More airborne spread - Printable Version
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More airborne spread - BostonCard - 12-03-2020
[tweet]https://twitter.com/zeynep/status/1334641685154902021?s=20[/tweet]
Take home message: indoors, neither being more than 6 feet nor being exposed less than 15 minutes will save you if you are downwind under the right circumstances.
This is why any indoor activity is highly risky, even at 25% capacity, or whatever it is that they are restricting it to these days.
BC
RE: More airborne spread - M T - 12-03-2020
(12-03-2020, 05:10 PM)BostonCard Wrote: Take home message: indoors, neither being more than 6 feet nor being exposed less than 15 minutes will save you if you are downwind under the right circumstances.
This is why any indoor activity is highly risky, even at 25% capacity, or whatever it is that they are restricting it to these days.
Why does indoor matter here? With any particular airflow in the right direction, wouldn't this have happened whether there were no walls or ceiling? Is there some laminar airflow that happens from fans that is distinct from natural breezes?
If someone finds the study's link, please post it. I'd like to see if they specify the times that the infector and infectees were without their masks.
I'm trying to understand whether it is airborne spread to lungs/eyes or airborne spread to food/drink and then to mouth.
I will point out that these distances are NOT too far for droplets to travel. I've forgotten the exact numbers, but I seem to recall that 1 m/s flow allows for this much distance. (It was mentioned in one of the UCSF Grand Rounds.)
Gee, too bad we can't do this tracing for each of the 753 new cases of COVID in SCC today. (That was satirical) I'd have to think you could do this for maybe one case a week or month.
RE: More airborne spread - BostonCard - 12-03-2020
Here's the paper (it was linked in the twitter thread)
https://jkms.org/DOIx.php?id=10.3346/jkms.2020.35.e415
Here's the section you are looking for
Quote:Restaurant A was located on the first floor of a six-story building totaling 96.6 square meters in size (9.2 × 10.5 m) without windows or a ventilation system. There were two doors in the restaurant, one at the front (door 1) and the other at the back (door 2). Two ceiling-type air conditioners were diagonally located at 3.2 m from the floor as shown in Fig. 3; they were fixed with wire and had been operating at the time the cases were in restaurant A. On CCTV, case A and his companion entered the restaurant at 16:00 on June 12 and finished their meals before case B (with case D) entered at 17:15 using door 2. Case B and his colleague sat at a table near door 2, at a 6.5-m distance from case A, who did not leave from his table or share his table with others. Cases A and B engaged in conversation with their respective companions without masks. At 17:20, case A went out of the restaurant A using door 1. In 2 minutes, case C and his companions (V6, V7) entered the restaurant A using door 1 and sat at another table 4.8 m distant from case B, where they remained for 21 minutes before case B left his table using door 1 at 17:43. The distance between case A and case B was 6.5 m, and the air flow direction at positions of both cases showed a maximum of 1.0 m/sec (3.6 km/hr) velocity measured by anemometer. The air flow between case B and case C showed a maximum of 1.2 m/sec (4.3 km/hr) over a 4.8 m distance. All positions such as guest tables, infectors, and infectees, ceiling air conditioners, and information for air speed and direction are shown in Fig. 3.
You are right that there is nothing magical about being outside, per se, but airflow does tend to allow much more diffusion away from the source than in an enclosed room. We should not assume that outdoor transmission is not possible, but at any given distance and level of protection, it seems much less likely.
BC
RE: More airborne spread - Genuine Realist - 12-03-2020
(12-03-2020, 06:39 PM)BostonCard Wrote: Here's the paper (it was linked in the twitter thread)In the outdoors, won't the air current be subject to more dispersal? Not the same vector?
