Yes—Babak Javid, Dirk Bassler, Manuel B Bryant
The likelihood of transmission of SARS-CoV-2 is far greater indoors than outdoors.1,2 So why do we argue here that masks, in some circumstances, should also be worn outdoors?
To be clear, we are not arguing that outdoor transmission represents a large transmission risk. Nor do we support policies that, for example, mandate masking outdoors when someone is alone or only with members of one household, since such scenarios, by definition, do not represent a transmission risk. Instead, we argue for a nuanced approach to mask wearing, where even outdoor mask wearing during the pandemic phase of Covid-19 is a normalised behaviour rather than a polarising one.
Estimates of indoor transmission risks compared with outdoors vary widely, but they are at least 4-20 times more likely than outdoors.2 Infection with SARS-CoV-2 is likely to depend on the amount of inoculated infective particles, which in turn mainly depends on two factors: the concentration of SARS-CoV-2 in the air and the duration of exposure.3 Situations where more infectious people share air with less dilution (wind and movement) for a prolonged time—such as in crowds and while queuing—will carry some risk of transmission, although this risk is lower than indoors because of the much higher and rapid dilution outdoors and the lack of accumulation.4