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Answer by Wei Dai200

Another reference (being reported in the news): Turbulent Gas Clouds and Respiratory Pathogen Emissions - Potential Implications for Reducing Transmission of COVID-19

Given various combinations of an individual patient’s physiology and environmental conditions, such as humidity and temperature, the gas cloud and its payload of pathogen-bearing droplets of all sizes can travel 23 to 27 feet (7-8 m). [...] Eventually the cloud and its droplet payload lose momentum and coherence, and the remaining droplets within the cloud evaporate, producing residues or droplet nuclei that may stay suspended in the air for hours, following airflow patterns imposed by ventilation or climate-control systems.

So indoors basically no distance is safe, outdoors maybe 10 meters is safe if people aren't up/down wind of each other.

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6Raemon
Thanks, this is great. I think the main followup question I have is "what's the rate of falloff for outdoors?" (given that my goal here is not "100% safe", its "the risk is comparable (i.e. within a factor of 2ish) to the usual background default level of micromorts", for the range of stuff humans typically do.)
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I'm not sure where the 6' number comes from, and I'm skeptical it really holds up as something I'd be comfortable maintaining for an extended period of time (If someone with c19 coughed at me from 6' away I would not feel very safe). I'm guessing the 6' is more like a quick rule for people who are only interacting briefly.

How much does it matter whether you're up/downwind? I've heard conflicting things about how airborne it might be.

I'm interested in this largely for "Okay, assuming we need to be careful about this for months at a time, what sort of practices could we use to maintain in-person social ties, indefinitely, without risk?" (i.e. going on long walks, visiting each other's house where 1-2 people hang out in the street or sidewalk and house denizens hang out on the porch, etc)

I'm guessing this has separate answers for "outdoor" and "indoor." 

Current Answers:

  • Indoors: Basically it's not safe.
  • Outdoors: 10 meters (30 feet) seems safe, if nobody is upwind/downwind of each other. But I'm unclear what the falloff range is.
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Wei Dai

200

Another reference (being reported in the news): Turbulent Gas Clouds and Respiratory Pathogen Emissions - Potential Implications for Reducing Transmission of COVID-19

Given various combinations of an individual patient’s physiology and environmental conditions, such as humidity and temperature, the gas cloud and its payload of pathogen-bearing droplets of all sizes can travel 23 to 27 feet (7-8 m). [...] Eventually the cloud and its droplet payload lose momentum and coherence, and the remaining droplets within the cloud evaporate, producing residues or droplet nuclei that may stay suspended in the air for hours, following airflow patterns imposed by ventilation or climate-control systems.

So indoors basically no distance is safe, outdoors maybe 10 meters is safe if people aren't up/down wind of each other.

[-]Raemon60

Thanks, this is great.

I think the main followup question I have is "what's the rate of falloff for outdoors?" (given that my goal here is not "100% safe", its "the risk is comparable (i.e. within a factor of 2ish) to the usual background default level of micromorts", for the range of stuff humans typically do.)

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Wei Dai

150

From "A choir group had 60 people show up for practice. Now 45 are sick.":

Ruth Backlund, a co-president at the Skagit Valley Chorale, said the group was monitoring public health guidelines at the time of the practice and had asked people to stay home if they showed even minor signs of illness. The group gathered in rows facing a piano and a choir director. They were all in individual chairs and had space to keep separated. Ms. Backlund had made sure there were extra soap dispensers in the bathrooms for people to wash their hands.

“Nobody was sick. Nobody touched anybody. Nobody shook hands. Nobody hugged everybody like you might do in a group. There was none of that,” Ms. Backlund said.

Given that they were spaced out and 1 asymptomatic person probably infected all 45 out of 60 in what must be a reasonably large room, it seems just impractical to keep sufficient distance to be safe indoors.

[-]Steven Byrnes50

Talking emits droplets and aerosols just like coughing does. In fact, I'm increasingly thinking that face-to-face conversations (especially without facemasks) are a major cause of asymptomatic transmissions. According to that theory, an indoor choir practice, with everyone singing loudly for extended periods, has to be one of the worst imaginable transmission risks. If the same type of super-spreader event happened in, say, a movie theater, I would be much more confused about it.