https://jkms.org/DOIx.php?id=10.3346/jkms.2020.35.e415
Here's the section you are looking for
Quote:Restaurant A was located on the first floor of a six-story building totaling 96.6 square meters in size (9.2 × 10.5 m) without windows or a ventilation system. There were two doors in the restaurant, one at the front (door 1) and the other at the back (door 2). Two ceiling-type air conditioners were diagonally located at 3.2 m from the floor as shown in Fig. 3; they were fixed with wire and had been operating at the time the cases were in restaurant A. On CCTV, case A and his companion entered the restaurant at 16:00 on June 12 and finished their meals before case B (with case D) entered at 17:15 using door 2. Case B and his colleague sat at a table near door 2, at a 6.5-m distance from case A, who did not leave from his table or share his table with others. Cases A and B engaged in conversation with their respective companions without masks. At 17:20, case A went out of the restaurant A using door 1. In 2 minutes, case C and his companions (V6, V7) entered the restaurant A using door 1 and sat at another table 4.8 m distant from case B, where they remained for 21 minutes before case B left his table using door 1 at 17:43. The distance between case A and case B was 6.5 m, and the air flow direction at positions of both cases showed a maximum of 1.0 m/sec (3.6 km/hr) velocity measured by anemometer. The air flow between case B and case C showed a maximum of 1.2 m/sec (4.3 km/hr) over a 4.8 m distance. All positions such as guest tables, infectors, and infectees, ceiling air conditioners, and information for air speed and direction are shown in Fig. 3.
You are right that there is nothing magical about being outside, per se, but airflow does tend to allow much more diffusion away from the source than in an enclosed room. We should not assume that outdoor transmission is not possible, but at any given distance and level of protection, it seems much less likely.
BC
Also, except in the case of a spectator sports event (which ain't happening), it would be unusual to be even five minutes in the company of anyone.
Griffins78 - Griffins78 - 12-03-2020
Much if not most of the “outdoor” dining is with partial walls and often roofs. The key difference from indoor is open ventilation. However, they use heat lamps and heaters which tend to blow the air around the area and often downward. Doh!
RE: Griffins78 - Genuine Realist - 12-03-2020
(12-03-2020, 09:45 PM)Griffins78 Wrote: Much if not most of the “outdoor” dining is with partial walls and often roofs. The key difference from indoor is open ventilation. However, they use heat lamps and heaters which tend to blow the air around the area and often downward. Doh!That's certainly the case at present and apparently at the French Laundry. However, until mid November, Castro Street dining was completely open air.
Griffins78 - Griffins78 - 12-04-2020
(12-03-2020, 09:52 PM)Genuine Realist Wrote:did they have heaters at night?(12-03-2020, 09:45 PM)Griffins78 Wrote: Much if not most of the “outdoor” dining is with partial walls and often roofs. The key difference from indoor is open ventilation. However, they use heat lamps and heaters which tend to blow the air around the area and often downward. Doh!That's certainly the case at present and apparently at the French Laundry. However, until mid November, Castro Street dining was completely open air.
RE: More airborne spread - M T - 12-04-2020
(12-03-2020, 06:39 PM)BostonCard Wrote: Here's the paper (it was linked in the twitter thread)Thanks!
https://jkms.org/DOIx.php?id=10.3346/jkms.2020.35.e415
Here's the section you are looking for
<excerpt removed>
You are right that there is nothing magical about being outside, per se, but airflow does tend to allow much more diffusion away from the source than in an enclosed room. We should not assume that outdoor transmission is not possible, but at any given distance and level of protection, it seems much less likely.
BC
While I do tend to believe that infections do happen across these distances (6.5m) and in less than 15 minutes (5m), there are several things that bother me about their analysis. In the text, the authors suggest that this incident is evidence of droplet transmission over a longer distance than normal. It may have been droplet transmission, but I don't think they've done a good job of showing it. There are several other possibilities for the source of the infection for A & C that they didn't rule out.
They didn't order the cases A, B, C based on genomes. They didn't find any difference among them. (I don't know how quickly minor changes happen in each individual so I don't know if they didn't look hard enough or if there shouldn't be any.) They treat case B as the index case apparently because he gets symptoms the next day. However, case D (B's table mate) is dismissed as an infector or infectee, saying the he got COVID the day before from another source. (No source is given for case B.) There is no mention as to whether Case D is distinct or the same genome as A, B, and C. No indication is given as to whether case B got infected the same time as D or what. As far as I know, D could be the index case and infected A, B, and C, or B infected A, C, and D.
