Looks comprehensive except they missed lower indoor humidity due to turning the heat on.
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Local outdoor heating units also decrease the humidity in their immediate vicinity to near zero, evaporating much or all of the liquid (I would assume) on virus clusters as the heated air in that region rises. In addition, vertical heaters induce torroidal convection currents and in a volume with a cover (e.g., an overhang) these currents stir the air with a length scale related to the height of the heater or cover. The recirculation movement is upward near the outside of the heating unit, nominally horizontal near the cover, and then downward and back, all around. Virus clusters that are actually drawn into the heating unit are most certainly destroyed. A vertical gas-fired heater might draw many of the small recirculating airborne particles into the burner at floor level, where they would burn or char, but an external natural convection torroid would still exist.
I’m talking about people turning on the heat in their homes, workplaces, etc. Outdoor heaters shouldn’t have much effect on humidity if there’s good ventilation. Most outdoor heaters are infrared, which have little effect on air circulation.
https://www.themonitor.com/2020/11/24/starr-county-emergency-order-urges-monitoring-humidity-levels/
I know that’s what you were talking about. I was supplementing it. (I’ve added the word “also” to my post to make that more evident.)
I was talking about heaters in semi-enclosed, covered outdoor spaces. Recirculation is reduced only if there’s a steady breeze flowing through.
If the particles dry out near the heater, they’re smaller, and much more readily follow airflow patterns. If the air is more or less still, tiny particles will follow the air currents.
All outdoor heaters are radiant heaters, including those with flames. I know of no outdoor heaters that use forced convection alone (a fan) to blow warm air upward – useless in an open space. Vertical electric radiant heaters are the worst; vertical combustion heaters are better; elevated heaters aimed downward are best. Just don’t sit right next to a vertical heater.
I doubt outdoor heaters have had a significant effect on cases.
I hope you’re right and there’s no evidence that they have had, but the physics are there. What then could possibly justify a ban on outdoor dining in LA? That your breath carries virus particles across the table to those with you at the same table?
In particular, fully unconfined tables with heaters, such as on a sidewalk, without any partial enclosure or cover, can’t possibly exacerbate case numbers. You’d have to sneeze directly into someone’s face, and how often does that happen?
The length scale for natural convection recirculation in such cases is essentially infinite even in still air, and there’s “flushing” if there’s any breeze at all. Dry active virus clusters in upward convection currents will simply rise to some height, move with the breeze, and decrease in concentration. In such cases, radiant heaters should actually help, and the closer to the heater the better.
In addition, when your breath is warm and the ambient air cold, your breath and any tiny particles in it will rise due to buoyancy, helping to prevent it from reaching others at the same table, “all in the family” or not. (In that connection, it’s worth noting that the human eye can’t see particles smaller than about 50 microns, so other than in special lighting conditions, you don’t usually see your “breath” unless the droplets you exhale are larger than that.)
The people making the rules seem only to know about one branch of science, to the exclusion of all others, including fluid dynamics. They should bring in a professor from one of the local universities who understands these things before clobbering outdoor dining.
I found The Monitor article (Starr County) in this post to be very interesting; in particular, that an ambient water vapor concentration of 10 gm/m**3 (which is reached at 72F and 50% relative humidity, for example, both comfortable levels) can have such a dramatic impact on reducing transmission. To be continued, I guess.
Another concern among some public health experts is people often come from different households to gather at outdoor restaurant dining tables. A safer approach would be to keep dining parties limited to members of one household at each table, experts say, but such an effort has been impossible to enforce.
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A good article. Surely those restaurant owners who have done what they had to but in the process taken advantage of the imprecise meaning of “outdoor” could at this juncture be offered the option to (ahem) re-think their layout, rather than closing down all outdoor dining in all restaurants in the county.
The problem is irresponsible customers, which discussion belongs here:
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Irresponsible patrons are another facet of the problem. Yes, perhaps that discussion belongs elsewhere, but the two threads overlap sometimes.
It’s hard to believe when seeing old posts like this that the FTC discussions on this topic go back so far. I remember when we were arguing what “real BBQ” was regionally, and where it was to be found locally…
I guess on T-Day, I’ll be thankful that 2020 is almost over. Time has passed both excruciatingly slowly and way too fast.
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Good preprint paper why a hard lockdown now could be beneficial longterm to have much less need for significant “control” mechanism
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That’s the “hammer and dance.” The problem in the US is that in places where lockdown resulted in low community spread, it was not followed by adequate TTI. Also, a lack of a national policy means any local gains are sooner or later erased by irresponsible travel.
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That’s why I hope Biden will show leadership and change things dramatically
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Nice tool to calculate risk in indoor settings (link to german newspaper but text is in English - just click the green button)
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And a good article in Boston Globe what went wrong over the last 9 months (a lot of obvious things but also topics which might not be this obvious, e.g. unions etc)
One thing that can be observed, just by looking the pictorial examples in this article, is that in almost every case, people in a corner or near a wall got infected. Corners and walls are locations in enclosed spaces where there are almost always trapped recirculation vortices, which by way of mixing with the room air and then trapping some of its particulate contents, will generally have higher concentrations of particles or fine droplets than other locations in the room.
Takeaway: If you’re in an enclosed space, don’t sit or stand too close to a corner or wall when others are present, for any length of time.
There can be no feedback control if people won’t cooperate with government guidelines, including travel.
On/off control is demonstrably unstable even without societal acceptance. I believe this has exacerbated the current “second wave” of viral spread in the US, simply due to our cultural, reflexive resistance to Big Brother.
It’s our culture, not our fault.
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You can close businesses but you can’t prevent, arrest, or cite and fine millions of people for violating a mandate that isn’t widely accepted; e.g., gatherings of friends at “speakeasy” residences.
In our society, at least in CA, “dimmer switch” feedback control might have prevented this second, astonishingly undamped surge in cases. The most extreme level of intrusive government, On/Off intervention, a full lockdown, is now once more being forced on the people of LA.
Has anyone here even read this article?
We just don’t know enough about Covid-19 yet or have detailed enough data. Notably, the sudden spike in LA doesn’t seem to be related to changes in behavior.
Certainly it would be more effective to ease or lift one restriction at a time and wait long enough to see what effect it has. E.g. reopen elementary schools and wait a month.
That would be a much more intelligent approach than what’s being done now.