[nudephil (OP)]

Here's a question from listener Jon:

If we are so concerned about airborne droplets as a method for spreading COVID, to the point where people are asked to wear masks, why are we testing for COVID with nasal swabs?  Surely if there is enough virus in our mouths and throats to infect others, there is enough to be detected?

Any response?

[Petrochemicals]

Thats where the virus abodes when one is in a state of infectious transmission.

[jon_bondy]

Thats where the virus abodes when one is in a state of infectious transmission.

If the virus "abodes" far back in the nasal passages, rather than in the lower nose and in the mouth, then why is it suggested that we wear masks?

[alancalverd]

It's all connected and nasal mucus is a good place to find the virus. Nasal mucus is found in the upper nasal passages and the back of the throat. Some tests swab the throat and nose on a single swab.

[jon_bondy]

The way I see it, either there is enough virus in the mouth to cause infection in others if I merely speak, or not.   If there is enough virus in my mouth to infect others, surely there is enough virus in my mouth for a swab to detect.

A lot of what I see going on regarding the pandemic "makes sense", but as to whether it is actually supported by scientific evidence, that is another matter.

I am sure that wearing a mask in social situations helps some.  As to whether that "helps some" is a significant aspect of disease prevention, I remain unconvinced.  It seems that panic and emotion are behind some of what we are being required to do. I would prefer scientific understanding, rather than "makes sense", if at all possible.

[Bored chemist]

It seems that panic and emotion are behind some of what we are being required to do.

It seems that way to you.

I would prefer scientific understanding, rather than "makes sense", if at all possible.

OK
https://www.hse.gov.uk/research/rrpdf/rr619.pdf

[jon_bondy]

Great article!  Thanks!

So, we can expect a reduction in aerosolized virus when using surgical masks by roughly a factor of six when exposed to a cough or sneeze.

I imagine that the effectiveness remains the same when the threat is just virus particles in the air absent a cough or sneeze.  Most of the time, when you are 2 meters away from an asymptomatic person who is not shouting, I would expect the virus threat to be minimal.  But with the surgical mask, it will 1/6th of that.

[alancalverd]

Shouting makes little difference. The pressure wave amplitude is larger than normal speech but the group velocity of exhaled particles doesn't change much  Coughs and sneezes are much more efficient at projecting aerosol entrapped in stable vortices.

Remember that a loose surgical mask principally protects others. To protect yourself you need a closefitting mask like a builder's dust mask or better.   

[Bored chemist]

So, we can expect a reduction in aerosolized virus when using surgical masks by roughly a factor of six when exposed to a cough or sneeze.

It's not that simple, but it's a good guess.

[evan_au]

...airborne droplets as a method for spreading COVID, ...why are we testing for COVID with nasal swabs?

Mucus in the nose and throat captures virus shed from the site of infection. It contains concentrated virus.
- Studies have shown that if you can produce phlegm/sputum, this comes from deeper in the lungs, and has even more virus; but you can't put cotton swabs down into your lungs! (at least, while the patient is conscious...)
- To produce virus-laden droplets, you need to get the patient to cough or sneeze; but that is exactly what we don't want, as it spreads the virus. Hence the advice that if you need to cough or sneeze, do it into your elbow.
- To produce aerosols, you only have to talk, sing or shout. But these droplets are very fine, perhaps 5 μm across and invisible to the human eye. They are very diffuse - researchers have used a laser rangefinder to detect that they are present and measure their distance.

To detect virus, you would need to capture lots of these droplets, or a huge number of aerosols out of a large volume of air. You need the largest possible sample so you can run a virus RNA detection machine on them.
- But these droplets and aerosols come from your throat and lungs. So a swab goes right to the concentrated source of the virus particles
- Without having to identify and capture them once they are spreading out in the air (and potentially infecting nearby people).

[Petrochemicals]

Thats where the virus abodes when one is in a state of infectious transmission.

If the virus "abodes" far back in the nasal passages, rather than in the lower nose and in the mouth, then why is it suggested that we wear masks?

My guess then john would be that the liquid runs down hill from the nasal pasages. Guessing again, testing is not at leisure to develop  viral multiplication and to be sure they go for the source to be sure if you are mildly infected.

[set fair]

We should all have been wearing masks once we knew about a/pre symptomativ spread. The mask will greatly reduce the forward momentum of expelled droplets. The now we know about aerosol spread we should be wearing masks is misleading. Probably used so they can pretend that they were right to say no to masks earlier. Aerosol spread was suspected on the Diamond Princess and discoverd in airconditiong vents in a hospital in Singapore early in March.