But before aerosols can get far, they must travel through the air thats near: meaning that they are a hazard at close range, too. And all the more so because, just like the smoke from a cigarette, aerosols are most concentrated near the infected person (or smoker) and become diluted in the air as they drift away.

A peer-reviewed study by scientists at the University of Hong Kong and Zhejiang University, in Hangzhou, China, published in the journal Building and Environment in June concluded, The smaller the exhaled droplets, the more important the short-range airborne route.

So what does this all mean exactly, practically?

Can you walk into an empty room and contract the virus if an infected person, now gone, was there before you? Perhaps, but probably only if the room is small and stuffy.

Can the virus waft up and down buildings via air ducts or pipes? Maybe, though that hasnt been established.

More likely, the research suggests, aerosols matter in extremely mundane scenarios.

Consider the case of a restaurant in Guangzhou, southern China, at the beginning of the year, in which one diner infected with SARS-CoV-2 at one table spread the virus to a total of nine people seated at their table and two other tables.

Yuguo Li, a professor of engineering at the University of Hong Kong, and colleagues analyzed video footage from the restaurant and in a preprint (not peer reviewed) published in April found no evidence of close contact between the diners.

Droplets cant account for transmission in this case, at least not among the people at the tables other than the infected persons: The droplets would have fallen to the floor before reaching those tables.

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Yes, the Coronavirus Is in the Air - The New York Times