It looks likely that most, or perhaps almost all, COVID-19 transmission is directly airborne. An infected person breathes out the virus in tiny aerosol droplets, which someone else later breaths in. But people also worry about becoming infected from touching surfaces contaminated with virus. This has led to work looking at how long virus can survive on surfaces.
A plot from some of the earlier work is shown below, it is work of van Doremalen and coworkers in 2020, and is for virus on a copper surface. The red points are for SARS-CoV-2, the virus that causes COVID-19 while blue points are for SARS-CoV-1 – the less infectious virus that gave us a scare (which we didn’t learn from) about 20 years ago.
You can see that the virus is gone after about 10 hours. We don’t – as far as I know – have any idea where this ten hours comes from, i.e., why it is 10 hours not 10 minutes or 10 weeks. This a bit of problem particularly as the way the data above was collected is perhaps closer to what happens when an infected person licks a surface, then when virus from the air settles on a surface. I think van Doremalen and coworkers took a small amount (50 microlitres) of solution* containing virus and just spread it on a surface. The small volume will then dry on the surface.
So the virus was in a solution (and so wet) when spread on a surface. The aerosol droplets an infected person breathes out, will dry when their breath mixes with the room air. Our breath is saturated with water and so until this air mixes with room air, droplets can’t evaporate. But once the air of the breath has mixed with room air, its humidity drops to the humidity in the room, and the droplets dry out almost instantly because they are so small. The already-dried tiny droplets then can fall onto a surface.
I don’t think we know if it matters for virus survival whether the mucus they are in dries before, or after, the droplet arrives at the surface, but it could do. So at the moment, all we have is some data on how long virus survives on surfaces licked by infectious carriers.
* The solution is I think not of mucus, which is what virus leaves the body in, but the broth biologist grow cells in, so that is another source of uncertainty. To do an experiment with virus, it needs to be grown. As viruses only reproduce inside cells, this means host cells must be grown, and they need feeding and so are immersed in nutritious broth.