How coronavirus is transmitted has crucial implications for how we stay safe.
You’re in the same room as someone who has coronavirus. How – exactly – do you catch it from them? Clearly, being in the same place as them is undesirable (let’s say they’re asymptomatic). How can the virus get from their body to yours?
A) Touching something they’ve touched.
B) Touching something they’ve breathed on.
C) Getting too close to them.
D) Breathing the same air as them.
If we are to limit the spread of the virus, we must be able to answer this question correctly. Scientists’ understanding is improving, but public knowledge (and school safety measures) aren’t keeping pace.
Two ways to get sick
First, the person needs to emit some particles of the virus: by coughing, sneezing, speaking, singing, breathing out. But how do those particles get to you? There are (more-or-less) two kinds of particles:
Droplets – large particles: too heavy to float. They sink to the ground gradually.
Aerosols – smaller particles: light enough to float. They drift around the room in the air.
If the virus is transmitted by droplets – well picture laughing aloud and accidentally spitting. It doesn’t go far. And it goes downwards (ballistically). If you’re too close to the infected person, these droplets might go into your mouth or eyes. And if you touch a surface these droplets are on, then eat, or touch your mouth or eyes without washing your hands, you may become infected.
If the virus is transmitted by aerosols – they can float anywhere. The infected person breaths them out, and you breathe them in. So, while droplets may not go further than two metres (and may be caught by screens and visors) aerosols can float beyond tape markings, around visors, above screens.
Droplets drop, aerosols fly: picture the difference between a rain drop and a will’o’the wisp
Almost all official guidance reflects a belief that the disease spreads through droplets (NHS guidance here). This makes touching surfaces and getting too close the main dangers (Options A-C above). It makes visors, screens, washing hands and cleaning surfaces the main ways to stay safe.
The problem: the evidence
The problem with believing that the virus doesn’t spread through aerosols is that many transmission events can’t be explained through droplet transmission alone.
The Canadian karaoke bar: the bar was operating at 30% capacity and a plexiglass screen separated singers. But this one bar produced at least forty cases (news report). (Singing produces aerosols; see also the Seattle choir practice in which one sick person infected 53 others, two of whom died.)
The Chinese bus: on the way to a Buddhist festival, one asymptomatic person (without a mask) was on a bus for around fifty minutes. The air was recirculating within the bus. A third of the passengers caught the virus (almost certainly from this journey), and they were as likely to catch it sitting near the infected person as sitting further away (peer-reviewed paper).
The Korean Starbucks: one infected person was sitting under an air conditioner in a confined branch. At least 56 people were infected (but employees, who wore masks, were not; news report).
The American hospital: researchers collected air samples in the rooms of infected patients. They found viable virus cells up to 4.8m away from the patients (preprint paper).
How many studies do you need to believe that the virus can float – it’s airborne – it doesn’t just fall the ground? This is enough for me.
It was also enough for 239 scientists to beg the World Health Organisation to rethink the question (the WHO originally said it wasn’t), and enough for the World Health Organisation to agree that it’s possible. Even the Chief Medical Officer is on the record as saying it’s possible.
How do you avoid coronavirus?
If coronavirus is spread by droplets, the answer is (relatively) simple: catch the droplets (face masks, screens, visors), keep your distance, clean surfaces, wash your hands. What we’ve been doing.
If – as I hope I’ve demonstrated – coronavirus can also be spread by aerosols, we need to think a lot more about the air we’re breathing.
- Minimise the production of aerosols: no singing, no shouting.
- Minimise their spread: face masks.
- Ventilate the space: open the windows and the doors (or be outdoors, or filter the air).
- Don’t inhale them: face masks again.
That doesn’t mean washing your hands and keeping a decent distance is a bad thing. Droplets may still play a role (and even if they don’t this should help with the flu, which spreads via droplets). But we need to worry a lot more about the air.
For schools, that means teachers and students need masks – in classrooms, corridors, everywhere – and the windows and doors need to be open, alongside physical distance where possible.
Why am I getting this from you?
You may wonder why this isn’t clear from government guidelines. I wonder too.
- Chinese health authorities announced it in February.
- The World Health Organisation got most of the way there in July.
- The UK government were advised on the consequences by a SAGE sub-committee in July; the Chief Medical Officer stated it as a fact in September.
- Most strangely, the US Center for Disease Control released updated guidance acknowledging that airborne transmission (not touching surfaces) was crucial last week:
Then – bizarrely – withdrew the guidance, claiming it was a draft.
What’s clear is that what we were originally told – it’s definitely not airborne – no longer holds. And that what we’re currently being told – wash hands, wash surfaces – is not enough to keep us safe.
You’re welcome to doubt, to question my credentials, or to improve on this evidence. But I’d ask, are you so convinced that aerosols don’t play a role in transmission that you refuse to take simple precautions: open the windows, wear a mask, don’t rely on washing your hands.
Breathing the same air as a person infected with coronavirus is dangerous. Please, put on a mask, and open the window.
Footnotes
If you want a fuller discussion of these questions, I strongly recommend Zeynep Tufekci’s superb essay (and her equally brilliant Twitter feed).
Michael Story has been a mine of examples of the changes in guidance (and lack of changes).
My previous discussion of masks in schools is here.