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6' "distancing": Myth? - Printable Version

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6' "distancing": Myth? - M T - 04-19-2020

I recently brought up the question of the temporal correspondence of the 6' distancing rule.   We're being told (just like we were told that we didn't need masks) that we need to keep at least 6' from others.  We, or at least I, presumed that meant that if we were 6' away, that we could more or less maintain that distance with minimal chance of getting infected from that person just over 6' away.

My question was what if I'm 30' behind someone, walking on the same path, and they cough.  Am I safe when I get to the point they were at?
Or, correspondingly, what if there is a 3mph soft breeze (walking speed) blowing my direction from someone who coughed 30' away?

The report on the restaurant infections showed people being infected apparently by the breeze from an air conditioner and its return flow, across a 6 meter room.  The conclusion was that the particles had traveled up to 9 meters (about 30') in the breeze of the AC,  and been concentrated enough to infect a person.   That led me to wonder just how long it took for that air to flow that far, and how long do the droplets in a cough float.

I don't like the answers.

Review of Aerosol Transmission of Influenza A Virus
Quote:Coughing or sneezing generates a substantial quantity of particles, a large number of which are <5–10 μm in diameter. In addition, particles expelled by coughing or sneezing rapidly shrink in size by evaporation, thereby increasing the number of particles that behave as aerosols. Particles shrunken by evaporation are referred to as droplet nuclei. This phenomenon affects particles with a diameter at emission of <20 μm, and complete desiccation would decrease the diameter to a little less than half the initial diameter

For spherical particles of unit density, settling times (for a 3-m fall) for specific diameters are 10 s for 100 μm, 4 min for 20 μm, 17 min for 10 μm, and 62 min for 5 μm; particles with a diameter <3 μm essentially do not settle. Settling times can be further affected by air turbulence.

I elided a paragraph about how the small sizes (aerosols) get pulled into the lower respiratory tract while larger droplets don't make it that far.

"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."

I was asked, "Don't the little eddies as you walk disperse the droplets?"  My answer was that the air you inhale comes from somewhere.

Factors involved in the aerosol transmission of infection and control of ventilation in healthcare premises
Quote:A sneeze can generate up to 40 000 droplets which can evaporate to produce droplets of 0.5–12 μm in diameter. A cough can generate about 3000 droplet nuclei, the same number as talking for 5 minutes. During normal breathing, exhalation can project droplets up to 1 m in room air, which may be inhaled by another person nearby whereas sneezing can project droplets several metres

The Size and Concentration of Droplets Generated by Coughing in Human Subjects
Quote:Results indicated the total average size distribution of the droplet nuclei was 0.58–5.42 μm, and 82% of droplet nuclei centered in 0.74–2.12 μm. The entire average size distribution of the coughed droplets was 0.62–15.9 μm, and the average mode size was 8.35 μm. The size distribution of the coughed droplets was multimodal. The size distribution of coughed droplets showed three peaks at approximately 1 μm, 2 μm, and 8 μm.
(droplet nuclei is the size of the dehydrated droplet)

The larger droplets comprise more virus than the smaller droplets.

What these articles don't indicate is how dispersed the droplets become over time.  The more dispersion, the more volume they affect, but also the lower the number of droplets you inhale.  I can imagine that becomes very dependent on small currents (generated by breathing, convection from warm bodies or warm food, or caused by movements of people).

I will remind you that the restaurant infections was said to suggest that aerosols (floating around the 6x18 meter room) didn't infect people in the other half the room.

I might just start taking 5AM walks (for enveloped viruses, higher humidity tends to inactivate viruses earlier), or continue walking in my backyard...

RE: 6' "distancing": Myth? - OutsiderFan - 04-20-2020

If face coverings aren’t worn, the virus spreads very easily in the air. And it doesn’t take coughing to spread it, though that and sneezing certainly sends it farther. There are plenty of proof points that show how easily it spreads in air and HVAC systems. Even just talking will send tiny particles into the air if mouth isn’t covered.

Everyone should cover mouth and nose when there is any possibility of encountering another person, even when outside walking a neighborhood or on trails.

RE: 6' "distancing": Myth? - oldalum - 04-20-2020

Nothing binary about inside or outside 6 feet; it's all a continuum of risk. And 6 feet is for someone who is stationary. Cyclists, runners, and walkers all have a slipstream that extends further. And no study has rigorously tested actual infectivity, just behavior of droplets and aerosols. Here's a pretty literate article on it, that discusses the bicycling/running pre-study that went viral: article

RE: 6' "distancing": Myth? - stupac2 - 04-20-2020

If you could actually get sick by walking 30' behind someone who has it, the R0 would be a lot higher. My understanding has never been that 6' is safe if it's sustained for a long time, but rather that it's a rule for thumb for transient interactions.

So it's not terribly surprising to me to see that sustained exposure (like in a restaurant) could spread it past those distances, but it would be surprising to me to see that walking behind someone outdoors could spread it, beyond "one in a million" type bad luck.

RE: 6' "distancing": Myth? - oldalum - 04-20-2020

30 feet is probably ridiculously safe. But the study referenced in the preprinted study discussed in the link above found a trail of particles 30 feet behind a cyclist going fast, which makes sense because it only takes a couple of seconds for a cyclist to traverse that distance at that speed. What I found interesting in the aerodynamics study is that the trail left behind a fast cyclist is only as wide as his or her shoulders.I wouldn't ride right behind another rider, you're inhaling too much of what they are exhaling.  And I wouldn't run right behind another runner, either. 

RE: 6' "distancing": Myth? - teejers1 - 04-20-2020

(04-20-2020, 10:06 AM)oldalum Wrote:  30 feet is probably ridiculously safe. But the study referenced in the preprinted study discussed in the link above found a trail of particles 30 feet behind a cyclist going fast, which makes sense because it only takes a couple of seconds for a cyclist to traverse that distance at that speed. What I found interesting in the aerodynamics study is that the trail left behind a fast cyclist is only as wide as his or her shoulders.I wouldn't ride right behind another rider, you're inhaling too much of what they are exhaling.  And I wouldn't run right behind another runner, either. 

Everyone should wear a mask at all times; even while sleeping at home.
We should never venture outside the confines of our dwellings.

It's the only way to be safe . . .

RE: 6' "distancing": Myth? - BostonCard - 04-20-2020

Your probability of becoming infected likely follows a sinusoidal dose/response relationship, where "dose" represents the number of viral particles you are exposed to.  At a high enough dose, the probability of being infected is nearly guaranteed; at a low enough dose, you are very unlikely to get sick.

The "dose" is dependent on a number of factors.  First off is how much the infected person is shedding.  Then it depends on where in the respiratory tract the virus is most shed (nasophayngeal shedding results means it is spread by droplets, while lower respiratory infections are aerosolized).  Then it depends a lot on how the virus filled particles are transported (which is based on whether they are breathed out or coughed or sneezed) as well as local ventilation patterns (indoor versus outdoor, winds or fans, etc).  The dose will be dependent on distance from the infected person to the susceptible person and by length of the contact, as well as other behaviors such as mask wearing (reduces risks to others, especially of large droplets generated while coughing/sneezine) and frequent hand washing, to minimize transmission from droplets that settled on the hands to the oropharynx.

All of this is to say that it's complicated and based on a lot of different variables.  Instead of looking at things as absolutes (safe/not safe) it is probably best looked at in a risk minimization sense; how can you minimize the risk to you and others.  Risks cannot be eliminated, but avoiding unnecessary contacts with others, maintaining as large a physical distance between you and others as possible, widespread use of face coverings, strict isolation of sick people, frequent hand-washing, etc. can all reduce risk.

BC