C[omp]ute

From: andrew cooke <andrew@...>

Date: Tue, 12 Jan 2021 17:52:10 -0300

There's a long and infamous tradition of physicists / engineers using simple
mathematical models to say something about complex systems (biology,
economics) and getting things horribly wrong.

I'm hoping to avoid that here by not drawing any strong conclusions.  Instead
I just want to throw some light on possible factors that people should take
into account when trying to decide what to do.


OK, so the question people keep asking in various places is: with Coronavirus
being a risk, is it safe to go cycling in a group?


If we assume that transmission is via someone breathing out virus-laden
particles that you breath in (which is, I think, roughly accepted) then you
can try to assess your risk yourself:

* The more people, the more likely someone is to be infected, so the higher
  your risk.

* Breathing out more energetically may increase the number of particles (if it
  increases as the volume increases, for example), which is likely a higher
  risk.

* Wearing masks hopefully filters out some of the particles, so other people
  not wearing masks increases your risk.  The type of mask is likely also
  important.

* How diluted the particles become as they move through the air is also going
  to be important.  In a closed, poorly ventilated space they will collect.
  In the open air they are more likely to be dispersed by the wind.  What
  happens when riding in a group?  Is the air 'captured' by the flow through
  the group or does it get dispersed?  Presumably a more coherent flow
  increases risk.

* Increased spacing between people gives more chance for the particles to be
  dispersed.

* Wearing a mask is also important when breathing in, so you not wearing a
  mask increases your risk.  And again the type of mask is likely important.

* Breathing in more deeply - as during exercise - may draw more of the virus
  more deeply into the lungs, increasing your risk.

Putting all this together, individuals riding separately, well separated,
wearing good quality masks, and not exerting too hard are likely pretty safe.
Change any of those variables and you increase risk.  So, at the other
extreme, many people in a compact group, without masks, working hard, hoping
to drag a "bubble" of air along for efficiency, are going to be more at risk.


Again, I am not a virologoist or an infectologist.  The above is just what
seems like common sense to me.

One example of how I may be incorrect is that, in the argument above, it
doesn't seem to matter who wears the mask (if only one person wears it, and it
filters out 90% of particles, it filters out 90% of particles either when
breathing out, or when breathing in).  This contradicts what I have read
elsewhere, which emphasises that masks are more important on "other people"
and less important for self-protection.

Andrew