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[-]Raemon20

That makes sense, and does update me against public gatherings (but, I'm thinking of things on the timescale of a months to a year. I'm guessing it'll get increasingly hard to keep people apart. I'm also thinking less in terms of groups of 60, and more more like groups of 2-5)

I was thinking of thresholds that were more like "at least 12 feet apart, maybe 20 feet", with nobody touching any objects. (Also, I'm assuming this is all outdoors)

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5Wei Dai
My point is that since 45 out of 60 people were infected and they were spaced out, the farthest person infected in that group must have been quite far from the source of the infection, and keeping a even longer distance to be safe is probably impractical for most indoor spaces. ETA: Didn't notice this part earlier. I would be much less worried outdoors where virus particles are more likely to disperse instead of hang around, but don't have any quantitative answers to offer.
3Raemon
Ah, okay yes that makes sense.
1jmh
I suspect the inside issue is something that will eventually have to be addresses via ventilation and filtration (as in planes) and attention to just how the air flows seems important here. That probably doesn't get us back to distances pre-COVID-19 but at least gets to some new workable normal. (Unless we're giving up direct social interactions and go to pure virtual reality solutions). Outside might still need some work I think. If you're thinking not overly crowded settings not nearly as much to worry about. However, things like open air markets, rallies, large spectator sporting events outside or even tightly packed streets may still be a bit problematic. I think a lot there will depend on infection density at that point. I would expect some type of cloud to still emerge from the crowd of people that may remain localized in a lot of weather settings.
3Raemon
I have roughly two use cases in mind here:  a) the sort of person who wasn't that trustworthy in the first place and was probably going to start hanging out with friends within a few weeks even if official quarantines weren't lifted, but who might follow basic precautions if they were spelled out clearly. b) small high trust networks where everyone has been quarantining (and documenting their quarantine), nobody has been interacting with anyone outside the network, etc. (but, still with a margin of error added so that a single person who's been exposed unknowingly doesn't end up auto-infecting everyone)

Steven Byrnes

90

You might try this article and the many references therein. Sorry I don't have time for a better answer at the moment :-)

[-]Raemon20

Thanks. Gave it a quick skim and will hopefully comb through and pull out useful bits later tonight.

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Raemon

70

I'm not yet sure how related this is to Wei_Dai's answer, but found a medium article exploring "how far away to be from others who are walking in front of you". It references a couple other non-english articles, and one english... translation (I think?) on urbanphysics.

Some researchers ran computer modeling of what happens to saliva

The researchers came to this conclusion by simulating the occurrence of saliva particles of persons during movement (walking and running) and this from different positions (next to each other, diagonally behind each other and directly behind each other). Normally this type of modelling is used to improve the performance level of athletes as staying in each other air-stream is very effective. But when looking at COVID-19 the recommendation is to stay out of the slipstream according to the research.

 

The results of the test are made visible in a number of animations and visuals. The cloud of droplets left behind by a person is clearly visible. “People who sneeze or cough spread droplets with a bigger force, but also people who just breathe will leave particles behind”. The red dots on the image represent the biggest particles. These create the highest chance of contamination but also fall down faster. “But when running through that cloud they still can land on your clothing” according to Professor Bert Blocken.

 

Out of the simulations, it appears that social distancing plays less of a role for 2 people in a low wind environment when running/walking next to each other. The droplets land behind the duo. When you are positioned diagonally behind each other the risk is also smaller to catch the droplets of the lead runner. The risk of contamination is the biggest when people are just behind each other, in each other’s slipstream.

 

On the basis of these results the scientist advises that for walking the distance of people moving in the same direction in 1 line should be at least 4–5 meter, for running and slow biking it should be 10 meters and for hard biking at least 20 meters. Also, when passing someone it is advised to already be in different lane at a considerable distance e.g. 20 meters for biking.

[-]Raemon40

There is a critique of this here. I haven't yet read it thoroughly.

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[-]jmh10

This suggests to me that we don't just want to consider distance. Time and speed are both elements here too. I think that is actually something people can understand intuitively if they get some basic information. Most people are not challenged with knowing where they need to be to catch the fly ball. Here they just need to have a reasonable sense of where not to be.

So the message really isn't X distance but several factors that can include a distance metric.

However, the other aspect here is not safe (pick you metric) versus not safe. It's a... (read more)

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[-]Ruby20

This came up on my Facebook feed. I have only glanced at a briefly, but is probably of interest here:

Belgian-Dutch Study: Why in times of COVID-19 you should not walk/run/bike close to each other.

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[-]Shmi10

The idea is not to remove almost all risk, but to reduce R0 below 1. Six feet is likely to help a lot with that.

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[-]Raemon60

That makes sense for essential grocery trips and similar things, as an overall public policy. 

The thing I'm trying to figure out is "if you wanted to see friends/neighbors without hurting each other on a regular basis (i.e. going on walks, or visiting each other's front porch), how much distance do you have to maintain to actually be confident that you wouldn't infect each other for months on end?"

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[-]Raemon20

(I'm assuming there's basically always some risk if you go out at all, but, like, at what point does the risk fall under one's usual exposure to micromorts?)

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[-]Shmi-30

I think 6 ft and everyone wearing masks (properly) to avoid infecting others, plus sticking to open spaces only should basically reduce the risk to that of venturing out alone.

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[-]Raemon60

What makes you think that?

I'm also particularly interested in the "people are not wearing masks" answer, since a) I expect masks to continue to be in short supply, b) masks are just really annoying and I expect people to not wear them.

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