Q: Can just wearing masks,  using hand sanitizers and maintaining physical distance protect us from Covid 19? I am getting invitations for celebrations, ceremonies, weddings etc.,  where lots of people gather. Can I attend these by taking the above mentioned precautions?

Krishna: The answer is "NO" according to virologists. They are proposing the Swiss cheese model for protection.

  The Swiss cheese model of protection From physical distancing to vaccines, there are many layers of defence that can protect us from COVID — but none of them is impenetrable. The multilayered ‘Swiss cheese’ model helps us to visualize how, when we combine all the strategies, no one hole lets the virus through. “It’s not really about any single layer of protection or the order of them, but about the additive success of using multiple layers, or cheese slices,” says virologist Ian Mackay, who has brought the Swiss cheese model to bear on COVID, illustrated with a stack of hole-riddled slices of the eponymous cheese. And the ‘the misinformation mouse’ can nibble away at any of those layers (1).

Avoiding crowded spaces is recommended. Avoiding  indoor activities where lots of people gather is advised. If you can't avoid them atleast limit your time there. And use big eye protection glasses too.

Okay, you wear a mask and use hand sanitisers. And maintain physical distance.  But do you know masks don't protect your eyes and the virus can enter your body through your eyes?

Do you know if the ventilation is not good, closed AC rooms are a big threat? 

Do the halls have lots of windows and doors? Are they open? How is the wind flowing? 

Air flow in the hall that affect fluid mechanics dictates how the virus spreads from person to person. 

They are too complex for common people to understand but here are a few details ....

Increasing evidence suggests that understanding airflows is important for estimation of the risk of contracting COVID-19. The data available so far indicate that indoor transmission of the virus far outstrips outdoor transmission, possibly due to longer exposure times and the decreased turbulence levels (and therefore dispersion) found indoors (2).

Transmission of respiratory diseases occurs via expiratory droplets produced by coughing, sneezing, speaking, singing and laughing.  The medical infectious disease community divides droplets into two classes: droplets larger than 5--10 μm in diameter are classified as respiratory droplets, whereas droplets smaller than 5 μm are referred to as aerosols. Droplets are considered to fall quickly to the floor close to the source, whereas aerosols are expected to remain airborne for long times. The cutoff between respiratory droplets and aerosols is somewhat arbitrary: in practice, droplets larger than 5 μm can remain in suspension for long times allowing them to be recirculated within or removed from the room.

Recent evidence suggests that asymptomatic/presymptomatic airborne transmission, particularly in crowded indoor environments, cannot be ruled out. At the early stage of the disease, upper respiratory tract symptoms and the presence of high concentrations of SARS-CoV-2 virus in oral fluids are common , supporting recent findings identifying speech droplets to be a potential cause of transmission. Conversational speech produces a wide range (submicron up to O(100 μm) of droplets) while the majority of aerosol particles in exhaled breath are <5 μm . However, the viral load associated with different aerosol sizes is unknown.

When droplets are exhaled they evaporate at a rate that depends on droplet size and composition, and the relative humidity and temperature of the air. The final size of exhalation droplets depends upon many factors including the initial size, non-volatile content, relative humidity, temperature, ventilation flow and the residence time of the droplet.

Consequently, the air flow patterns within a space are crucial for determining the distribution, transport and fate of any airborne contaminants. Predicting these flow patterns is extremely challenging since they depend critically on both the boundary conditions (e.g. the location of inlet and outlet vents) and on the internal dynamics of the fluid, particularly buoyancy forces associated with temperature differences. This should be contrasted to, say, aerospace where flow round an aerofoil does not depend on the dynamics of the air, and geophysical fluid dynamics where boundary conditions are often unimportant. Further, flows in buildings and other enclosed or semi-enclosed spaces often take place in very complex geometries, making computation of these turbulent flows particularly challenging.

Various forms of ventilation that affect the viral spread:  various typical forms of ventilation: mixing ventilation, natural and mechanical displacement ventilation, and wind-driven ventilation.

How people talk, laugh or cough all effect viral particle movement. Also how people move, walk too dictate these terms. Then temperature and humidity of the space too are vital. 

So  room flows are ‘turbulent’ in the sense that spatiotemporal variations of the flow are larger than the mean flow. They take place in complex geometries where the placement and sizes of inlets and outlets determine overall flow patterns, superimposed on which are significant perturbations associated with often transient events such as the movement of occupants, the opening and closing of doors, and (for naturally ventilated buildings) variations in the external conditions. The dispersal of a second phase in such an environment is complicated, as droplets are released over a continuum of sizes and they evaporate and reduce in size with time. However, the scientific analysis suggests that airborne transmission of the virus can occur in particles with fall speeds that are lower than typical velocities found in the room and so are advected through the space effectively like a passive tracer.

In that case it seems reasonable to consider  as a marker for air that has been exhaled. Indeed, it has been shown that CO2 concentration can be linked to the probability of infection.

