Monday, April 6, 2020

A possible SARS-Cov-2 proxy

Let us presume that you wish to prevent contracting, or spreading, this virus. Obviously you should avoid those behsviors and environments most conducive to the spread. Unfortunately, our benighted experts know so little about how that works that we are forced to avoid all contact whatsoever.
What we would like is a pair of VR glasses that simply showed us the virus, in the air or on surfaces, wherever it happened to be. We don't have that. Is there a next best thing?

How it spreads

Basically we don't know. In the absence of knowledge, health experts are assuming it spreads the same way as a cold or flu. Chances are it does; but the coronavirus spreads a lot faster and further than those. 
The first thing about it is that it has a long asymptomatic incubation period, during which some of which the patient is infectious. In fact, from the statistics I've seen, something like half of those infected never show symptoms at all. So there are a lot more people walking around thinking they're fine, but actually spreading the virus.
But it's likely that's not the whole story.  People catch it from those they never touch and who never sneeze. It probably has other vectors than a normal cold. 

Choir practice

I believe the case of the Skagit Valley Chorale is instructive. After a practice session March 10, about half the choir came down with Covid-19. And yet:
In light of the coronavirus outbreak, Comstock said they greeted each singer with hand sanitizer at the door, they were individually spaced out during rehearsal, each singer used their own sheet music, and they avoided shaking hands or hugging.
 “During the entire rehearsal, no one sneezed, no one coughed, no one there appeared to be sick in any way,” she said.
What did happen? Two and a half hours of singing. That's a lot of deep breathing of shared air. The infection rate among the Skagit Valley singers was extremely high, probably twice as high as on the Diamond Princess cruise ship where the passengers were cooped up together for over a month.

Bad Air

The only reasonable inference seems to be that the aerosol theory, which holds that the virus spreads not so much from visible droplets from a cough but microscopic ones perhaps containing just one virus, might be the major vector. 
The virus itself is about 0.12 μ, has a Reynolds number in air of 3e-8 and a settling velocity of 3e-4 cm/s. That's 0.0000067 mph. With any air circulation it will remain in the air indefinitely. So when people keep breathing the air in a closed space, it will build up and build up. 
We have to start thinking of the virus as a gas. My guess is that it's the elevated concentration of SARS-Cov-2 virus that led to the high infection rates. 
So what we need, to determine the danger level of a given environment, is a gas detector that measures SARS-Cov-2 virus concentration. But we don't have one.
What we do have, however, is a proxy. People also exhale CO2. And we do have CO2 detectors. 
So let me propose that a CO2 detector can be used as a worst-case measure for  SARS-Cov-2 concentration in the air. Worst-case because of course there may be many people breathing who are not spreading the virus. But if you're outside and the CO2 level is about 400 ppm, you're probably safe. If you're inside and it's 2500, open windows or turn on that attic fan. 
In other words, yes, a ventilator can save your life. But it's not necessarily the kind of ventilator people have been talking about.


  1. Good to see you post, and interesting insight, as always.

    Are there any other places you post at? The activity here isn't very consistent, and I'd hate to miss new content or books.

  2. As an HVAC engineer, I see at least a couple of issues with this approach.

    One is that, while the virus is very tiny, a CO2 molecule is 1000 times smaller. CO2 is subject to gas diffusion dynamics, whereas the virus is too large. So CO2 is not a good physical model for how viral particles would spread through a space.

    Another is that commodity HVAC CO2 sensors are just not very good. I have seen many cases of new-out-of-the-box sensors reporting nonsense, like 200 PPM, or 800 PPM in a room that has been empty and ventilated for hours.

    CO2 sensors have been around for a relatively long time. I suspect that it is now possible to make one that is cheap, accurate and reliable - but the manufacturers have no incentive to do so until they are forced to, and HVAC as an industry has a very poor record of holding manufacturers or installers responsible for achieving expected performance. Until that changes, they'll probably keep making the same not-very-good sensors.