You might think it doesn't matter whether B or D (or both) is the source of the virus in this restaurant. As far as the distance and air flow, it doesn't matter much. However, they make arguments for droplet transmission based upon the direction people are facing.
1) There is no consideration mentioned that case A (the case they were investigating) was infected by someone who was asymptomatic at some place other than this restaurant. (All people in the restaurant at the time were identified & tested, so it wasn't an asymptomatic person there.) The fact that there were 4 people who developed COVID in the restaurant within a short time makes it likely it was in the restaurant.
2) There is no mention of identifying and testing anyone who was in the restaurant shortly before A entered. There was no mention of identifying and testing everyone who was there while C was in the restaurant (after B & D left). In other words, they didn't investigate that A and C may have been infected separately; they presumed a single infection incident.
3) "...we used the EISS of the KDCA to gather data from June 2 to June 15. The results showed only one (case B) of 538 confirmed domestic cases with a tracking map overlapping that of case A during that period." Wait a minute! Why didn't case D show up in that data? My guess is they didn't go back a week later after more cases were available in EISS to determine if other infectors might be involved.
4) "Secondary" cases A & C were apparently reasonably close outside the restaurant, where A exited the restaurant by door 1 two minutes before case C entered by that same door. They could have been exposed to an unknown (asymptomatic) case outside.
5) "In addition, the visitors sitting at tables with cases A and C (V1, V6, and V7) were not infected with COVID-19 because they faced away from the infector’s face. These findings strongly suggest that this outbreak occurred by droplet transmission exceeding a 2 m distance and excluded contact and fomite transmission."
5a) Obviously the direction you're facing doesn't exclude contact and fomite transmission. That's excluded only if you know the infector and can disprove contact. We don't know when the tables were prepared before the cases arrived. A single waitstaff that left prior to A's arrival might have handled the dishes, tableware, condiments, etc. As I already mentioned, A and C could have had contact with a common third-party outside the restaurant.
5b) The claim that table mates were not infected "because" of the direction they are facing is presuming what they seem to want to claim.
5c) "away from the infector's face" assumes that B is the infector, not D.
5d) In contradiction to what their text says, their diagram shows V6 facing the same direction as case C. Is it just luck that V6 didn't get infected, but cause, not luck, that V1 and V7 didn't?
5e) There is no argument whether eddies around a person's head in a 1m/s airflow would or would not allow for preferential or non-preferential inhalation of virus particles.
6) "Air speed and direction at several specified positions were precisely measured using a portable anemometer (Kestrel 2500; Nielsen-Kellerman Co. Boothwyn, PA, USA) on June 24 and July 2." Uh, that device does NOT measure air flow direction. Where ever they measured air flow direction at several specified positions, those directions are not shown in this paper. They show wind speed at 5 positions. If you look at the room diagram, they show the air flowing out from the A/C units (that are 3.2m above the floor (presumably that is their low point, not a center or top measurement), roughly 2m above the head of someone seated) in 4 directions in straight lines (I think that is too simplistic) and then, generally, turning around. They don't indicate where the A/C's inlet is, so it is hard to judge the overall 3D flow in the room. (Assume that north is to the top of the room diagram) They draw arrows that suggest the air flows across the 6m from B to A, but that seems uncertain since the airflow to the units' intakes isn't shown. In particular, the initially-east-flowing air from the unit near door 2 is shown as turning to the southwest which seems unlikely since it should be impacted by the north flowing air from the other unit. I would think it would tend to go to the north or northwest.
7a) Their sequence of events has case B (and D) walking by case C (within about 3m) as B exits but that is not mentioned as the possible cause of C getting infected.
7b) While B (and D) entered the restaurant by door 2, his path to his table is not documented. Potentially, upon entering, he walked to the counter by the front door (door 1) by either the aisle next to A or further away from A, before being taken to his table. This path should have been noted as it could be a much closer (1 meter) interaction.
8) If aerosols are involved, C's exposure doesn't end when B (or D) leaves the room.
It would have been effective if they had produced body-temperature smoke at B's position and videoed its flow in the room. Does it flow from B to C and A in the few seconds before droplets would fall? Or, they could have used some sort of droplet producer (atomizer?) using colored water (again, at body temperature) and measured its arrival at the locations of C and A.
