20200529

Coronavirus Hates the Outdoors


PSI Blog 20200601 Coronavirus Hates the Outdoors


Pardon the teleology, but I just had to do a little off-topic speculation on our current predicament. It is becoming clearer every day that air-borne viruses do not do well in the outdoors. Mom’s advice to “get some fresh air” seems well-taken. The energy-saver’s advice to tighten up every window and door seal might just give those little buggers plenty of time for you to breathe them in. The advice to put on that mask when you leave the building might be backwards—maybe you should put it on when you go inside. Instead of “sheltering in place,” maybe we should “shelter out place.”


There now are plenty of data showing that being inside with carriers during a pandemic is not a good idea. The coronavirus is spread many ways, but it looks more and more like the finest aerosols (less than 5 um) are the top culprits in most cases. The mainstream press is finally waking up to the aerosol problem.[1]


A choir practice in Washington State was the site of a huge outbreak and offers one of the strongest pieces of evidence for airborne transmission. Satoshi-K/E+ via Getty Images

1.    The first indication ignored for far too long was the 2.5-hr Skagit County, WA choir practice in which 53 of 61 singers became infected with COVID-19 by a single carrier.[2] The time from exposure to onset was 3 days, 12 days for hospitalization, and 14 days to death for the two that died.
2.    A single carrier in Guangzhou, China dining at a restaurant infected four at her own table along with five others at adjoining tables.[3]
3.    A single carrier in South Korea partied at three nightclubs, infecting 54 people.[4]
4.    In another incident, a “super spreader” in South Korea infected 37 people in a church.[5]
5.    After an employee got the virus, a huge grocery in China had 8,244 shopper visits and only 2 (0.02%) infections, while the 120 employees had 11 infections (9%), showing that duration and closeness of contact was important.[6]
6.    Two buses in China “brought people to the same temple, where they mixed and mingled. But who was most at risk of getting sick? Those who rode the bus with an infected person. Twenty-four out of 67 people on that bus got sick. No one on the other bus did.”[7] Lesson: Close quarters and duration.

Meanwhile, “in a study of 1,245 cases that occurred across China from January 4 to February 11, only two cases were traced to contact with an infected person out of doors.”[8]

To get infected, you only have to be exposed to someone’s breath for less than the 15 minutes. The breath aerosol can stay in the air for hours. Where ventilation is poor, as in a bar or bus, that aerosol remains in the air and is replenished continually by the infected person. Six feet of separation is not enough, particularly when the air is continually stirred up by the motion of others in a small enclosed space. “Super-spreaders” typically do not cough or sneeze on every one, they simply breathe, filling the trapped air with tiny particles that take a long time to settle even when not stirred. A runner or biker going fast past you is extremely unlikely to do that.

Conclusion: Ventilation

Indoor air bad; outdoor air good. That is why we have many more colds and flu in winter than in summer—it is not simply due to the temperature—it is what the temperature makes us do to ourselves—breathe bad indoor air. Being with a large group outside on a windy day would be much less risky than being with the same group on a calm day.  Athletics played outdoors would be much less risky than those played indoors, etc. Voting in a well-used booth verges on suicide, while voting at a table outside might be as safe as mailing a ballot; teaching classes outside would be safer than teaching inside; political demonstrations outside would be safer than those inside.

Again, the key to all this simply is ventilation, and plenty of it. Note that in the restaurant case, there was an exhaust fan on the left side of the room and an air conditioner on the right (Figure). It was 79oF outside [9] and the investigators assumed the air conditioner was on even though swabs of the conditioner and the exhaust fan indicated no virus. The infection pattern does not support air flow from right to left. My conclusion: The air conditioner either was turned off or was insufficient. Looks like we need more powerful ventilators before we get sick so we won't need them later.

In sum: We should avoid breathing used air.



Figure. Sketch showing arrangement of restaurant tables and air conditioning airflow at site of outbreak of 2019 novel coronavirus disease, Guangzhou, China, 2020. Red circles indicate seating of future case-patients; yellow-filled red circle indicates index case-patient. Modified from: Lu and others (2020) https://wwwnc.cdc.gov/eid/article/26/7/20-0764_article#tnF1.




[2] Hamner, Lea, and others, 2020, High SARS-CoV-2 Attack Rate Following Exposure at a Choir Practice — Skagit County, Washington, March 2020: Morbidity and Mortality Weekly Report, v. 69, no. 19, p. 606-610 [Here is the summary: https://go.glennborchardt.com/Skagit-summary]
[7] Ibid.


1 comment:

Glenn Borchardt said...

Here is an update on the Guangzhou restaurant virus spread and its wimpy air conditioner. Turns out the AC was inadequate as we surmised:

https://go.glennborchardt.com/AC-virus