Worrying about inhaling more than the smell of coffee

Way back in the innocent days of 2018, I had a very mild tussle with that part of the BBC that runs BBC Bitesize, which is the part of the BBC’s website dedicated to providing educational resources for school children (original blog post here). The BBC’s webpage originally claimed that the smell of coffee could travel across a (then open) coffee shop, via diffusion. This is not correct, it is carried by air currents, and the BBC did update the webpage.

The BBC are not the only people ignorant of these currents, they are surprisingly complicated, and I think not studied very much*. But they are now very topical, as they may be carrying corona virus from an infected person to other people in the same room, who may then become infected. This is called airborne transmission, and has been surveyed in part of a very recent review on the role of fluid flow on the life cycle of the corona virus.

The exact mechanism by which corona virus particles spread from one person to another seems likely to be complex and to occur by multiple ways (air currents, surfaces, etc). But the corona virus particle is only about 100 nanometres (= one ten thousandth of a millimetre across) across and particles this small can easily be convected up and down and across spaces via air currents. So transmission this way is physically possible**, but whether it accounts for, say, 0.01 % of infections or 60 % of infections, is unknown.

A lot of work is now going into this and related tricky physics/biology problems. Answers would help us go back to as it was in 2018, when we could walk into coffee shops without a care in the world.

* Although there are entire papers dedicated to studying the “human thermal plume” our hot bodies set up.

** Across distances greater than 2 m, droplets micrometres across and larger fall to the ground under gravity over distances of a metre or so or less.

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