Infectee A finished his meal before presumed infector B (or D) entered. So no path from infector via particle/aerosol to food to infectee. Whether A was still drinking a drink isn't mentioned.
RE: More airborne spread - teejers1 - 12-04-2020
(12-04-2020, 03:30 AM)M T Wrote: Thanks!
While I do tend to believe that infections do happen across these distances (6.5m) and in less than 15 minutes (5m), there are several things that bother me about their analysis. In the text, the authors suggest that this incident is evidence of droplet transmission over a longer distance than normal. It may have been droplet transmission, but I don't think they've done a good job of showing it. There are several other possibilities for the source of the infection for A & C that they didn't rule out.
They didn't order the cases A, B, C based on genomes. They didn't find any difference among them. (I don't know how quickly minor changes happen in each individual so I don't know if they didn't look hard enough or if there shouldn't be any.) They treat case B as the index case apparently because he gets symptoms the next day. However, case D (B's table mate) is dismissed as an infector or infectee, saying the he got COVID the day before from another source. (No source is given for case B.) There is no mention as to whether Case D is distinct or the same genome as A, B, and C. No indication is given as to whether case B got infected the same time as D or what. As far as I know, D could be the index case and infected A, B, and C, or B infected A, C, and D.
You might think it doesn't matter whether B or D (or both) is the source of the virus in this restaurant. As far as the distance and air flow, it doesn't matter much. However, they make arguments for droplet transmission based upon the direction people are facing.
1) There is no consideration mentioned that case A (the case they were investigating) was infected by someone who was asymptomatic at some place other than this restaurant. (All people in the restaurant at the time were identified & tested, so it wasn't an asymptomatic person there.) The fact that there were 4 people who developed COVID in the restaurant within a short time makes it likely it was in the restaurant.
2) There is no mention of identifying and testing anyone who was in the restaurant shortly before A entered. There was no mention of identifying and testing everyone who was there while C was in the restaurant (after B & D left). In other words, they didn't investigate that A and C may have been infected separately; they presumed a single infection incident.
3) "...we used the EISS of the KDCA to gather data from June 2 to June 15. The results showed only one (case B) of 538 confirmed domestic cases with a tracking map overlapping that of case A during that period." Wait a minute! Why didn't case D show up in that data? My guess is they didn't go back a week later after more cases were available in EISS to determine if other infectors might be involved.
4) "Secondary" cases A & C were apparently reasonably close outside the restaurant, where A exited the restaurant by door 1 two minutes before case C entered by that same door. They could have been exposed to an unknown (asymptomatic) case outside.
5) "In addition, the visitors sitting at tables with cases A and C (V1, V6, and V7) were not infected with COVID-19 because they faced away from the infector’s face. These findings strongly suggest that this outbreak occurred by droplet transmission exceeding a 2 m distance and excluded contact and fomite transmission."
5a) Obviously the direction you're facing doesn't exclude contact and fomite transmission. That's excluded only if you know the infector and can disprove contact. We don't know when the tables were prepared before the cases arrived. A single waitstaff that left prior to A's arrival might have handled the dishes, tableware, condiments, etc. As I already mentioned, A and C could have had contact with a common third-party outside the restaurant.
5b) The claim that table mates were not infected "because" of the direction they are facing is presuming what they seem to want to claim.
5c) "away from the infector's face" assumes that B is the infector, not D.
5d) In contradiction to what their text says, their diagram shows V6 facing the same direction as case C. Is it just luck that V6 didn't get infected, but cause, not luck, that V1 and V7 didn't?
5e) There is no argument whether eddies around a person's head in a 1m/s airflow would or would not allow for preferential or non-preferential inhalation of virus particles.
