What do you think of the advice about 1.5m or 2m spacing?
I don't have a background in evaporation or biology so I can't say much about whether the distance could be larger or even smaller.
Obviously there is still risk at those distances but how much do you think it reduces the risk?
I think I can say it will almost certainly reduce the risk. How much the risk reduces with distance I don't know, and I don't think you can answer that very easily. But generally the further you stand from an individual, the greater opportunity you give for the droplets to lose all the initial energy they attain from the sneeze.
You can think of this like water being ejected from a hose when you are cleaning the car or gardening. If you put your finger right near the nozzle of the hose, all the water will hit your finger. Now move your finger away from the nozzle. Less water will hit your finger because the water has lost some of the pressure energy and also it has dispersed outwards away from the central axis of the hose pipe.
Continuing the analogy, the greatest distance the water travels from your hose pipe depends on:
- how much you turn the tap on therefore giving more energy to the water (analogous to how violently someone sneezes)
- The angle relative to the horizontal in which you hold the hose which determines the initial direction of the water (analogous to the tilt in someones head at the moment they release the droplets when they)
- Whether or not it is windy
Remember, we have not considered evaporation in this argument.
Is it appropriate advice?
As I said earlier, I can't really say much about the specific distance, but it is definitely going to help - I can't see how it won't. I have no doubt we will see people argue the distance could be further, but really it is like answering "how long is a piece of string" simply because it is difficult to consider each and every different environmental circumstance in which someone sneezes e.g. they sneeze at home with air con off, they sneeze at home with air con on, with windows open/closed etc.
But one can simplify these problems (e.g. using non-dimensional numbers) and talking to people with the right knowledge. A lot of people have experience in both dispersion and heat transfer problems associated with droplets. Examples include:
- combustion industry such as injecting sprays of fuel droplets into hot turbulent gas inside an engine
- cloud physics where you study rain formation in clouds, where rainwater can disperse, freeze (snow) or perhaps evaporate as well
- industrial processes like spray drying whereby you dry liquid droplets into fine powder within turbulent gasses
- atmospherical scientists who may study droplet emission problems, rather than just solid particles
So we have the people and the knowledge to come up with answers. I hope that the people in biology do tap into that experience.
EDIT: Added the bit in bold.