Keeping two meters apart might not be far enough to stop the spread of coronavirus from sneezes and coughs, according to a new study (3). Researchers at Loughborough University have created a mathematical model which shows that droplets can reach more than 3.5 meters, without a facemask, significantly increasing the distance needed to stay safe. Researchers found that the largest droplets consistently travelled further than two meters. It is due to a phenomenon known as a buoyant vortex—the turbulent motion of hot, dense air that we eject together with the droplets when we cough or sneeze. The paper also suggests that the trajectories of the droplets are significantly affected by the way people tilt their heads when they cough or sneeze.

Therefore, guidelines suggesting two meters physical distancing limits may not be adequate to prevent direct transmission via droplets of large size.

The model also shows that the smaller droplets are carried upwards by this mini-vortex and take a few seconds to reach a height of four meters. At these heights, building ventilation systems will interfere with the dynamics of the cloud and could become contaminated. It is apparent that tilting the head downward as we cough or sneeze, significantly decreases the range for the majority of droplet sizes.

 When several factors decide outcomes, they follow the interplay of scientific rules and routes and exactly fit into the reaction realities.

So if an infected and pre-symptomatic/asymptomatic person is attending the function you are attending, with an ill-fitting mask or without a mask, if it 's organised inside a hall, if the ventilation is poor, if there is a complex picture of air flow and fluid mechanics, if the temperature and humidity favours virus containing aerosols to move long distances, there is no guarantee that the virus cannot lodge inside your eyes or your nose/mouth while you are eating.

That is why the scientists are asking you to avoid crowded spaces. Listen to them. 

If you still want to take chances and spread/get the virus, it is up to you. 

But get ready for second and third waves of covid. Science  strikes back when you ignore it.

 (See below  updates to know how face shield is also useless)

Footnotes:

1. https://www.nytimes.com/2020/12/05/health/coronavirus-swiss-cheese-...

2. https://www.cambridge.org/core/journals/journal-of-fluid-mechanics/...

3. E. Renzi et al. Life of a droplet: Buoyant vortex dynamics drives the fate of micro-particle expiratory ejecta, Physics of Fluids (2020). DOI: 10.1063/5.0032591

Updates:

Face shields no match for sneeze vortex rings

Do face shields provide enough protection to the wearers against COVID-19 if they don't also wear a mask? Spoiler alert: no. But researchers at Fukuoka University in Japan are working to create face shields safe enough to be worn alone.

Sneeze vortex: Streamwise velocity distribution along the vertical cross section and the three dimensional vortex structure

In Physics of Fluids,, Fujio Akagi and colleagues describe their work to gain a better understanding of what happens to the airflow around a face shield when someone nearby sneezes. Sneezes are intriguing, because they produce a fluid phenomenon known as vortex rings.

"A vortex ring is a donut-shaped vortex that is generated by an instantaneous ejection of fluid from a circular orifice," said Akagi. "This resembles bubble rings made by dolphins."

These vortex rings can capture microscopic particles, which sneezing also generates. What happens when a face shield wearer is exposed to a sneeze from an infected person standing 1 meter (39.3 inches) in front of them?

"The vortex rings generated by the sneeze capture the microscopic droplets within the sneeze and transport them to the top and bottom edges of the face shield," said Akagi, adding that droplets travel to the face shield wearer quickly—within 0.5 to 1 second after the start of the sneeze.

If this arrival time is synchronized with inhalation, the shield wearer will inhale the droplets," Akagi said.

The researchers made three findings everyone should know:

First, droplets of sneezes are transported not only by the high velocity airflow caused by sneezing, but also by the vortex rings generated by sneezing.

Second, microscopic droplets transported by these vortex rings can get inside the shield through its top and bottom edges.

Third, face shields alone are not highly effective to prevent COVID-19 infection.

By gaining a better understanding of face shield weaknesses, researchers believe the protection can be enhanced by reducing the flow getting inside the shield.

"We are currently developing and demonstrating several improved shields," said Akagi. "We want to contribute to keeping people safe from infection, and believe that one day in the near future, medical workers will be able to prevent infection using only a face shield and a regular mask or, ideally, with only a face shield."

Source: "Effect of sneezing on the flow around a face shield," Physics of Fluids, aip.scitation.org/doi/10.1063/5.0031150

https://phys.org/news/2020-12-shields-vortex.html?utm_source=nwlett...

Qs People asked me based on this article:

Q: So how can we really end this pandemic?

Krishna: By following what scientists say .... wearing masks, keeping physically distanced, avoiding crowded situations, doing things outdoors more than indoors and washing hands frequently, well ventilating all public indoor spaces and of course taking vaccines. 

 Q: Can we go out attending functions as soon as  we take vaccines?

Krishna: NO! Your system needs time to develop immunity. That varies from person to person. Then you will have to take the second dose too.

So you still have to take all precautions.