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In the COVID-19 pandemic, many jurisdictions are now requiring that masks be worn in public, often including both indoor and outdoor settings. However, many sources (including widely cited preprints by Qian et al. and Nishiura et al.) also suggest that outdoor transmission of COVID-19 is much less likely than indoor transmission, which has led people to move some activities outside. In addition, some studies have apparently investigated how far droplets can spread due to the breathing/speaking/etc. of people with or without masks.

Has there been any study on the relationship between these factors? For instance, has anyone studied or modeled the concentration of droplets/aerosols that would be present for masked/unmasked people in indoor/outdoor areas? What is the relative effect of masks vs. outdoor environment on the likelihood of transmission (either of COVID-19 or other previously-studied diseases)?

(Please note that I'm not asking about the mechanical function of the mask, which clearly would be unchanged indoors or out. What I'm asking about is the effect of the mask on the actual risk of transmission, relative to or considered in the context of an indoor/outdoor environment.)

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    Related: To what degree do facemasks protect against COVID-19?
    – Dan Dascalescu
    Jul 25, 2020 at 3:12
  • You're engaging in a base rate fallacy here or you're just confused about different measures of risk. Relative risk is (roughly) the same (assuming no interaction terms) regardless of the base rate. Absolute risk however is obviously not independent of the base rate. But studies usually quantify the former (relative risk) because deriving the latter (absolute risk) in a given/different setting is usually a case of trivial arithmetic (again assuming no interaction terms/effects).
    – Fizz
    Mar 9, 2021 at 9:34
  • So nobody is going (get funded) to conduct studies in umpteen different settings for the same treatment/measure unless they have a good reason to suspect/suggest there is some interaction resulting in a non-linear effect, i.e. that deducing the absolute risk (from the relative one and the base rate) is not a mere matter of arithmetic.
    – Fizz
    Mar 9, 2021 at 9:37
  • N.B. there's e.g. a study on the effect of masks "on top" of the (assumed) base transmission rate, but it's nothing more than calculations basically, for various numbers assumed for the two inputs (mask relative effectiveness and base transmission rate).
    – Fizz
    Mar 9, 2021 at 10:47

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As to your title question, I'm not aware of "data" but intuitively the effectiveness of a mask (as defined by the mask's ability to filter or capture) should be the same regardless of the environment.

A mask rated at 95% will filter out 95% regardless of the concentration present outside the mask.

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  • I'm not asking about the effectiveness in terms of ability to filter or capture particles, I'm asking about effectiveness in terms of chances of getting the disease.
    – BrenBarn
    Jul 25, 2020 at 3:47
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    The current thinking is that principle method of spreading the disease is by inspiration of virus particles. A mask, used indoors or outdoors, functions the same, by filtering or capturing the virus particles. A mask's capability of performing that function remains unchanged indoors or out.
    – BobE
    Jul 25, 2020 at 19:39
  • Again, I'm not asking about the mechanical function of the mask. I'm asking about its impact in achieving the goal of preventing disease spread. For instance, sunscreen performs its function of blocking UV rays indoors or out, but that doesn't mean its relevance for preventing sunburn is the same indoors or out, because indoors there is less UV radiation to block. Likewise if virus concentrations are very low outdoors for independent reasons (e.g., dilution), the practical impact of masks may be less than indoors.
    – BrenBarn
    Jul 25, 2020 at 19:58
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    Well then, the question you really wanted answered is something like: Is there data on the relevance of SARS-Cov2 transmission outdoors when subjects are masked versus unmasked? If so, then at minimum, I suggest you replace the word "effectiveness" with relevance in your title question.
    – BobE
    Jul 26, 2020 at 2:50
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    Effectiveness is the ratio or percentage between two conditions. You seem to be asking about the effectiveness of mask/no mask when applied in an outdoor setting as contrasted with the effectiveness of mask/no mask when applied to an indoor setting. The effectiveness (ability to block particles) of mask/no mask is independent of setting. - Do you question this?
    – BobE
    Jul 26, 2020 at 14:53

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