How likely are you to get COVID in a sauna?

We don’t know the answer to this question. Over the two years of the pandemic there has been a fair amount of debate on how far COVID-19 can spread, with some people, incorrectly, thinking that it rarely travels more than a metre or two across a room. But we have clear examples, such as in a restaurant in Guangzhou, where COVID-19 spread across a room. So, I hope this debate is settled, you can catch COVID-19, and almost certainly, flu, from across a room. We also know that masks offer some protection, with FFP2/N95 masks offering much more protection than cloth masks.

But there are still huge gaps in our knowledge of how airborne diseases such as COVID-19 and flu spread. For example, viruses are delicate and many are destroyed when the droplets they are in dry out. An infected person breathes out the virus in microscopic droplets of mucus, that dry quickly as they leave the infected person’s hot humid breath and drift across a room. So naively, you might think saunas are very dangerous places to be, as their the air is so humid that the tiny droplets don’t dry out and so all the viruses survive.

Maybe this is true, maybe it is not, studies of the effect of humidity on viruses are all over the place. Maybe saunas keep viruses alive for longer, maybe the heat rapidly kills them. Your guess is as good as mine.

To give you some idea of the field of virus survival in the environment, I can quote from a 1991 review:

ADV [a virus that affects pigs] suspended in liquid fomites [particles with viruses] reached 99.99% inactivation at the following times:

– well water : 7 days

– chlorinated water : 2 days

– urine from swine on subtherapeutic medicated feed : 14 days

– manure pit effluent : < 1 day

– anaerobic lagoon effluent : 2 days

EC Pirtle and GW Beran, Reviews Sci Tech 10, 733 (1991)

And no I don’t understand why manure effluent is so destructive to this virus but not urine, maybe it is the pH?

The problem here is what we don’t understand the mechanism by which, in this case, pig manure effluent destroys virus. So we have no idea if it is general, maybe pig manure is generally toxic to all viruses, including SARS-CoV-2. But we don’t know, so it is hard to justify a trial against SARS-CoV-2, especially as consumers may generally prefer sweet-smelling hand-gel. And without knowing what is the magic ingredient, we can’t extract it and so develop a nicer smelling sanitiser that uses this and only this from manure.

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