by Tom Chivers
Monday, 10
January 2022
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07:00

Repeat after me: Covid doesn’t spread on surfaces

Will we ever move beyond outdated advice?
by Tom Chivers
Watch out for those fomites!

On every table in every cafe, on every counter-top in every store, there is a bottle of disinfectant. Every shop door has a “please disinfect your hands” sign with a little hand gel dispenser. “These premises are cleaned regularly,” say proud little signs in train stations.

It’s more than two years since the virus started spreading, and we still think it does so by touching things.

For the record: it pretty much doesn’t and we’ve known as much for a while. Almost exactly a year ago, Nature was asking: “COVID-19 rarely spreads through surfaces. So why are we still deep cleaning?”

It was, at the beginning of the pandemic, a reasonable thing to think. We didn’t know much about how it spread and we went on what we thought we knew about other coronaviruses and the flu. What we thought was that it spread via contact — via “fomites”, infected surfaces. We were urged to wash our hands regularly. (I remember going to a gig in March 2020, and being amazed that men were washing their hands after going to the toilet! Unprecedented.)

But it became clear, relatively quickly, that that wasn’t how it worked. A Lancet Infectious Diseases paper way back in August 2020 pointed out that the claims about how long Covid lasted on surfaces was overstated, based on studies that used unrealistic concentrations of viral particles. 

As far as I know, there have been no solidly confirmed cases of fomite transmission (although this somewhat gross “snot-oral transmission” incident is a possible). No doubt there have actually been many tens of thousands — most cases are never fully traced — but they are rare. Instead, SARS-Cov2 spreads through people breathing or coughing out tiny droplets which each carry a payload of viral particles. It is the concentration of those particles in the air which is the risk factor, not how long it has been since a surface was cleaned. 

Things aren’t quite as mad as they were in 2020, when our children’s school shut down on Wednesdays (in the brief period when it was open at all) for a “deep clean” and all the local playgrounds were fenced off in case someone got the virus from licking a swing, or when people were decontaminating their Ocado deliveries. But it’s still there. Amazingly, the UK government’s guidance is that “Regular cleaning plays a vital role in limiting the transmission of COVID-19,” and that at home and at work, “As a minimum, frequently touched surfaces should be wiped down twice a day.” The US CDC has similar advice. (The WHO used to, but has changed it, although Facebook still recommends that you “clean hands often and disinfect frequently touched surfaces at home”, quoting the WHO as a source.)

This isn’t to say that you shouldn’t clean things or you shouldn’t wash your hands. You should! The prevention of faecal-oral transmission of pathogens, through hand-washing and increased hygiene, is one of the great reasons why waterborne illnesses are so reduced in the Western world (along with improved sewerage). But you presumably did that before the pandemic, and you didn’t feel the need to sanitise your hands every time you went into Uniqlo or disinfect every surface in your house twice a day.

Instead, what we should be doing is improving ventilation, keeping windows open and meeting outdoors where possible. And, for a start, changing the bloody government advice. It turns out that the miasma theory of disease actually had a lot going for it.

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  • When you breathe out normally it emerges as an inverse emulsion, also known as an aerosol. In a simplistic way this is like a solution of water vapour in air. You can see this when people breathe out in cold air; this fogging is caused because the air emerging from the body is warmer than the surroundings.

    As a PhD student, my supervisor was a world expert on aerosols and he had a big tank about 2m long. He pumped out coloured warmer aerosol into the tank and you could watch it rise – warmer air rises. This is where the 2m idea comes from. If somebody breathes out, the aerosol is probably above your head by the time it reaches you. A mask on either person makes little difference.

    But, if someone coughs, it is different. The cough can push out actual droplets of water, not an aerosol. The droplets are heavier than air and they sink quickly. If you are 2m away you are probably safe but if you are closer than 2m you can get a blast of droplets in your face. Better that the person who coughs (or shouts or anything that propels droplets of water) wears a mask to contain those droplets.

  • Who actually coughs at other people? “Wow, lucky this cloth caught all my infected particles that I involuntarily sprayed in your direction”.

  • When all else fail, I do a rain dance to awaken the Rain spirit. This is how humans operate. That is why we have religions and prayers. The whole human culture is built on rituals.

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