coronavirus-%282019-ncov%29-outbreak

https://www.who.int/publications/i/item/advice-on-the-use-of-masks-in-the-community-during-home-care-and-in-healthcare-settings-in-the-context-of-the-novel-coronavirus-%282019-ncov%29-outbreak


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References All

Brewster, David J., et al. "Consensus statement: Safe Airway Society principles of airway management and tracheal intubation specific to the COVID-19 adult patient group." Med J Aust 16 (2020).

Dhand, Rajiv, and Jie Li. "Coughs and Sneezes: Their Role in Transmission of Respiratory Viral Infections, Including SARS-CoV-2." American Journal of Respiratory and Critical Care Medicine ja (2020). Feng, Shuo, et al. "Rational use of face masks in the COVID-19 pandemic." The Lancet Respiratory Medicine 8.5 (2020): 434-436. Frieden, Thomas R., and Christopher T. Lee. "Identifying and interrupting superspreading events—implications for control of severe acute respiratory syndrome coronavirus 2." (2020). Hendrix, M. Joshua. "Absence of Apparent Transmission of SARS-CoV-2 from Two Stylists After Exposure at a Hair Salon with a Universal Face Covering Policy—Springfield, Missouri, May 2020." MMWR. Morbidity and Mortality Weekly Report 69 (2020). Hinds, William C. Aerosol technology: properties, behavior, and measurement of airborne particles. John Wiley & Sons, 1999. Hui, David S., et al. "Exhaled air dispersion during coughing with and without wearing a surgical or N95 mask." Plos one 7.12 (2012): e50845. Jayaweera, Mahesh, et al. "Transmission of COVID-19 virus by droplets and aerosols: A critical review on the unresolved dichotomy." Environmental Research (2020): 109819. Johnson, D. F., et al. "A quantitative assessment of the efficacy of surgical and N95 masks to filter influenza virus in patients with acute influenza infection." Clinical Infectious Diseases 49.2 (2009): 275-277. Marr, Linsey C., et al. "Mechanistic insights into the effect of humidity on airborne influenza virus survival, transmission and incidence." Journal of the Royal Society Interface 16.150 (2019): 20180298. Morawska, Lidia, and Donald K. Milton. "It is time to address airborne transmission of covid-19." Clinical Infectious Diseases (2020). https://academic.oup.com/cid/article/doi/10.1093/cid/ciaa939/5867798



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Miller, Shelly L., et al. "Transmission of SARS-CoV-2 by inhalation of respiratory aerosol in the Skagit Valley Chorale superspreading event." medRxiv (2020). Riley, Richard L., et al. "Air Hygiene in Tuberculosis: Quantitative Studies of Infectivity and Control in a Pilot Ward: A Cooperative Study Between the Veterans Administration, The Johns Hopkins University School of Hygiene and Public Health, and the Maryland Tuberculosis Association." American Review of Tuberculosis and Pulmonary Diseases 75.3 (1957): 420-431. Tellier, Raymond, et al. "Recognition of aerosol transmission of infectious agents: a commentary." BMC infectious diseases 19.1 (2019): 101. Van Doremalen, Neeltje, et al. "Aerosol and surface stability of SARS-CoV-2 as compared with SARS-CoV-1." New England Journal of Medicine 382.16 (2020): 1564-1567. Woolhouse ME, Dye C, Etard JF, Smith T, Charlwood JD, Garnett GP, et al. Heterogeneities in the transmission of infectious agents: implications for the design of control programs. Proc Natl Acad Sci U S A. 1997;94:338–42. CDC https://www.cdc.gov/mmwr/volumes/69/wr/mm6928e2.htm?s_cid=mm6928e2_w

New York Times

Most People With Coronavirus Won’t Spread It. Why Do a Few Infect Many?, New York Times, June 30, 2020.

https://www.nytimes.com/2020/06/30/science/how-coronavirus-spreads.html Washington Post

The outbreak that didn’t happen: Masks credited with preventing coronavirus spread inside Missouri hair salon. Washington Post, June 17, 2020

https://www.washingtonpost.com/business/2020/06/17/masks-salons-missouri/

“Spate of new research supports wearing masks to control coronavirus spread”, Washington Post ,June 13, 2020

https://www.washingtonpost.com/health/2020/06/13/spate-new-research-supports-wearing-masks-control-coronavirus-spread/

WHO


https://www.cdc.gov/mmwr/volumes/69/wr/mm6928e2.htm?s_cid=mm6928e2_w

https://www.nytimes.com/2020/06/30/science/how-coronavirus-spreads.html

https://www.washingtonpost.com/business/2020/06/17/masks-salons-missouri/





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