Molecules are very small. So small that every breath we take contains about 2.4 times 10 to the 22nd power of them. The vast majority of these are oxygen and nitrogen, with lesser amounts of argon and carbon dioxide. When random mixing in the atmosphere is accounted for, we can calculate that every breath we take contains some molecules that have once been exhaled by Albert Einstein, Napoleon or Jesus.

Thats all very interesting, but the fact that every breath we take has at least five molecules that appeared in Abraham Lincolns last breath has no practical significance. However, the number of coronavirus particles that are expelled in every breath of the asymptomatic infected stranger with whom we may share airspace in a room, bus, airplane or car is of significance.

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Unfortunately, as with almost every other aspect of the SARS-CoV-2 virus, we have very limited data. A guesstimate can be made based on some mouse experiments with the original SARS virus. The infective dose in that case turned out to be only a few hundred viral particles, which would suggest that humans could be infected by as little as a few thousand inhaled particles, or virions, although we have to keep in mind one of the dictums of virology that mice lie and monkeys exaggerate.

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The Right Chemistry: Why being passed by a jogger is low risk - Montreal Gazette