https://www.erinbromage.com/post/the-risks-know-them-avoid-them
Good article that sums up some of the bits I've posted on here in the past 2 months like the Chinese restaurant, LA choir and ventilation being key indoors and adds some other examples like a call centre and indoor sporting event
This is potentially key information for places of work opening up soon and not walking into a huge second wave. Washing hands, cleaning your phone and keeping 1-2 metres apart won't help indoors and most is spread at home.
I had a look at that article. First of a lot is not referenced.
A Cough: A single cough releases about 3,000 droplets and droplets travels at 50 miles per hour. Most droplets are large, and fall quickly (gravity), but many do stay in the air and can travel across a room in a few seconds.
A breath: A single breath releases 50 - 5000 droplets. Most of these droplets are low velocity and fall to the ground quickly. There are even fewer droplets released through nose-breathing. Importantly, due to the lack of exhalation force with a breath, viral particles from the lower respiratory areas are not expelled.
How far a droplet travels really depends on quite a few factors. Saying most droplets are "large" doesn't mean anything in this context. Saying droplets fall quickly due to gravity alone is only half the picture. Some of the droplets will also settle faster because of the additional momentum (and consequently a velocity component) they gained during the sneeze/cough etc. Think of it like firing a gun downwards - the bullet will fall because of gravity and also the energy imparted onto the bullet by the mechanism in the gun. Compare that to just dropping a single bullet from your hand from a great height and watching it fall due to gravity.
But even if that cough or sneeze was not directed at you, some infected droplets--the smallest of small--can hang in the air for a few minutes, filling every corner of a modest sized room with infectious viral particles. All you have to do is enter that room within a few minutes of the cough/sneeze and take a few breaths and you have potentially received enough virus to establish an infection.
If there is a negligible amount of motion in the air, it is very unlikely the smallest droplets will fill quickly every corner of a modest sized room. Molecular transport processes take a long time to transport contaminants across large distances. This is discussed in the seminal book by Lumley in Chapter 1 in "A first course turbulence", see page 8-9 [
here]. If there is a small degree of motion, then it is possible. But you need to also then consider that greater motion may also lead to greater evaporation.
Small sized droplets (less than 5 micron) can be considered those which follow the air flow well but they can also evaporate very quickly, see for example [
here]. Small droplets in this context can also be called "tracers", again, the meaning is that they "trace" or follow the air flow (its instantaneous streamlines) very well. Droplets greater than this size may not follow the air very well. So if a group of air molecules goes in one direction, the "larger" droplet in that same region may not go in the same direction.
The important point from the paper is:
very high gas flow rates can .... increase droplet evaporation.
Water fast evaporation can be a serious problem when long droplet lifetimes are needed. It is for this reason that it is seldom used as fluid for tracer droplets.
after a short time and depending on the carrying gas humidity, most of the water will have evaporated,
They measured the velocity of air, using these "tracer" droplets when the air flow was 10m/s. So effectively the droplets were moving at 10m/s and evaporation was an issue. 10m/s is about 22mph. So you can imagine how problematic evaporation could be at 200mph, as is the claimed velocity of the droplets during a sneeze. Evaporation is not my background, but it is definitely something which is being ignored by articles that I am reading. I would like to see an article which explains this process specifically, rather than articles trying to explain dispersion properties which are based on obscure simulations or very niche experiments.
By the way, how far two droplets separate is a topic called "pair dispersion", more specifically inertial pair dispersion and the wealth of papers in this topic may also give some insight regarding how a cloud of droplets disperses (and separates).