Does the six foot rule work indoors? The answer is in a smoke filled bar


When people imagine social disturbances, they usually think of the “6-foot rule”.

It is true that staying 6 feet away from other people can reduce the chances of a person having a cough with coronovirus-rich breaths in your eyes, nose or mouth. Most of these drops are very small to see, and people are pushing them out in the air all the time – when they scream, talk, or even just breathe.

But the 6-foot rule is not responsible for all risks, especially indoors.

Think of walking into a room where someone is smoking a cigarette. The closer you are to the cigarette, the stronger the smell – and the more you are breathing. That smoke drenches in the air. Over time, it will not matter where you are in the room. Smoke will be everywhere.

Cigarette smoke contains particles that are similar in size to small respiratory droplets emitted by humans – which remain in the air for the longest time. While this is not a complete analogy, showing how cigarette smoke passes through different environments, both indoors and outdoors, can help to see how virus-laden droplets swirl in the air.

As professors who study fluid dynamics and aerosols, we are exploring how Kovid-19 rotates and poses its risks. The 6-foot rule is a good benchmark that is easy to remember, but it is important to understand its limitations.

86 year old rule

The 6-foot rule goes back to a paper published in 1934 by William F. Wells, who was studying how tuberculosis spreads. Wells hypothesized that small breath droplets evaporate quickly, while larger ones rapidly fall to the ground, following a ballistic-like trajectory. They found that the travel of any of the droplets was about 6 feet before going away or evaporation.

While that distance may reduce risk, it does not provide a complete picture of the risk of infection with the SARS-CoV-2 virus.

When people exhale, they expel respiratory droplets with a wide range of sizes. Most are smaller than 10 microns in diameter. Due to evaporation, they can reduce to about 40% of their original diameter or be smaller.

However, the droplets will not evaporate completely. The reason for this is that they consist of both water and organic materials, possibly including the SARS-CoV-2 virus. These small droplets stay in the air for minutes, posing a danger to whoever comes in contact with them. When suspended in air, these droplets are commonly referred to as aerosols.

Computer models describe how respiratory droplets operate under different conditions. Credit: K. Liu, J. Salinas, M. Allahari, N. Zehib and S. Balachander / University of Florida.

Ventilation matters

The risk of infection is highest next to the person who has the virus and decreases with distance. However, the way in which the air droplets mix in the air and the resulting concentration affects the distance required to safely avoid exposure.

Outside, the combination of physical disturbances and face covering provides excellent protection against virus transmission. Think of being near a smoker again. Smoke can be carried more than 6 feet away by air, but the high concentration of smoke usually does not build up outside because the smoke is quickly diluted by large amounts of air. A highly effective strategy for avoiding smoker smoke is to avoid the smoker from being directly down. This is also true for respiratory drops.

Inside the house, the picture is very different.

Very light room air streams from fans and ventilation units can carry respiratory droplets more than 6 feet apart. However, unlike outdoor, most indoor locations have poor ventilation. This allows the concentration of small airborne respiratory droplets to build up over time, reaching all corners of a room.

A simulation shows the trajectories of droplets emitted by someone in a room with mixed ventilation. Sincerely: Gooderz Ahmadi and Mazar Salmanzadeh / Clarkson University.

When indoors, the risk of infection depends on variables such as the number of people in the room, room size, and ventilation rate. Loud, shouting or singing can also produce very large concentrations of droplets, increasing the risk of associated infection.

It is not surprising that most of the “Superspreader” events have infected a large number of people, including commercial conventions, crowded bars, funerals, and choir practice.

Strategies to stay safe

In pre-Kovid-19 bars, some people are concerned about respiratory infections from small virus-laden droplets that accumulate indoors because their virus load is usually the cause of the infection.

With SARS-CoV-2, the situation is different. Studies have shown that Kovid-19-positive patients, even those who are asymptomatic, carry more of the virus in their oral fluids. Respiratory infections are possible when air droplets, singing, and so on are inhaled by these patients during a conversation.

There is no safe distance in a poorly ventilated room, unfortunately. Good ventilation and filtration strategies that bring in fresh air are important to reduce aerosol concentration levels, such as opening windows to clean smoke-filled rooms.

In addition, mask or face covering should be worn in public indoor environments at all times. They both expel the concentration of respiratory droplets into the room and provide some protection against infectious aerosols.

Finally, because the risk of infection increases over time, it is also important to limit the amount of time spent inside public spaces.

The 6-foot Social Distancing Guideline is an important tool to counter the spread of Kovid-19. However, as more activities go indoors with the arrival of coolers this season, implementing safety measures that you can use to avoid cigarette smoke will be necessary.

Byron Arath is an Associate Professor of Mechanical Engineering and Andrea Farrow Professor of Civil and Environmental Engineering at Clarkson University. Gudrez Ahmadi is a Professor of Mechanical Engineering and Suresh Dhaniyala is a Baird D. Clarkson Distinguished Professor of Mechanical and Aeronautical Engineering at the same institute.

This article first appeared on Conversation.

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