Let's say 15 minutes after a person coughs or sneezes, where does the droplet go? Up in the air and down to the ground? Is it different inside a building?

  • Are you referring to Covid-19? Commented Mar 30, 2020 at 3:04
  • @America Yes, that's what I'm referring, too Commented Mar 30, 2020 at 3:06
  • we have a tag ;) you could add coronavirus tag too Commented Mar 30, 2020 at 3:07

1 Answer 1


Data suggests droplets remain in the air up to 3 hours, and longer for droplet nuclei. Where they go depends on the local temperatures, humidity and air flows.

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The image shows a cough on the left and a sneeze on the right.

Currently, the term droplet is often taken to refer to droplets >5 μm in diameter that fall rapidly to the ground under gravity, and therefore are transmitted only over a limited distance (e.g. ≤1 m). In contrast, the term droplet nuclei refers to droplets ≤5 μm in diameter that can remain suspended in air for significant periods of time, allowing them to be transmitted over distances >1 m (Stetzenbach, Buttner & Cruz, 2004; Wong & Leung, 2004). Other studies suggest slightly different definitions, with ranges for “large” droplets, “small” droplets and droplet nuclei being >60 μm in diameter, ≤60 μm in diameter and <10 μm in diameter, respectively (Tang et al., 2006; Xie et al., 2007). The concept is that the naturally and artificially produced aerosols will contain a range of droplet sizes, whose motion will depend significantly on various environmental factors, such as gravity, the direction and strength of local airflows, temperature and relative humidity (which will affect both the size and mass of the droplet due to evaporation).

Oops, breathing and talking can produce droplets yet health authorities continue to claim that masks are not needed (to stop wearers from broadcasting droplets into the environment)

Humans can produce respiratory aerosols (droplets) by several means, including breathing, talking, coughing (Figure C.1, A), sneezing (Figure C.1, B) and even singing (Wong, 2003; Toth et al., 2004).

We are being told that the main droplets are the larger ones being emitted but talking for 5 mins can produce droplet nuclei the same as a cough

Published data have suggested that sneezing may produce as many as 40 000 droplets between 0.5–12 μm in diameter (Cole & Cook, 1998; Tang et al., 2006) that may be expelled at speeds up to 100 m/s (Wells, 1955; Cole & Cook, 1998), whereas coughing may produce up to 3000 droplet nuclei, about the same number as talking for five minutes (Cole & Cook, 1998; Fitzgerald & Haas, 2005; Tang et al., 2006). Despite the variety in size, large droplets comprise most of the total volume of expelled respiratory droplets. Further data on the behaviour of droplet dispersion in naturally generated aerosols are needed.

Small droplets fall to the ground where they dry up before they reach the ground but some become droplet nuclei on their way down which allows them to remain airborne

In the classic study of airborne transmission, Wells (1934) was able to identify the difference between disease transmission via large droplets and by airborne routes. Wells found that, under normal air conditions, droplets smaller than 100 μm in diameter would completely dry out before falling approximately 2 m to the ground. This finding allowed the establishment of the theory of droplets and droplet nuclei transmission depending on the size of the infected droplet. The Wells evaporation-falling curve of droplets (see Figure C.2) is important in understanding airborne transmission and transmission by large droplets. Wells' study also demonstrated that droplets could transform into droplet nuclei by evaporation.

And what happens to droplet nuclei

Droplet nuclei floating on the air may be carried by the movement of air. Entrainment of air into neighbouring airspaces may occur during the most innocuous daily activities; for example, as a result of people walking, or the opening of a door between a room and the adjacent corridor or space (Hayden et al., 1998; Edge, Paterson & Settles, 2005; Tang et al., 2005, 2006). In addition, the air temperature (and therefore air density) differences across an open doorway will also cause air exchange to occur between the two areas, providing a second mechanism to allow air into other areas (Tang et al., 2005, 2006) (see Figure C.3).

which is why they use negative pressure rooms when doing a procedure that can form aerosols


  • Excellent answer especially explanation how enough talking is equivalent to a cough. When coughing into the sleeve though I assume it absorbs most droplets. Commented Mar 30, 2020 at 3:36
  • Well, talking lacks the same energy added to the droplets so i would think not more dangerous but maintain that 2 m separation distance. Commented Mar 30, 2020 at 3:38
  • Sorry I had revised my comment making your reply somewhat confusing to others. Commented Mar 30, 2020 at 3:39
  • It stays in aerosol form for 3 hours and with modern air tight buildings it probably doesn't matter. Interestingly I have hearsay testimony a local firefighters have noticed lots of swollen eyes taking people to hospital lately. I'm wondering if eyeballs can encounter these droplets too and they are not only just breathed in. Commented Mar 30, 2020 at 22:45
  • I think that's wrong. It can be droplets for 3 hours but aerosol much longer. yes, the red eyes are conjunctivitis from virus infecting the eyes which is why everyone is wearing eye protection. Commented Mar 30, 2020 at 22:59

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