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I'm reluctant to talk about this, but my experience as an exposure scientist (particle air pollution, not infectious disease) leads me to think there's no reason to wear a mask outside, as long as you aren't in close proximity (<2m) to another person for an extended period. 1/
It's likely that Covid is transmitted by aerosol. That means that infected people breathe out virus particles which are suspended in the air, and uninfected people can breathe them in and catch the disease. 2/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7413047/
I've done a lot of research on second-hand tobacco smoke (SHS). We know that smoking even one cigarette indoors leads to SHS persisting in the air for hours, with levels of PM2.5 (small particles linked to heart disease) elevated for up to five hours. 3/ https://academic.oup.com/ntr/article-abstract/16/10/1365/2509361
By contrast, if you smoke a cigarette outdoors the particles disperse very quickly into the atmosphere. We talk about air exchange rates indoors as a measure of ventilation effectiveness - times per hour all the air in a room is replaced. Outdoors this is effectively infinite. 4/
You can still be exposed to SHS outdoors. But you need to be in close proximity to a source (a smoker) and your exposure will be lower (as particles don't hang around in the room for hours for you to breathe in, but disperse almost instantly instead). 5/
(That's a simplified version of a complex question - for instance, what does it mean to be inside vs outside? What about a bar terrace with a fabric covering but one side open to the air? What about normalising smoking? Here I'm talking about being outside in the open air.) 6/
It's useful to think about the difference between a hazard and a risk. A hazard is something that can cause you harm (eg. an asymptomatically infected person). A risk is the chance that a hazard will cause you that harm. 7/
Risk is affected by exposure to the hazard (an infectious person breathing out virus particles). Lots of factors can affect that exposure - time, distance, respiratory rate (how fast you breathe), wearing a mask (or the "hazard" - a person - wearing a mask). 8/
Our hazardous person running past you on a crowded street might enter a 2m radius around you briefly (ideally not, but it does happen). You're exposed to them. But they run past you in a few seconds, so your exposure is brief and the risk is low. 9/
By contrast, spending an extended period of time in a crowded shop or another indoor space (particularly where ventilation is poor) means you're exposed to virus particles in the air for a much greater length of time, even if you're further from infected people. 10/
You might be exposed to a lower concentration of particles than someone running past you breathing heavily. But that only lasts for a few seconds. In the shop you're exposed (at a lower level) for much longer. You'll breathe in far more particles. 11/
So even if the risk is small, why not remove it by saying "wear masks outside just in case"? Because this argument leads to all kinds of ineffective policymaking and takes up space from important policy changes (eg paying people to self-isolate). 12/
To be clear, I'm not a virologist (my undergrad was microbiology but that's a long time ago!) Aerosolised Covid doesn't necessarily work exactly like other fine particles (eg could a "short sharp" dose cause infection more easily?) That's why I'm reluctant to talk about it! 13/
But I thought I'd share a perspective from my own discipline, not as a corrective but as a point of view. One way or another: stay home as much as you can and wear a mask when you have to do essential shopping/other indoor activities outside the home. 14/