6) "Air speed and direction at several specified positions were precisely measured using a portable anemometer (Kestrel 2500; Nielsen-Kellerman Co. Boothwyn, PA, USA) on June 24 and July 2." Uh, that device does NOT measure air flow direction. Where ever they measured air flow direction at several specified positions, those directions are not shown in this paper. They show wind speed at 5 positions. If you look at the room diagram, they show the air flowing out from the A/C units (that are 3.2m above the floor (presumably that is their low point, not a center or top measurement), roughly 2m above the head of someone seated) in 4 directions in straight lines (I think that is too simplistic) and then, generally, turning around. They don't indicate where the A/C's inlet is, so it is hard to judge the overall 3D flow in the room. (Assume that north is to the top of the room diagram) They draw arrows that suggest the air flows across the 6m from B to A, but that seems uncertain since the airflow to the units' intakes isn't shown. In particular, the initially-east-flowing air from the unit near door 2 is shown as turning to the southwest which seems unlikely since it should be impacted by the north flowing air from the other unit. I would think it would tend to go to the north or northwest.
7a) Their sequence of events has case B (and D) walking by case C (within about 3m) as B exits but that is not mentioned as the possible cause of C getting infected.
7b) While B (and D) entered the restaurant by door 2, his path to his table is not documented. Potentially, upon entering, he walked to the counter by the front door (door 1) by either the aisle next to A or further away from A, before being taken to his table. This path should have been noted as it could be a much closer (1 meter) interaction.
8) If aerosols are involved, C's exposure doesn't end when B (or D) leaves the room.
It would have been effective if they had produced body-temperature smoke at B's position and videoed its flow in the room. Does it flow from B to C and A in the few seconds before droplets would fall? Or, they could have used some sort of droplet producer (atomizer?) using colored water (again, at body temperature) and measured its arrival at the locations of C and A.
Infectee A finished his meal before presumed infector B (or D) entered. So no path from infector via particle/aerosol to food to infectee. Whether A was still drinking a drink isn't mentioned.
Am in awe of MT's breakdown of report - most all of this stuff is way over my head.
My takeaway (from the beginning of pandemic): don't dine inside.
I have eaten outdoors at restaurants with wife and then family a few times in last 8 months. Seemed fine and pretty risk-less.
Maybe we just got lucky. But my older brother, far more social, has had dinner with friends lots of times and has so far been virus-free.
RE: Griffins78 - Farm93 - 12-04-2020
(12-03-2020, 09:45 PM)Griffins78 Wrote: Much if not most of the “outdoor” dining is with partial walls and often roofs. The key difference from indoor is open ventilation. However, they use heat lamps and heaters which tend to blow the air around the area and often downward. Doh!There are a lot of memes on this "outdoor" topic.
In a lot of the USA you can eat in a pop-up tent like structures including those sophisticated types that people often use for "outdoor" weddings. As it is cold in many places, the solution is to drop those vinyl walls to keep warmth and block the wind.
The hybrid dining offers a new danger too. Most of the structures are in parking lots or in street side parking spaces, so it becomes possible to get hit by drivers. Ultimately the vinyl probably keeps the virus in, but can't keep wayward drivers out.
RE: More airborne spread - JustAnotherFan - 12-07-2020
Small Data, Big Implications. What we can learn from the latest indoor dining study.
https://zeynep.substack.com/p/small-data-big-implications
RE: More airborne spread - M T - 12-07-2020
(12-07-2020, 04:56 AM)JustAnotherFan Wrote: Small Data, Big Implications. What we can learn from the latest indoor dining study.
https://zeynep.substack.com/p/small-data-big-implications
"First, just reading the study is an exercise in what it means to do a study really, really well,"
That's not what I thought!!
That doesn't mean that droplet spread over that distance wasn't what happened, it was that they left other explanations unchecked and made assumptions that they don't justify, at least in the study.
RE: More airborne spread - JustAnotherFan - 12-07-2020
(12-07-2020, 08:22 AM)M T Wrote:(12-07-2020, 04:56 AM)JustAnotherFan Wrote: Small Data, Big Implications. What we can learn from the latest indoor dining study.
https://zeynep.substack.com/p/small-data-big-implications
"First, just reading the study is an exercise in what it means to do a study really, really well,"
That's not what I thought!!
That doesn't mean that droplet spread over that distance wasn't what happened, it was that they left other explanations unchecked and made assumptions that they don't justify, at least in the study.
I know. I appreciate your post a few up in the thread that highlighted your concerns.