I would expect that when stationary, the aerosol particles have more time to accumulate whereas when walking they don't, and thus walking would offer a pretty nice reduction of risk. Along the same lines, I would expect jogging and biking to offer an even larger risk reduction.

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James Grugett

Nov 16, 2020


My impression is that as long as you are outside, that would be sufficient ventilation, and so walking or biking wouldn't make much difference.

One other factor though could be that if you stand really close to someone, talking loudly could send some bits of saliva over directly, in which case walking and not facing each other would be an improvement.

My impression is that as long as you are outside, that would be sufficient ventilation, and so walking or biking wouldn't make much difference.

Hm yeah, on second thought I think you're right.

I'm not convinced – I'd like to see a deeper model backing this up rather than just "my impression is that". My impression is the opposite; does that change your mind at all?
2Adam Zerner3y
Yeah I'd also like to see a deeper model. I don't feel much confidence and your opinion does shift my beliefs, but I still err on the side of thinking that there's enough ventilation outside where walking or biking wouldn't make much of a difference.
I'm sorry for being terse in my initial reply; I was in a hurry. What I truly meant to point to was the idea that Zvi keeps hammering home that there is no binary of 'safe' and 'not safe' actions. You asked whether walking would reduce risk relative to being stationary, but the question James answered was essentially whether being outside was 'safe enough.' (I want to note that this is James's first ever LW contribution, so it's an understandable mistake.) So, given your initial question, I don't think you should have updated on James's answer at all. Being outside offers lower COVID risk than being inside, because there's more exchange of air. A windy day is in turn probably less risky than a day when the air is stagnant, for the same reason. Presumably you could simulate windiness by walking, biking, or jogging – the main open question for me there would be, like, if you're directly behind the other person, maybe all the air they breathe out is breathed in by you just seconds later, which seems bad. This comment is also not an answer to your question, because I think I'm just restating the model in your initial post. The important/interesting part of your question is to what extent. Hopefully someone more informed than me comes along and can give you hard numbers :)
2Adam Zerner3y
No problem at all, and I see what you're saying. When I read James's answer the big reason why I updated is because it prompted me to think it through again, and in thinking it through it now seems to me that if you're reasonably distanced, walking wouldn't provide much additional benefit vs being stationary.
I think it depends on what "outside" means, it is not one homogenous category. I believe you cannot assume to have sufficient ventilation just because you are in the open air (e.g. in a small courtyard). However, in a setting where you are able to run (not in circles) or even bike most of the benefit should come from ventilation even without moving.