Naked Science Forum

Life Sciences => Physiology & Medicine => COVID-19 => Topic started by: Drmarkf on 03/05/2020 17:18:28

Title: How much does transmission reduce with distance?
Post by: Drmarkf on 03/05/2020 17:18:28

I have read in several media reports that current thinking by the scientific advisory group in the U.K. in favour of maintaining a 2m social distancing guideline as we move towards easing lockdown, rather than reducing to the WHO guideline of 1m, is based on “the risk of 2 seconds exposure at 1m from a case being equivalent to 2 minutes at 2m”.

Now, I’ve just done some literature searching for evidence on that, including going through several curated paper collections from academic institutions, without turning anything up.

Can anyone please direct me to any relevant studies? I imagine these could be epidemiological studies based on the new phone-based proximity apps, or else people could have looked at numbers of virus particles reaching particular distances in some sort of experimental study, but I can’t find anything strictly relevant on either.

Title: Re: How much does transmission reduce with distance?
Post by: alancalverd on 03/05/2020 18:08:07

If you exhale 12 times per minute, there is roughly a 40% chance that you will do so in any 2 seconds.

at 2 m radius your exhalate will be dispersed in 8 times the volume of 1 m radius, and the density of exhaled aerosol also decreases with distance due to the viscosity of air, at about 1/r³, so the doserate at 2 m from the source is about 1/64 of that at 1 m.

This the risk of infection in 2 minutes at 2m from an infected  person is about the same as 2 seconds  at 1m.

Coughing and sneezing increases the risk immediately downwind as the low-speed dispersal mechanisms are dominated by vortex projection. 

Title: Re: How much does transmission reduce with distance?
Post by: Drmarkf on 03/05/2020 18:56:07

Interesting, thanks, so this is a theoretical calculation, obviously with a host of assumptions.

It seems reasonable, however, as a starting point if there are no empirical data. As ever, this sort of thing is being bandied around as though it was rigorous and based on experimental evidence.
Do you by any chance have a reference that shows it is indeed the basis for the U.K. view?

Analysis of the app data (or, at least, the app data from those countries where sufficient individualisation is possible) should provide one way of getting that data. Whether enough people will feel sufficiently trusting of our lovely government to allow them anywhere near their tracking data is entirely another matter, of course.

I’ll say no more, but I’ll think a lot very loudly  ;-)