It doesn’t matter if you’re downstream or upstream from the infected person, only whether you’re sharing recirculated air. Virus particles accumulate in the air over time, as shown by the person who was infected on the bus 30 minutes after the infected person got off. And as you said, the more virus particles, the more infectious the air is.
Asymptomatic carriers are a major source of infections, and they’re not coughing or sneezing.
Here it is, direct from the State of California and CalOSHA, the guidelines for restaurants to reopen.
I’ve skimmed them and I’m not sure how doable they are for many restaurants. Sorry, Robert, no mention of A/C but there are 2 bullet points pertaining to fresh air and air circulation
There’s been some mention of restaurant air circulation in restaurants with wall-mounted a/c units, vis-à-vis virus spread. (E.g., a video on CBS news today.) These small a/c units probably have dust filters, but those filters are only intended to trap dust that would interfere with the a/c efficiency, as in a home furnace or central a/c. I think.
I did notice this (brief) comment in the CalOSHA guidelines:
“Consider installing portable high-efficiency air cleaners, upgrading the building’s air filters to the highest efficiency possible, and making other modifications to increase the quantity of outside air and ventilation in all working areas.”
Interesting to compare the CA with the WA guidelines:
Especially this:
9: “If the establishment offers table service, create a daily log of all customers and maintain that daily log for 30 days, including telephone/email contact information, and time in. This will facilitate any contact tracing that might need to occur.”
In the Guangzhou restaurant, only two of the seven sitting at the table in front of the air conditioner got infected, vs. three of the four people at the table at the opposite end of the room.
California at the state level has been doing a pretty crappy job so far compared with the SF Bay Area health officers.
You might want to take a look at the CBS video, or the report by the researchers that it references (at UC Davis and Univ. Oregon). The report title in part is …“Built Environment Considerations…” The 3D simulation is that of a small 4-table restaurant, not the one in Guangzhou where A1 was the original source and people at three tables were infected. It’s a different situation and therefore an interesting and relevant addition to this kind of information.
The CBS video has the title “How a restaurant’s ventilation system can affect the spread of the coronavirus”.
P.S. My apologies, the infected person in the CBS video was not directly under the a/c unit (although that was the case on the bus).
Simulations are only as good as their assumptions, and experts have been making a lot of bad ones. A four-seat restaurant’s not a very common real-world scenario.
The Hunan bus had 13 seats. The infected person was sitting in row 12, and infected passengers in rows 5, 6, 8, 9, 11, and 13.
Do you have a cite for where the air conditioner vents were on the bus? I haven’t seen that.
Here’s a preprint of a second study of the Guangzhou restaurant. Excellent 3-D simulation on page 10.
Very good point, Robert. No I don’t. I looked at the diagram of the bus on the link you posted (repeated here) and assumed that the a/c was in the very back of the bus, which would be pretty much right above the infected person in row 12.
Outstanding! This, in my opinion, is an excellent example of a truly scientific study, incorporating both 3D CFD (including a widely-accepted turbulence model), and experimental data collection in a very carefully-constructed full-scale physical simulation; in conjunction with thorough analysis and comparison of both. Their concluding remarks and discussion are very clearly written and direct.
Transmission of the covid due to the airflow patterns in restaurants is something we very much need to fully understand right now. IMO, this should be required reading by all restaurant owners and designers of their ventilation systems. Retrofitting the HVAC systems in restaurants as needed should be at least as high a priority as many of the other 10,000 steps that have been government-mandated to limit transmission in CA restaurants so that they can re-open.
Return vents are often at the back of the bus, but supply vents are usually toward the front or spaced throughout the bus. Sometimes they’re over every seat, like on a plane.
OK, so if someone who’s infected is seated near the return vent in the back of a bus, particles/droplets could be drawn into the return and then spread throughout the bus, or deposit residue on the surfaces of air ducts for later spread.
That was originally my thinking when I said, “don’t sit near an air return” (in a restaurant). If you’re infected, you could infect others.
It doesn’t matter where in the cloud of recirculating virus particles you sit.
I can’t agree with that, but I’m willing to agree to disagree.
If you’re infected and sneeze directly under a wall-mounted horizontal a/c return vent, I think the probability of transmitting particles through the ventilation system is much greater than if you’re further away, because in the latter case, turbulent diffusion (i.e., decreased concentration) will have diluted the cloud as it expands, before reaching the a/c return.
Turbulence plays a major role in dispersing/diffusing droplets and particles – i.e., spreading them out and reducing their overall volumetric concentration. The large-scale (room-size) source-generated vortices in Guangzhou established the largest-scale, primary vortex structures, but those have to have induced secondary, smaller vortices due to shear between them, and then on and so forth, until the vortical eddy structures finally got so small (near-microscopic) that they were kinematically stopped and dissipated by aerodynamic drag, a process referred to as cascading. This is an ongoing process in a “steady-state” flow; that is, one in which a general “picture” of the flowfield doesn’t change with passing time.
In steady state, turbulence production and dissipation balance, and all scales of hydrodynamic mixing proceed at established, constant rates, progressing in scale and kinetic energy content through the turbulence cascade process. The Guangzhou restaurant airflow wasn’t in a true steady state condition, but largely so over time.
Below some concentration level (and exposure time), even if you’re right inside the cloud you won’t get infected. Which is why the servers didn’t get infected in the Guangzhou restaurant. The amount of virus that gets into your lungs is a function of the concentration of particles in the air you breath; your rate of breathing; and the amount of time you’re exposed; i.e., the overall dose of infectant. The servers weren’t exposed to as much nor for as long as those who were dining in the area of TA, TB, and TC.
That all seems on point except I don’t know how you can look at the diagram on page 10 of the second study of the Guangzhou restaurant and still imagine that it matters how close an infected person is to the air conditioner.
The studies make it clear that in a restaurant, where people can’t wear masks, the length of a meal is plenty of time for one asymptomatic person to create a high enough level of concentration.of virus particles to infect half the people sharing the same semi-stagnant volume of recirculated air.
Also, over the period of time that an infected “index” person is dining in a restaurant, there’s a good chance that they will have coughed (or spoken loudly if the place is crowded) more than once, meaning multiple releases of virus-containing aerosols.
I can’t cite a source, but I believe that I read somewhere that droplets expelled by asymptomatic carriers contain far fewer virus particles than those from symptomatic carriers. (But, emphasis on the “I believe” part.)
People don’t need to to cough, sneeze, or talk to spread the virus.
There are two things I’d add to the already infinite list of regulations/guidelines for restaurant re-openings in CA: (1) Turn down the volume of the music; and (2) where possible, add acoustic treatment.
In general, greatly reduce the overall interior noise level.
Why? Because in a noisy environment people have to talk loudly to be heard, and as people start to talk in loud voices, everyone talks even louder. In some restaurants and pubs, people are almost shouting at one another.
It’s been shown that the number of droplets expelled from your mouth when you talk increases as you talk louder. Quieting things down so that people talk more softly would reduce the number of droplets being generated and released into the air by an infected person when they talk, thereby reducing transmission at the source.
Yes, vocal noise is certainly reduced with reduced occupancy. What’s not clear is how much occupancy can be realistically reduced for restaurants to remain viable. Any amount reduced occupancy would help; people wouldn’t have to talk as loudly to be heard.
In many cases, though, the “seed” for loud conversations is background music that’s played at excessive levels. If I were Gov. Newsom, I’d say that there should be no background or live music played in any restaurant. For now at least.
Also, those hard surfaces that became the vogue in many restaurants greatly exacerbate noise levels from all sources, and aggravate the need to talk more loudly. A noisy atmosphere may be considered “lively”, with people talking in loud voices over music and the general din, but that “lively” ambience may well be contributing to unnecessary speech-generated, potentially infectious droplets in restaurants.
If the air is recirculated, how loud people talk might not make much difference. If there’s sufficient flow of fresh air, loud talking might be no more dangerous than talking at all.
Awareness of the indoor air issues seems to be increasing, so maybe someone will actually study that.
As with any airborne infectious isolation room (AIIR), the air in a COVID-19 patient room should not be recirculated. To conserve energy, most hospitals are designed to intake 25% of outdoor air and recirculate the remaining 75%. To achieve 100% intake of outdoor air poses a significant issue, as most hospital facilities do not have an infrastructure designed with this capability. It is important to conduct a thorough analysis of the hospital’s existing heating, cooling, fan output and controls. The total system impacts need to be understood and considered before attempting to increase isolation spaces. This is a difficult issue but one that is absolutely necessary in the case of COVID-19.
https://www.builderonline.com/design/indoor-air-quality-becomes-a-bigger-issue_o
So you don’t think that the number of source droplets makes any difference? That’s what I’m talking about: reducing the amount of virus-bearing droplets at the source(s).
